REVIEW

CRYSTAL OF

CHEMISTRY

MONOBASIC

AND

CARBOXYLATES

STEREOCHEMISTRY OF

THE

TRANSITION

METALS M.

A. P o r a i - K o s h i t s

UDC

548.3

The importance of carboxylates in organic, inorganic, and analytical chemistry, chemical technology, biochemistry, and other branches of chemical science is fairly well known. Similarly, there is no need to emphasize specially the role of structural data on compounds of this class, as one of the most important components of an understanding of their functional characteristics and as the general basis of their s t e r e o c h e m i s try. It is not at all accidental that the number of structural publications on carboxylates in recent y e a r s has been increasing extremely sharply. These publications cover compounds of different chemical classes and varied structures. A particularly large number of papers have been devoted to the complex carboxylates of the transition metals. The accumulated material urgently r e q u i r e s systematization, which is necessary for the analysis, comparison, and correlation of the r es ul t s obtained, that is, in other words, for the development of the general principles of the s t e r e o e h e m i s t r y of carboxylates. The available data make it possible to approach the question of the role of various factors in the pr ef e r ence for a given type of structure in earboxylate complexes and in the choice of a particular structural function by a carboxylate ligand. It is to these two questions, as applied to monobasic carboxylates, that the pr es e nt review is devoted. It is based on papers presented by the author at the F i r s t All-Union Conference on Inorganic Crystal Chemistry in Zvenigorod in 1977 and at the Thirteenth All-Union Chugaev Conference in Moscow in 1978. I.

SYSTEMATIZATION

OF MONOBASIC

CARBOXYLATES In t e r m s of general chemistry, it is convenient to divide carboxylates into four groups. Firstly, they can be divided into R-active and R-inert, that is, containing or not containing active centers for coordination to a metal in the substituent R of the carboxyl group. Secondly, in each of these types it is possible to distinguish mono- and polybasic carboxylates. The R-active mono- and polybasic earboxylates are the so-called complexonates. Their systematization and the general principles of their s t e r e o c h e m i s t r y have already been examined in a number of publications and reviews by the author and co- w or ke r s [1-5]. The R - i n e r t polycarboxylates have been systematized by A. S. Antsyshkina [6]. The R - i n e r t monocarboxylates, examined in the present review, formally make up 1/4 of this field by structural chemistry, but in fact approximately 70% of all available structural material corresponds to these compounds. The carboxylate anion RCOO- can be regarded as the last t e r m in the s e r i e s of anions of oxyacids, a r ranged in o r d e r of decreasing charge:

sio -, PO -, co -, Rcoo-. The replacement of one of the oxygen atoms of the carbonate ion by the inert group R alters the situation significantly, however, firstly by increasing the polarizability of the oxygen atoms and secondly by shielding part of the space around the anion from interaction with cations. This determines the specific features of the s t r u c ture of carboxylate complexes and the variety of structural functions of carboxylate anions. Repeated attempts have been made to list and systematize the structural functions of the RCOO- anion, but the authors have generally r e s t r i c t e d themselves to an analysis of only unidentate and individual cases of N. S. Kurnakov Institute of General and Inorganic Chemistry. Translated f r o m Zhurnal Strukturnoi Khimii, Vol. 21, No. 3, pp. 146-180, May-June, 1980. Original article submitted December 18, 1979.

0022-4766/80/2103-0369507.50 9 1980 Plenum Publishing Corporation

369

R 1-a 1 O~[;~.O/M

a-2-a R 2-sa

f O//C~.O

j M-M

~ a-2-s ~ s-2s~ k,s zls[~J/Z~

~176

2~

/

R a-3-sa

~

4~

R i-s

M~.O~'~O.#'M

e

/,

s-J-sa

o.#-o" /

/R s-g-sa[M]/ R 8z-a

--~

RPseud~

~ Jfs

,

M

M

M

Fig. i. Structural functions of the c a r b o x y l a t e anion RCOO- in c o m p l e x e s , and their s y m b o l s .

Fig. 2. Schematic r e p r e s e n t a t i o n of the original units of c a r b o x y l a t e s belonging to different f a m i l i e s : [) Sc(HCOO)3, II) Cu4(PhCOO) 4, I/I) [Eu(i-Nic) 2" (H20)4]NO3, IV) Re2CI4(CH3COO)2, V) Sc(CH3COO)3, VI) Cu2" (CH3COO)4(H20) z. bidentate coordination of this anion. An e x a m p l e is provided by the w o r k of Alcock and c o - w o r k e r s in1976 [7], w h e r e 6 or 7 m o d e s of coordination a r e mentioned. In fact, t h e r e a r e m a n y m o r e , since the typical m o d e s of coordination of the c a r b o x y l group include terdentate coordination (and in s o m e c a s e s a t e t r a d e n t a t e function of the carboxyl group is encountered), and since it is n e c e s s a r y to distinguish coordination with a n d without the f o r m a t i o n of a m e t a l - m e t a l bond.

370

i. P r i n c i p a l S t e r e o e h e m i c a l F a m i l i e s of Complex Carboxylates with Bridging RCOO

TABLE

Grou )s No.

Family

F o r m u l a of r e f e r e n c e compound

I

Scandium f o r m a t e

Sc(HCOO) 3

II

Copper (I) benzoate

Cu4(PhCOO) 4

III

V

Europium iso-nico- [Eu(i-Nic)z(HzO)4] (N%) tinate nitrate Rhenium(Ill) acetate RezCI4(CH3COO) 2 dichloride Scandium acetate Sc(CH3COO) 3

VI

Copper (II) acetate

IV

Cu2(CHaCOO)r

Literature cited

i0 ii

12

2

13

Structural c h a r a c t e r i s t i c s

F r a m e w o r k , network, and (less f r e quently) chain p o l y m e r s with single a r l t i - a n t i o r a n t i - s y n bridges* C l u s t e r s (oligomers) with single s y n - s y n bridges Chain p o l y m e r s and linear oligom e r s with double s y n - s y n bridges~ Clusters (oligomers) withdouble syn - s y n bridges Chain p o l y m e r s and linear oligom e r s (in p a r t i c u l a r , dinuclear c l u s t e r s ) with triple s y n - s y n bridges$ Dinuclear c l u s t e r s , linear oligom e r s , and chain p o l y m e r s with quadruple s y n - s y n bridges.$

* For chain p o l y m e r s also with single s y n - s y n bridges. Rotation of some of the RCOO groups with the production of 32-coordination, including a syn-anti bridging function, is possible. TABLE 2. Scandium Acetate Family (chain and dinuclear s t r u c t u r e s with triple s y n - s y n and s y n - a n t i carboxylate bridges) No.

Compound

of the

Sc(CI%COO)s [Ca(CH3NH3+CH2CO0-) a]C12 La(H~O)2(i-Nic)s Nd(H20)2(NHa+CHzCOO-)a.C13' u 9H~O Ce(H~O)(CHaCO0)s [Ru2(Et2NCS2)~](BFa).C~H~O Dimer:

Structural function

ig.

~itera-

c--2--c, c--2--c, 3~

5b

[t21 [t41 Ii5l [161

c--2--c, 32, 32 c--2--c, 3~, 32

5c 5d

[t71 [18]

C--2--C

5a

c--2--c C--2--C

The g e n e r a l s c h e m e of the s t r u c t u r a l functions of the carboxylate anion RCOO- in coordination compounds is shown in Fig. 1. The top row gives unidentate RCOO-, the second bidentate, the third terdentate, and the fourth tetradentate. Bidentate bridging carboxyl groups may have t h r e e conformations: a n t i - a n t i . a n t i - s y n , and s y n - s y n . The second row shows all possible c a s e s of bidentate coordination: a n t i - a n t i , a n t i - s y n with attachment to the M atoms by one oxygen atom, a n t i - s y n with attachment by two oxygen atoms, s y n - s y n Without a m e t a l - m e t a l bond, s y n - s y n w i t h a m e t a I - m e t a l bond, the limiting case of s y n - s y n coordination to one m e t a l atom (chelate coordination), and the special c a s e of a s y m m e t r i c pseudo-cheLate coordination. The other rows of the s c h e m e can be c o n s i d e r e d s i m i l a r l y . AI1 c a s e s of coordination with m e t a l - m e t a l bonds a r e a r r a n g e d along the diagonal: coordination with the f o r m u l a of metal-containing rings on the right, and c a s e s of the purely bridging function of RCOO on the left. T h e r e a r e a total of 18 different ways in which the carboxylate anion can f o r m bonds with m e t a l atoms; of these, at least 14 have actually been found in c r y s t a l s . Some, for example those denoted 2-sa and 4, have been found only in single s t r u c t u r e s . Mention may be made of the symbols used. The principal figure gives the total coordination capacity of the carboxylate anion (subsequently the somewhat slang t e r m "dent[city" will be used), the subscript gives the :171

Ph

0

/

C

c

a

b

oc

c~

0 OdrnT ~ )

d

C

OCHj

Odrnf

f

Fig. 3. Examples of cluster s t r u c t u r e s with single s y n - s y n carboxylate bridges: a) Cu4(PhCOO)4, b) Ma~u4-O)(CH3COO)6, M=Be, Zn, c)Coc(~-O)~" (HCOO)6, d)Co3(dme) 3~u3-C[) ~3-SO4)(CF3COO)3, dme=C2ttaO2Me2, e)[(PyH)3" (MoO)2(NCS)4(~-O)2(ttCOO)]. H20, f) Cd3(dmf)2(/~ 2-C19H13N302)(CH3COO) 2.

L

0 Pd c

R o

x

Pd

R

0

b

c

Fig. 4. Examples of c l u s t e r s t r u c t u r e s with double s y n - s y n carboxylate bridges: a) Re2X4(CH3COO)2, X = C[, I, b) RezCI4(CH3COO)2L2, L = H20 , OS1VIe2, OCHNMe3, OPPh3, c) Pd3(CII3COO)6, d) Pt4(CH3COO)8, e){Cu2(CHqCOO)2} ~. number of m e t a l atoms joined to the RCOO group, a and s r e p r e s e n t anti and syn, and [M] indicates the p r e sence of a m e t a l - m e t a l bond. The letters a and s standing on different sides of the principal figure indicate attachment of the m e t a l atoms to different oxygen atoms of the RCOO group, and the l e t t e r s a and s standing on the same side indicate attachment to the same oxygen atom. The bidentate cyclic group 21 and the cyclic p a r t s of the polydentate groups 32 or 43 do not r e q u i r e letter symbols: The c o r r e s p o n d i n g O - M bonds always have the syn orientation. Neither the unidentate nor the chelate carboxylate ligands contain c h a r a c t e r i s t i c configurational features which would r e q u i r e the development of a special s y s t e m a t i z a t i o n of the s t r u c t u r e s of the complexes which they f o r m . It might be possible to divide the group of compounds with unidentate RCOO into two subgroups, with the attachment of the carboxyl groups according to type 1-a or 1-s. In the g r e a t m a j o r i t y of actual c a s e s , however, 1-s coordination is o b s e r v e d . The specific f e a t u r e s of chelation in c o m p l e x e s of divalent copper axe noteworthy. One of the two C u - O bonds always occupies an equatorial position, and the other an axial position, of the s q u a r e bipyramid or p y r a m i d of copper, and the a s y m m e t r y of chelation is immediately r e vealed in the nonequivalence of the two C - O bonds [this coordination can be r e g a r d e d as i n t e r m e d i a t e between types l - s and 21 (see Fig. i)]. The situation is quite different with bridging ligands. The actual bridging function, together with the s y n - a n t i conformation of the ligand, d e t e r m i n e s to a considerable extent the g e o m e t r i c s i m i l a r i t y or difference 372

in the basic fragments of the crystal structures of these carboxylates. It is therefore natural to distribute the structural material among definite groups (families) of compounds, related by the geometry of the binding of the metal atoms by the carboxyl ligands. As the simplest formal classification, it might be possible to propose a division into i~finitely polymeric and oligomeric (cluster) structures, and within each of these two subdivisions into structures with single, double, triple, or quadruple bridges between each pair of metal atoms. M

\

/M.,.,.

u/

/0--

\o

\~..~/o'

/

I

g l. - R

II

0/)'I"-0

/ 'S

P C

\C-i~

o~ h/0 Ill

n i] 0 O\ J- I \_R\

2 -(:'

~-

_

C" C-~,

,"

~....,,// IV

As in any other formally geometric classification, however, certain logical difficulties arise here. First of all, it is obvious that the concepts of oligomer and cluster are not completely equivalent. The term cluster is usually understood to mean an oligomer containing metal-metal bonds, or in which they can at least be assumed. It is of course possible to distinguish noncluster and cluster oligomers. Often, however, the two have extremely similar structures, and in many cases the question of the presence or absence of a metalmetal bond is disputed and requires additional time-consuming physicochemical and quantum-chemical studies. On the other hand, there are a fairly large number of polymeric structures made up of interconnected oligomeric (in particular cluster) units, having structures analogous to those of the oligomers (clusters) found in systems containing isolated units. It is not appropriate to describe these structures within different families. Finally, it sometimes happens that compounds with very similar compositions, in particular compounds obtained from the same solution, have to be put in different families when formal classification characteristics are used, and this naturally is not very desirable. Various other less important difficulties and ambiguities also arise. Thus taking as a basis the above classification characteristics (polymeric and cluster structures; single, double, triple, and quadruple carboxylate bridges), the author has tried as far as possible to modify the classification in such a way that compounds having similar compositions and/or basic structural elements with similar structures do not appear in different stereochemical groups. Analysis of the available structural material makes it possible to distinguish from these criteria six principal "stereochemical families" - groups of structures related according to the specific features of their principal fragments. These families are listed in Table i, and corresponding examples are shown in Fig. 2. These include two families with single carboxy[ate bridges: Each pair of metal atoms is joined by only one carboxyl group. The first family consists of framework, layer, or chain structures with anti-anti or antisyn bridges. This is the scandium formate family (M. K. Guseinova [8]). The second family consists of cluster or (more widely) oligomeric structures with syn-synbridges. The most striking representative is tetrameric copper(1) benzoate (Drew [9]). The first family contains two subfamilies: a) framework and network structures with anti-anti and antisyn carboxyl bridges; and b) chain structures with anti-anti, anti-syn, and (less frequently)syn-syncarboxylate bridges. In the second family- cluster structures- the bridges generally have the sya-syn structure.* It is thus possible in principle to speak of a continuous transition from chain polymers with syn -syn bridges (assigned to the first family) to linear oligomers and dinuclear clusters (assigned to the second family), Examples of cluster structures with single syn-synbridges are given in Fig. 3. The second pair of families is that with double earboxylate bridges: Eachpair of metal atoms is drawn together by two c a r b o x y l s . One of t h e m i s a f a m i l y of chain s t r u c t u r e s with double s y n - s y n b r i d g e s . ~ l i s is the e u r o p i u m isonieotinate n i t r a t e f a m i l y (Aslanov [10]). The second is the f a m i l y of c l u s t e r s t r u c t u r e s with double s y n - s y n b r i d g e s . The s i m p l e s t r e p r e s e n t a t i v e is the dinuclear rhenium(III) a c e t a t e dichloride ( K o z ' m i n [ii]). It is a l s o convenient to include the f i r s t of these f a m i l i e s those i s o l a t e d - u n i t dinuclear c a r b o x y l a t e s which a r e known to contain a m e t a l - m e t a l bond and which genetically a r e r e a d i l y distinguished f r o m the c o r r e s p o n d * Slight r o t a t i o n of the c a r b o x y [ group, t r a n s f o r m i n g it f r o m the bidentate s - 2 - s into the t e r d e n t a t e bridgingcyclic 32, is also p o s s i b l e . 373

a

~

c

M

~

d

~ ~ 1 7 6

Fig. 5. Variationsof the structuralpatternsin the scandium acetatefamily: a) Sc(CH3COO).~,b) Nd(NI-13CH2COO)3(H20)2CI3" H20, c) Ce(CH3COO)3HzO, d) [Ru2 (Et2NCS2) 5] BF 4" C3tI60. ing chain s t r u c t u r e s as the " t e r m i n a t i o n s " of these chains. On the other hand, it is p o s s i b l e to include in the family of c l u s t e r s t r u c t u r e s not only o l i g o m e r s with directly i n t e r a c t i n g m e t a l a t o m s (including d i m e r s ) but also their s t e r e o c h e m i c a l analogs in which m e t a l - m e t a l bonds a r e not obvious, or even known to be absent f r o m the magnetic and other c h a r a c t e r i s t i c s , but which f r o m s t r u c t u r a l and g e o m e t r i c c o n s i d e r a t i o n s a r e close analogs of t r u e c l u s t e r s , and a l s o p o l y m e r i c chains made up of c l u s t e r f r a g m e n t s (Fig. 4). This division is of c o u r s e e x t r e m e l y a r b i t r a r y , and in f a c t the two f a m i l i e s m e r g e continuously into one another via the dinuclear c o m p l e x e s . The fifth f a m i l y c o n s i s t s of chain and dinuclear s t r u c t u r e s with triple syn - s y n b r i d g e s . This is a g a i n a scandium family, but now not the f o r m a t e but the a c e t a t e (Antsyshkina [12]). H e r e , a p p a r e n t l y , it is not a p p r o p r i a t e to distinguish a s e p a r a t e f a m i l y of c l u s t e r s t r u c t u r e s , since triple bridges a r e f o r m e d only in the s i m p l e s t binuclear c l u s t e r s , and the latter can be r e g a r d e d as f r a g m e n t s r e m o v e d f r o m chains of analogous s t r u c t u r e . M o r e o v e r , few such s t r u c t u r e s a r e known at p r e s e n t . Finally, the sixth family c o n s i s t s of dinuclear c o m p l e x e s and chain s t r u c t u r e s with quadruple s y n - s y n bridges. This is again a copper a c e t a t e family, but now divalent copper (Niekerk and Schoening [13]). The situation h e r e is the s a m e as in the fifth f a m i l y , the only difference being that m o s t of the s t r u c t u r e s studied a r e binuclear c l u s t e r s , and only a few a r e chain s t r u c t u r e s . Thus with i n c r e a s e in the n u m b e r of e a r b o x y l a t e bridges, the f a m i l i e s of p o l y m e r i c and c l u s t e r s t r u c t u r e s gradually a p p r o a c h one another; in s t r u c t u r e s with single bridges they a r e divided c o m p l e t e l y c l e a r l y , in s t r u c t u r e s with double bridges they a r e divided s o m e w h a t a r b i t r a r i l y , and in s t r u c t u r e s with triple and m o r e p a r t i c u l a r l y quadruple bridges it is i n a p p r o p r i a t e to divide them. It is a l s o understandable that t h e r e m a y also e x i s t c a r b o x y l a t e s with bridging RCOO groups, the c r y s t a l s t r u c t u r e s of which cannot be a s s i g n e d to any of the f a m i l i e s Listed. In different s t r u c t u r e s belonging to the s a m e family, the s t r u c t u r a l function m a y be modified. As an example we can consider the s c a n d i u m a c e t a t e f a m i l y (Table 2 and Fig. 5). In the a c e t a t e itself, all t h r e e RCOO groups joining a given p a i r of m e t a l a t o m s a r e bidentate. The s a m e applies to lanthanum isonicotinate [15], but the coordination n u m b e r of the m e t a l is i n c r e a s e d f r o m 6 to 8 by the a t t a c h m e n t of two w a t e r m o l e cules. In t r i s ( g ~ c i n e ) d i a q u o n e o d y m i u m chloride [16], one of the t h r e e c a r b o x y l groups is c o n v e r t e d f r o m a bidentate bridging group to a t e r d e n t a t e b r i d g i n g - c y c l i c group, and the coordination numb er is i n c r e a s e d to 9.

.374

25 ~7

] Single

Triple

Quadruple

Double

I

V

u

III

21, 22, 22(@2H20 )

8 8

{Er(H20)2(i-Nic) 3}1~

{Uu(H20)~(~-~io)+]

22, 22(@4H20 )

21, 2~, 22(@2H,zO)

8

{Sm{H~O)~(Nic)s}a

a~

22, 22, 22

] z 2, 2 2, 2 2

I

6

6 o

{S~

{So(HCOOM3 |

compound

]vii

t0

23

20

t2

8

22, 32, 32(@2H20 ) 32, 32, 3~(~-H20)

9 t0

Ln=Er, Y, Ho

{Ln(H20)2(CH,~COO)a}2

9

21, 21, 3~(-~-2H20)

2 l, 2 i, 21 , 32(~-H20 )

22, 22 , 32, 32

21, 22, 3~(-~2H~O)

t0

9

{co(H~o)~(cH3coo);}z 10

{La(H20) (CH 3COO)3}2oo

{U(CH3COO)J.o

Ln=La, Pr

{Ln(H~O)2(Nic)a}z

28,29

27,24

26

24,25

22

20,21

t6 22, 22, 32(@2I-I20)

{(Nd(H20)2" 9(NrI3cr~2coo)~+~ } ~

t9 17

33, 33 , 33 32, 32, 32(@H20 )

8

literatrite cited

Metal

{Ce(H~o)(cu~coo)3}~

{Gd(HCOO)3}3o~

compound

1Yf2 EN of structural function of RCOO vi2

F u n c t i o n o f t h e C a r b o x y [ G r o u p o n t h e R a d i u s of t h e C o m p l e x - f o r m i n g

[CN of structural function ~iter$ ure of RCOO ci,~ed IM1

D e p e n d e n c e of t h e S t r u c t u r a l

Stereo-I I chemi- [Multiplicity' cal. [of the RCOO family Ibridge

( r M l < r M z)

T A B L E 3.

In c e r t a i n a c e t a t e [17], one w a t e r molecule is lost, but another c a r b o x y l group is c o n v e r t e d into a bridgingcyclic group. All these s t r u c t u r e s a r e chain s t r u c t u r e s . In the r u t h e n i u m d i e t h y l d i t h i o c a r b a m a t e [Ru2(S2CNEt2)5]BF a" C3H60 [18], the chains b r e a k down into d i r e c t s , the functions of the t h i o c a r b a m a t e groups a r e the s a m e as those of the a c e t a t e groups in the p r e v i o u s example, and the coordination n u m b e r of the m e t a l is d e c r e a s e d to 7. In each of the f a m i l i e s listed, it is possible to distinguish definite s u b f a m i l i e s of s t r u c t u r e s , s i m i l a r to one another in their s e c o n d a r y g e o m e t r i c or e l e c t r o n i c c h a r a c t e r i s t i c s . Thus in the family of p o l y m e r i c s t r u c t u r e s with single c a r b o x y i a t e bridges, as a l r e a d y noted, it is p o s s i b l e to distinguish a s u b f a m i l y of f r a m e w o r k and network s t r u c t u r e s and a subfamily of chain s t r u c t u r e s . In the f a m i l i e s of c l u s t e r s t r u c t u r e s with single and double s y n - s y n b r i d g e s it is p o s s i b l e to distinguish s u b f a m i l i e s of dinuclear, t r i n u c l e a r , t e t r a n u c l e a r c l u s t e r s , etc. (examples in Figs. 3 and 4). The family of s t r u c t u r e s with quadruple bridges can be subdivided a c cording to the multiplicity of the m e t a l - m e t a l bond. On the whole, a f a i r l y (but not e x t r e m e l y ) branched s y s t e m a t i z a t i o n is obtained, into which, as shown by e x p e r i e n c e in r e c e n t y e a r s , the r e s u l t s of new s t r u c t u r a l studies of c a r b o x y l a t e s can be r e a d i l y fitted. With the a c c u m u l a t i o n of e x p e r i m e n t a l s t r u c t u r a l m a t e r i a l it will obviously be p o s s i b l e to obtain f u r t h e r details of the g e n e r a l s c h e m e and to distinguish new s t e r e o c h e m i e a l f a m i l i e s or s u b f a m i l i e s a m o n g the c a r b o x y l a t e s . II.

FACTORS

THE

STRUCTURAL

OF IN

DETERMINING FUNCTION

CAR13OXYLATE

COORDINATION

LIGANDS COMPOUNDS

The chief f a c t o r s influencing the s t r u c t u r a l function of RCOO- anions in c o m p l e x e s a r e r e a d i l y listed. ~hey a r e quite obvious. They a r e : the nature of the m e t a l M, its size and e l e c t r o n i c c h a r a c t e r i s t i c s ; the individuality of the substituent R of the c a r b o x y l group, its s i z e and e l e c t r o n i c p a r a m e t e r s ; the nature of the a c c o m p a n y i n g ligands L, and again their size and e l e c t r o n i c c h a r a c t e r i s t i c s ; the s t e r e o c h e m i c a I r e l a t i o n s h i p between the t h r e e components M, RCOO, and L; and finally the c h e m i c a l h i s t o r y of the compound and its conditions of s y n t h e s i s . H e r e , however, ~wo s e r i o u s r e s e r v a t i o n s axe n e c e s s a r y . F i r s t l y , there a r e unfortunately at p r e s e n t no s y s t e m a t i c data on the dependence of the s t r u c t u r a l function of c a r b o x y l groups on the con ditions of s y n t h e s i s , concentration and acidity of the solvent, e t c . , although this question d e s e r v e s the m o s t s e r i o u s attention. Only a few disconnected f a c t s a r e known. Thus in the p r e s e n t r e v i e w this p r o b l e m in g e n e r a l will not be considered. It should be e m p h a s i z e d , however, that its solution r e q u i r e s v e r y close and continuous cooperation between synthetic c h e m i s t s , p h y s i c a l c h e m i s t s studying the p r o p e r t i e s of s u b s t a n c e s in solution and in the solid state, and c r y s t a l c h e m i s t s . Secondly, although the n u m b e r of m o n o b a s i c m e t a l c a r b o x y l a t e s for which s t r u c t u r a l studies have been c a r r i e d out is f a i r l y l a r g e (close to 200), it is n e v e r t h e l e s s p o s s i b l e only r a r e l y to d i s tinguish p u r e s e r i e s , that is, s e r i e s in which only one p a r a m e t e r is v a r i e d s y s t e m a t i c a l l y while the other f a c t o r s r e m a i n unchanged. T h e r e a r e as y e t few such s e r i e s . The p r e s e n t r e v i e w will t h e r e f o r e deal only with trends (qualitative r e g u l a r f e a t u r e s ) and s o m e t i m e s only tentatively. III.

THE

PART

OF

THE

PLAYED

METAL

BY

THE

NATURE

ATOM

The i m p o r t a n c e of the s i z e c h a r a c t e r i s t i c s of the c e n t r a l c o m p l e x - f o r m i n g a t o m is f a i r l y obvious. The l a r g e r the r a d i u s of the m e t a l , the l a r g e r its coordination n u m b e r , and hence the g r e a t e r the tendency for the denticity of the c a r b o x y l ligand to i n c r e a s e . This is i l l u s t r a t e d in Table 3. In this c a s e it is p o s s i b l e to make a " p u r e r' c o m p a r i s o n - to c o m p a r e the s t r u c t u r e s of compounds not only of analogous c o m p o s i t i o n but also belonging to the s a m e s t o i c k i o m e t r i c f a m i l y . The m e t a l s with s m a l l e r r a d i u s a r e on the left, and those with l a r g e r r a d i u s on the right. On the left, the c a r b o x y l a t e g r o u p s a r e bidentate, and on the r i g h t they a r e chiefly t e r d e n t a t e ( t r i p l y - b r i d g i n g or b r i d g e d - c y c l i c , depending on the f a m i l y to which the compound belongs).

376

TABLE 4. Distances C - O c o o r d and C - O t e r m i n a l in Complexes of d- and p-Elements with Unidentate RCOO

No.

Dista nces

Compound

Difference

Literature cited

C-Ocoord

c-Oter m

1,250)

Ni(m-Cl-B z O) ~.4H 20

1,24(l) 1 24 0 ) t,27(1) i ,255(5) i ,247(9) t,26t(4) t 248(4)

0,01 0,08 0,00 0,0t7 0,013 --0,011 0,022

37

t,29(i) i,27(t) i,272(5) 1,260(9) 1,250(4) t,270(4)

t0 tt

C u0,-Tol)2(H20)~(CttaCOO)z. H20 Cu(a-Pic) 2(C1CH~CO0)2 Cu(a-eic)2(C12CttCO 0)2 Cu(CaN2Ha)(CHaCOO)~ Cu(CaNHs)(C1CH2CO0)~ Cu(A NC)2(H20)(HCOO) ~'

t,267(8) 1,28(i) 1,25(2) 1,30(2) 1 282(4) t ,259(2)

t ,247(9) 1,2i(t) t,19(2) t,21(2) i ,229(4) 1,230(2)

0,020 0,07 0,06 0,09 0,053 0,029

42 43 44 45 46 47

12 i3 t4

(NMea)[A1Mea(CHaCO0) ] Si(CHsCOO)a Sn(CHaC6 HS)a(cHa CO0) K[Sn(HCOO)al

i6 t7

Ca[Sn(CHsCOO)3h BzO(CHaCO0)4

1,23(2) t,t96(6) t,2i(4) 1 ,i90(2) 1,199(2) t ,245(2) 1,242(I0) 1,206(4) t,222(4)

0,09 0,178 0,i0 0,ii0 0,093 0,10i 0,04t 0,t34 0,108

48 49 50

i5

i,32(2) t,374(3) t 31(4) 1,300(2) 1,292(2) 1,346(2) 1,283(3) t 34O(4) i ,330(4)

Co(SCN2C2H4)(CttaCO0)2 Co(C3N~H~)(CHsCO0)2 Ni(H20)~(CH.1CO0)o. NiPY2(H.zO)2(CHaCO0)2

6 7 8 9

38 39 40 4i

51 52 53

* ANC = anthracene-9-earboxylate.

TABLE 5. Dependence of the Approximate Ranges of M - M Distances in High- and Lowspin Di- and TrinucIear Cluster Complexes on the Electronic Configuration of the Metal* i

Spin state

s

3' .3

* For

Electronic Metal an( [M-M disconfigura- oxidatior tance, A state Ition d n do

SgIII

N4,35

dl

~3,7

dz

Ti III VnI

d7

Co ~I

~2,8

ds

Ni II

~2,75

d9

CuII

2,6--2,7

d8

Ni II

N 2,55

d7

Rh II

2,35--2,45

d~

Re III

2,2--2,3

d4

MoII

2,05--2,i5

exceptions

3,3--3,35

see text.

377

Another r e g u l a r f e a t u r e can be followed equally clearly: an i n c r e a s e in function with i n c r e a s e in the coordination number (CN) of the metal (generally its radius). This is natural: The l a r g e r the coordination number, the s m a l l e r atom and hence the s m a l l e r the s w a i n in the f o u r - m e m b e r e d metal-containing This is also illustrated by the data in Table 3.* If however

the tendency to adopt a chelate associated with an i n c r e a s e in the valence angles at the c e n t r a l carboxylate ring.

we consider the available structural data for

the purely carboxylate complexes [M(RCOO)n ]• it is found that in them, the chelate function of the type 21 appears only beginning f r o m CN = 7 (for example, (Me4N)[Sn(CH3COO)5] [7]). In s e r i e s of the same type, their number generally i n c r e a s e s with i n c r e a s e in the radius of the metal. Examples are provided by the nicotinates of Ho, Sm, Pr, and La, where the number of such groups is 0, 1, 2, and 2, r e s p e c t i v e l y [30, 20, 21, 20]. In the h e x a - a c e t o c o m p l e x of the large metal thorium (in (CN3H6)2[Th(CH3COO) 6] [31]), all six groups are bidentatecyclic. If however the i n c r e a s e in the radius of the metal is not accompanied by an i n c r e a s e in its coordination number, this tendency should in principle be r e p l a c e d by its opposite: In octahedral complexes, i n c r e a s e in the distance M - O leads to a d e c r e a s e in the angle O - M - O and an i n c r e a s e in the s t r a i n in the m e t a l - c o n t a i n ing ring. Here, however, e l e c t r o n i c f a c t o r s also operate. On going f r o m the high-spin compounds of the t r a n s i tion metals of the fourth period to the low-spin compounds of the fifth and sixth periods, the strength (degree of covalent c h a r a c t e r ) of the M - O bonds i n c r e a s e s , together with an i n c r e a s e in the r e s i s t a n c e of octahedral complexes with 21 carbexyl groups to angular distortion. Thus in Co(HzO)4(CH3COO) 2 [32] the acetate groups are monodentate, whereas in Re(CO)2(P2hs)(CH3COO) [33] and [Ru(PMezPh)4(RCOO)](PF6) , where R=CH3 [34] and Me3N [35], they f o r m r e l a t i v e l y stable metal-containing rings. On the other hand, on going f r o m right to left a c r o s s the fourth period f r o m Ni, Co, and Fe to Sc and Ca, there is a s h a r p i n c r e a s e in the polarity of the bends between the metal and its environment, and it again becomes m o r e indifferent to distortions of the valence angles. In a b r o a d e r sense, the role of the e l e c t r o n i c p a r a m e t e r s of the c e n t r a l atom in the choice of s t r u c t u r a l function for the anion RCOO- can be r e p r e s e n t e d in the f o r m of the following general tendency. In the most ionic compounds the alkali and alkaline e a r t h metal carboxylates a tendency towards potydenticity is observed: Each anion RCOO- tends to surround itself by the l a r g e s t possible number of cations (naturally, in a c c o r d a n c e with size r e q u i r e m e n t s ) . An example is provided by the strontium acetate Sr(Ch3COO)(CH3COS) [36]. The ligand is tetradentate, and f o r m s bonds with three s t r o n t i u m atoms, including one p s e u d o - m e t a l - c o n t a i a i n g r i n g - "pseudo" because the S r - O bond is predominantly ionic, and also because the ring is nonptanar. -

-

In compounds of the r a r e e a r t h elements, the basic predominant function of the carboxylate ligand is terdentate bridging-cyclic; a purely cyclic function is also typical (see Table 3). With i n c r e a s e in the covalent c h a r a c t e r of the M - O c a r b bond in compounds of the d transition metals, the tendency towards cyclization dec r e a s e s , and the bidentate bridging function of RCOO becomes the main function (see Tables 6-10). This applies p a r t i c u l a r l y to the low-spin compounds of the d metals. When transition metals a r e r e p l a c e d by nontransitton elements, there is a significant i n c r e a s e in the tendency towards the unidentate attachment of carboxylate ligands. The available s t r u c t u r a l data r e l a t e chiefly to boron, aluminum, silicon, and tin. At the same time, the M - O c a r b bond becomes m o r e covalent. This is indicated by the i n c r e a s e in the difference in the lengths of the C - O bonds with coordinated and noncoordinated oxygen atoms of the carbexyl group. Table 4 gives a s e r i e s of compounds with unidentate groups: f i r s t l y , the high-spin compounds of cobalt and nickel with unstable polar M - O c a r b bonds; then various compounds of divalent copper, in which the equatorial bends a r e m o r e covalent and s t r o n g e r ; and finally the carboxylates of nontransition metals. The diff e r e n c e in the C - O distances to coordinated and noncoordinated oxygen i n c r e a s e s in the sequence: It amounts to 0.02 A in the cobalt and nickel compounds, an average of 0.05 ,~ in the copper complexes, and an average of 0.11 A in the complexes of nontransition metals, corresponding to the difference in the lengths of a single and a double bond. For the d transition metals, as already noted, the bidentate bridging function of the carboxyl groups is most typical. This explains the fact that s t r u c t u r e s of the cluster type a r e widely encountered in the c a r bexylates of the transition metals. F i g u r e s 2-4 show various examples of dinuclear, t r i n u c l e a r , t e t r a n u c l e a r , and hexanuclear c l u s t e r s , including compounds with single, double, triple, and quadruple bridges. *Only the groups 21, and not

q 78

3z, a r e chelating.

T A B L E 6. R h - R h and R h - L a x

D i s t a n c e s in D i n u c l e a r C o m p l e x e s

Rhodium(K)

1

Rh~(CHsCOO)4(H~O)2

2

Rh~(CFaCOO)a(H~O)~

2,3855(5) 2,396(2) 2,409(2) 2,3i65(20) 2,3963(2) 2,397(2) 2,403(3) 2,4196(4)

3 4 5 6 7 8 9 l0 tt

[Rh~(CHaCOO)a(H~O):IC1Oa.tI~O* RhdCHaCOO)a(Py)~ (CN~H~)~[RhdCHaCOO)~CI~] Rh~(CHsCOO)~(NHEt2)~ Rh~(CH~COO)a(CO)~ Rh~(CHaCOO)r Rh2(CHaCOO)a(P(OPh)~)~ Rh~(CHaCOO)a(PPha)~ Rh~(CHaCOO)~(P(OMe)a)~

2,4450) 2,449(2) 2,4555(3)

12

Rh~(CHsCOO)~(NO)(NO~)

2,4537(4)

* Electronic configuration d ~-d

L-M-M-L

L-M-=M-L / ~

d~(~L,WM) %

/

/ ")'~ .\~* (~-M)

2,3i0(3) O L 2,280 ) 0 L 2,260 ) 2,220 ) O L 2,227(3) NL 2,57(1) C1 2,308(3) N L 2,092(4) CO 2,42(t) PL 2,418(3) PL 2,47(4) PL 2,437(5) PL fi,933(4) NO

2,430(3)

~2,010(4) NO 2

59

60

6t 62 63 64 65 66 66 67 66

65

L-M(HCOO)4M-L

M(RCOO)4M

)

~-i._.~ o " ""7--~

\ X I

-

--

~uZe cited

7.

,,..

e'I %M-M) 02U ",,\

Litera-

Distances, M-M M-Lax

CompounJ

Noo

b~,

~,

~,"/ . . . . . . /// ",/,1'

//". ~ (~-M) b ~,, '

i,

bl.

*

~T'~(M-M).ego / \_~x.\- ~ - ' f P ' - - ' / / " - - - - \ - ~ ".-,,'.- / --- --e(.-L) r \ ~II+"~'~'

\~"

"E.

), ....... ~/M t - M~ni-e"n-# u ~;(M-MM-L) a;,g --

II

6'(M-L,M-M) --

+

....

O2u IF.

6(M-L'M~M) ~;'g

" " /-~."~ \ 6
~tM-~/ "

. . .

/

aZu

-/ ~

'~------:--

-t +

-- - - - - - - \

X ~/(M-L,M-MJGo, )

a19

',]"

-.:. dg ~

"

II

~r~eu~,

\\6 01~

al.9

O: J@ \ ~"A:-II'-/~ /. . . ~ " "};, e 2u

'~;'~._// eu

\ II':~

6(M-L'M-M)II

1 . . . .

7-

i+.~

~ M M

}Y 01. 9

~

"44"

Z(RCOO) I

I

l/

III

IV

Fig. 6. Diagrams giving the order of the valence MO "servicing" chiefly the axial fragment L-*Rh-Rh -~- L in the rhodium carboxylates Rh2(RCOO)4L2: I) according to Porai-Koshits and Antsyshkina; II) according to Cotton and co-workers; Ill) according to Norman and Kolari; IV) according to Cristoph and Koh. The m o s t c h a r a c t e r i s t i c s t r u c t u r e f o r m a n y t r a n s i t i o n m e t a l s is the d i n u e l e a r l a n t e r n c o m p l e x with four b r i d g i n g c a r b o x y l g r o u p s , often s u p p l e m e n t e d by two a p i c a l d o n o r ligands L (VI in Fig. 2). The m e t a l - m e t a l d i s t a n c e s in c l u s t e r s t r u c t u r e s lie in a v e r y wide r a n g e : f r o m 2 to m o r e than 4 (Table 5), E x a m i n a t i o n of the d i s t a n c e s t y p i c a l of v a r i o u s t r a n s i t i o n m e t a l s r e v e a l s two t r e n d s : In the h i g h spin c l u s t e r s , the m e t a l - m e t a l d i s t a n c e d e c r e a s e s a s the e l e c t r o n i c c o n f i g u r a t i o n of the m e t a l b e c o m e s r i c h e r in e l e c t r o n s , and in the h i g h - s p i n c l u s t e r s it d e c r e a s e s as the e l e c t r o n i c c o n f i g u r a t i o n of the m e t a l b e c o m e s p o o r e r in e l e c t r o n s ( f r o m d 8 to d 4). It is a m i n i m u m f o r the c o n f i g u r a t i o n d ~, m a k i n g p o s s i b l e the f o r m a t i o n of a q u a d r u p l e m e t a l - m e t a l bond, w h i c h h a s p l a y e d s u c h an i m p o r t a n t r o l e in the r e s e a r c h w o r k of the Soviet s c i e n t i s t V. G. K u z n e t s o v and the A m e r i c a n c h e m i s t F. A. Cotton. T h e r e a r i s e s the q u e s t i o n of defining the m e t a l - m e t a l d i s t a n c e s at w h i c h it is p o s s i b l e to s p e a k of a d i r e c t bond between t h e m , and those at w h i c h it is not. T h e r e is no doubt that s u c h bonds e x i s t in l o w - s p i n c o m p l e x e s of the d4-d 8 e l e c t r o n i c c o n f i g u r a t i o n s . T h e r e is equally no doubt that t h e s e bonds a r e not f o r m e d i n 379

the high-spin compounds of scandium and titanium, with m e t a l - m e t a l distances exceeding 3.6 A. The question is not so obvious in the c l u s t e r s of vanadium with distances of the o r d e r of 3.3 A, and is fairly c o n t r o v e r s i a l in the high-spin d i m e r s of cobalt and nickel with distances of 2.7-2.8 A and of copper with distances of 2.6-2.7 A. To d e t e r m i n e the nature of the bonding in the carboxylate c l u s t e r s of these metals, it is n e c e s s a r y to i n t r o duce v a r i e d p h y s i c o c h e m i c a l data - the magnetic and r e s o n a n c e c h a r a c t e r i s t i c s , their t e m p e r a t u r e dependence, etc., together with a s e r i o u s t h e o r e t i c a l analysis of the e l e c t r o n i c s t r u c t u r e s of the c l u s t e r s . All this f o r m s the subject of special study.* o

Only a few special points concerning the dinuclear carboxylates of Cu(II), Rh{II), Re(III), Mo{II) and Cr(II) will be dealt with. 1. D i n u c l e a r

Copper

(II)

Carboxylates

The dinuclear lantern complex s t r u c t u r e of LM(RCOOt)ML was d i s c o v e r e d in 1953 for Cu2(CH3COO) t(HI20)2 [13]. These complexes a r e much m o r e widely encountered among compounds of divalent copper than among the neighboring metals of the fourth period - Zn, Ni, Co, Fe and 1Vln. In p a r t i c u l a r , the difference in the s t r u c t u r e s (and h y d r a t e composition) of the acetates of these m e t a l s is symptomatic: Cuf(CI.I3COO)4(I,I20) 2 is a c l a s s i c a l r e p r e s e n t a t i v e of dinuclear complexes, w h e r e a s the acetates of Ni, Co, and Mn have the composition M(I-I20)4(CH3COO)2, the complexes are m o n o m e r i c , and the acetate groups a r e unidentate. The diff e r e n c e is even m o r e distinct in the general data on the composition, i n f r a r e d s p e c t r a , and magnetic s u s c e p t i bility of the metals. F o r the adducts of the carboxylates of Mn, Fe, Co, and Ni with the m o s t v a r i e d R and L, the m o s t typical r a t i o s n M : n L a r e 1 : 2 or 1 : 4 [54-57] (see below for the exceptions), w h e r e a s copper with the same R and L generally gives the 1 : 1 compounds, for which physicoehemical data indicate a dinuclear c o m plex s t r u c t u r e . The unique tendency of divalent copper to f o r m dir -Aear complexes is d e t e r m i n e d by its e l e c t r o n i c configuration d s and the accompanying static J a h n - T e l l e r effect. H e r e , three f a c t o r s apparently operate s i m u l taneously. 1) Judging f r o m the available p h y s i c o c h e m i c a l data (magnetic, ESR), d i r e c t C u - C u interaction in these c o m p l e x e s is e i t h e r a b s e n t or v e r y weak. The coordination of copper must be a s s u m e d to be square pyramidal. This coordination is generally much m o r e widely encountered in Ca(K) compounds than in compounds of neighboring divalent metals. The c o r r e s p o n d i n g hybrid state is energetically m o r e favorable for copper than for Fe(II), Co(II), Ni(II),and Zngi). 2) In the s q u a r e p y r a m i d of copper(II), the apical bond is always much longer (4+ 1 coordination). The i n c r e a s e in the length (decrease in the strength) of this bond r e d u c e s considerably the s t e r i c hindrance in t.he contacts between the atoms ORCOO and the atoms of the apical ligands (see below for m o r e details) and makes possible the attachment, in the apical positions, of the m o s t v a r i e d molecules, including large branched m o l e cules. 3) At the same time, this construction makes i t possible to p r e s e r v e strong bonds with all eight oxygen atoms of the RCOO groups. Thus in those c a s e s where the accompanying ligands cannot compete with ORCOO for a position among the four strong C u - l i g bonds (onlyfour, and not six, as in the complexes of neighboring metals), dinuelear lantern complexes a r e p r a c t i c a l l y always formed. 2. D i n u c l e a r

Rhodium

(II)

Carboxylates

Carboxylate lantern complexes with a c l e a r l y defined m e t a l - m e t a l b o n d w e r e f i r s t detected in 1962 in a study of the s t r u c t u r e of dirhodium t e t r a - a c e t a t e dihydrate Rh2(CH3COO)4(H20) 2 [58].~ The dinuclear c a r b o x y l a t e s of rhodium(II) with the composition Rhf(RCOO)4Lf, and the corresponding distances R h - R h and Rh--Lax, a r e given in Table 6.

* The t e m p e r a t u r e dependence of the magnetic p r o p e r t i e s indicates that the exchange i n t e r a c t i o n in c l u s t e r s of V, Co, Ni, and Cu takes place chiefly through bridging groups. This however does not exclude a weak d i r e c t bond between the m e t a l atoms. The t e r m "lantern complex" appeared at the s a m e time; the r o l e of candle is played by the e l e c t r o n s drawing the m e t a l atoms together.

380

TABLE 7. Metal-Metal Distances in Various Dinuelear fates with the d 4 Electronic Configuration of the Metal

Carboxy-

Distances, No.

- -

Compound

l 2 3 4

M-M

M- Lax *

Literature cited

Re2(CI'[aCOO)~Brz Re2(CH.~COO)~.CI.o l~e2(PhCO0)4C12.2CHaC1 Re2(n-CaH~CO0)a(Re04)~

2,234(i) 2,236(i) 2,235(2) 2,251(2)

2,60 2,45 2,49 2,t8

Br C[ CI

OJleO~

76 76 77 78

5 6 7 8 9 i0

R%CI~(i-C.~I{TCOO)~(FieOa) (NH4)dRe2Cla(HCOO)~CI.~I Re-.Cla(CH.~COO)2(OSMeo]~ R%CI~(CItaCOO)2(OCHNMc2)2 R%Cla(C[IaCOO)_~(OPPh.q)2 R%CI~(CH3COO)2(H20)~

2,259(3) 2,260(57 2,237(i) 2,239(2) 2,236(2) 2,224(5)

2,28 2,7t 2,35 2,35 2,36 2,50

0 T~eO, CI OL OL OL 1I20

79 8O 8I 82 83 84

ii t2

Re..CI~(CH~COO)~_ Re2Ia(PhCO0)~

2,2it(3) 2,i97(i)

2,88

13 14 15 t6

M%(CF,~C00)aPy2 M%(NH~CH~C00)a(SOa)!" [M%(NH,~CH~CO0)4CI~]CI2 Mo~(PhCOO)~(%K~-aOa)2

2A29(2) 2,1i5(i) 2,107(i)cp 2,t00(i)

2,55 2,93 2,84cp 2,66

t7 18 i9 20 2t 22

M%(PhCO0), M%(CH.~CO0)4 Mo2(HCO0)4 MO2(CFaCO0)~ M%(CM%CO0)2 MoCr(CH~CO0)4

2,0960)

2,88

2,093(1) 2,09t(2) 2,090(4) 2,088(i) 2,050

2,65 ~ 2,65 2,72 ~ 2,90 2,55 ~

il 85 86 87

Npy 0SO, Cl O

88

89 9O 91 92 93 90 95

~The data on the M - L a x distance a r e rounded off to hundredths of an i . O atom of the carboxyl group of a neighboring complex.

a

,

I'I~C

/

\

, N,r c ~ c o

b

/

~

R'---.- c -

\

II--C

2,21 12,8

F i g . 7. I n t r a m o l e c u l a r s t e r i c h i n d r a n c e R . . 9 M a n d M . . . the a n t i - a n t i and s y n - s y n c o n f o r m a t i o n s .

M for

The a u t h o r s of the f i r s t s t r u c t u r a l s t u d y of Rh~(CH3COO)4(K20) 2 p r o p o s e d a m o d e l f o r the s e q u e n c e of m o l e c u l a r o r b i t a l s of the l i n e a r f r a g m e n t L - - R h - R h - - L [68]: c r ( M - - L ) a' (M - - L) a (M - - M) n ( M - - M) 5 (M - - M) 5* (M - - M) ~t* (M - - M)

HO0

I g*L(M~"- L) (V* (M - - L) (l* (M - - M),

a c c o r d i n g to w h i c h the R h - - R h bond i s to be r e g a r d e d a s f o r m a l l y a s i n g l e bond ( s c h e m e I in F i g . 6).* F o u r bonding o r b i t a l s , l o c a l i z e d c h i e f l y in the m e t a l - m e t a l r e g i o n ((r, t w o ~ , and 5), t h r e e a n t [ b o n d i n g o r b i t a l s , l o c a l i z e d in the s a m e r e g i o n (t* and two 7r*), a n d two or-bonding o r b i t a l s , c o n c e n t r a t e d p r e d o m i n a n t l y b e t w e e n the l i g a n d s L a n d t h e m e t a l a t o m s (HOe i s the h i g h e s t o c c u p i e d o r b i t a l , a n d LUO i s the l o w e s t u n o c c u p i e d * A n a l m o s t i d e n t i c a l s c h e m e w a s l a t e r p r o p o s e d by D u b i c k i a n d M a r t i n [69].

381

orbital).

Since all the (~-orbitals a r e f o r m e d chiefly by linear combinations of the s - and dz2-AO of the two

m e t a l a t o m s and the p z - o r b i t a l s of the axial ligands, the effect of the MO denoted (r ( M - M ) and ~* ( M - M ) also extends to the r e g i o n M - - L , and the effect of ( , ( M - L ) , r o ' * ( M - L ) and (r, * ( M - L ) extends to the c e n t r a l M - M region, and this d e t e r m i n e s the mutual t r a n s - i n f l u e n c e of the R h - R h and R h ~ - L bonds. Since the M - M bond is m o r e covalent than the donor L - - M bond, the , t a n s - i n f l u e n c e of the c e n t r a l bond should p r e d o m i n a t e . In other words, the c e n t r a l bond is f a r t h e r strengthened by the d e c r e a s e in the s t r e n g t h of the p e r i p h e r a l L ~ M and M - - L . In [70] this was f o r m u l a t e d as an i n c r e a s e in the s t r e n g t h of the single bond, bringing the R h - R h bond c l o s e r to a double bond (which in g e n e r a l is t e r m i n o l o g i c a l l y u n s a t i s f a c t o r y ) . The d e c r e a s e in the length of the R h - R h bond c o m p a r e d with the usual value for a single bond (2.6-2.7 A) naturally f a c i l i t a t e s the c o n t r a c t ing action of the bridging c a r b o x y l groups. Somewhat later, F. A. Cotton and c o - w o r k e r s p r o p o s e d another scheme, a c c o r d i n g to which the multiplicity of the R h - R h bond had to be r e g a r d e d as equal to 3 [71, 72]. A c c o r d i n g to this s c h e m e , of the anti bonding orbitals localized in the m e t a l - m e t a l region, only one, ~,* is filled. The other e l e c t r o n s a r e a r r a n g e d in the (r-bonding and (r-antibonding o r b i t a l s localized in the L - - R h and R h - - L r e g i o n s (scheme II).~ In [72], p r e f e r e n c e was given to the second s c h e m e , chiefly because of the unexpectedly s h o r t length of the R h - R h bond in Rh(II) a c e t a t e (2.3855(5) A) [59]. It is possible, however, to put f o r w a r d a number of a r g u ments in s u p p o r t of the f i r s t s c h e m e for the sequence of the MO. The chief of these is the f u r t h e r d e c r e a s e in the R h - R h distance when one e l e c t r o n is lost (in the cationic c o m p l e x R h I I - R h III in [Rh2(CH3COO)4(H20)2]C104" H20, the R h - R h distance is 2.316(2) /~) [61]. This m e a n s that the last occupied level is antibonding, and localized in the r h o d i u m - r h o d i u m region. This c o r r e s p o n d s to the f i r s t and not the second MO s c h e m e ~ [63]. The r e s u l t s of an ESR study of the 1 : 1 adducts of dirhodium t e t r a k i s ( t r i f l u o r o a c e t a t e ) in solution [73], and the x - r a y e l e c t r o n s p e c t r a (XES) for the guanidine complex of dirhodium dichloride t e t r a - a c e t a t e , also a g r e e with the view that the R h - R h bond is a single bond. In 1978, N o r m a n and Kolari c a r r i e d out a t h e o r e t i c a l calculation for dirhodium t e t r a f o r m a t e dihydrate in the SCF-Xc~-SW approximation, including in the system all the atoms of the complex, including the formate groups [75]. Allowance for the equatorial ligands naturally increases the total number of molecular orbitals and mixes the AO of the equatorial ligands into those ~-, r and 5-MO which "initially" related to the central fragment. Scheme III distinguishes only those molecular orbitals of the complex which are localized chiefly on the central linear fragment. In the sequence obtained, ff(M -L)(r'(M- L)r M)~(M- M)6 (M- M)~r* (M- M) ff*(M - M) I~*(M - M), adefinite rearrangement of the occupied orbitals has taken place compared with scheme I, HO0

LUO

but the m a i n r e s u l t r e m a i n s the s a m e : The last two occupied MO have antibonding c h a r a c t e r in the M - M r e gion, and the bond r e m a i n s f o r m a l l y a single bond. 3.

Dinuclear

Rhenium(III)

and

Molybdenum(II)

Carboxylates

Table 7 gives data on the m e t a l - m e t a l d i s t a n c e s in v a r i o u s dinuclear c a r b o x y l a t e s of m e t a l s with the d 4 e l e c t r o n i c configuration. For Re(Ill), data a r e also given for c o m p l e x e s in which s o m e of the bridging c a r boxyl groups have been r e p l a c e d by t e r m i n a l halogen a t o m s , and also data on lantern c o m p l e x e s without apical ligands. * F. A. Cotton and c o - w o r k e r s s t r i c t l y distinguish only the c e n t r a l R h - R h f r a g m e n t , without allowance for the axial ligands L. They t h e r e f o r e o p e r a t e d with two nonbonding MO (rn and r d i r e c t e d along the axis outwards f r o m the c e n t r a l f r a g m e n t R h - R h , and accordingly had not an 1 8 - e l e c t r o n but a 1 4 - e l e c t r o n s y s t e m , filled in the o r d e r t

c ( M - - M) ~ (M - - M) 6 (M - - M) % (In8

,

(M

--

M) I a* ( M - M) (~* (M - M)"

HO0 LUO F o r c o m p a r i s o n with s c h e m e I, the axial ligands w e r e taken into account in s c h e m e II; this r e q u i r e d the r e p l a c e m e n t of the two nonbonding o r b i t a l s a n and a n by two bonding o r b i t a l s r ( M - L ) and a ' ( M - L ) and two antibonding o r b i t a i s ( , * ( M - L ) and (~'*(M-L). Their exact position is of c o u r s e not known, but Cotton's idea c o r r e s p o n d s to any sequence in which they a r e all situated below r and a * ( M - M ) . The f i r s t s c h e m e p r o v i d e s an equally n a t u r a l explanation for the f u r t h e r d e c r e a s e in the length of the M - M bond to 2.28 ~ o n going to the (larger) Ru a t o m in Ruz(C3HzCOO)tC1 (chain s t r u c t u r e with bridging CI a t o m s between the dinuclear f r a g m e n t s ) . H e r e , the e l e c t r o n i c configuration is d4-d 5. The l o s s of t h r e e e l e c t r o n s p r e s e r v e s one unpaired e l e c t r o n each in the h i g h e s t occupied levels closely situated to one another (which c o r r e s p o n d s to the magnetic m o m e n t of the compound), i n c r e a s i n g the f o r m a l multiplicity of the bond to 2.5 with an i n c r e a s e in its s t r e n g t h due to the t r a n s - i n f l u e n c e with the axial v - C 1 a t o m s . 382

TABLE 8. C r - C r and C r - L a x D i s t a n c e s in Dinuclear Carboxylares of Divalent C h r o m i u m No,

Distances, Cr-Cr C~-Lax

Compound

Axial ligand *

2~388(2) 2~44(t) ! O IICO0 2,47(7) from neighbor 2,288(2) 1 2,327(41 Oncoo from neighbor

Crc(CM%CO0)~

C%(CItaCOO)4

Literatllre cited 108 t09

(NII4)4[Cr:(gOa)4(II~O)~t . H~O Cr.,(CHaCOO)4(ILO)e Cr.~(HCO0)4(II~O)~ ., 3 "l/3tt~O

2,2t4(i) { 2,300(3' 2,362(1) 2,272(3~, 2,373(2) 2,268(41 2,360(2) 2,2t0(6)

It20 H.,O

t it} 59 111

Cr,,(CH.~C00)4(NC~Hn)2 Cr2(HCOO)4(NCsH!)a

2,342(2) 2,338(7) 2,408(t) 2,308(3)

NC:,H1, NC~H4

tt0 108

Ch(anth~acene-9-carboxylate)4" (Me0CHaCtI.20Me)

2,283(2) 2,283(5)

~in..~O(Me)

108

9 10 tt

Cre(CH.~gOO)4(CltaCOOtl)z Cr2(PhCOO)a(PhCOOH)2 Crs(HCO0)~.2II20

2,300(t) 2,306(3) 2,352(3) 2,295(7) 2,45t0) 2224(2)

CH~O(MB) CH~COOH PhCOOH Cr(H~_O):,

t10 t08 108

12

Cr=,(CF.~CO0)a(Et20)~

L541(1) ,)244(3)

gt~

t08

.(Hcoo>.

* The atom through which the bond with Cr is f o r m e d is underlined.

.

~-4o~

br

Fig. 8. Contacts O R C O O . . . CH a tn the chain of dinuelear c o m p l e x e s in the s t r u c ture of Cra(CMEaCOO) 4, The m e t a l - m e t a l bond in c o m p l e x e s with the d4 configuration is a quadruple bond; it is f o r m e d by the filling of the bonding MO ~ ( M - M ) a, ~ ( M - M ) 4, g ( M - M ) 2. The i n c r e a s e in the multiplicity of the bond leads to a further d e c r e a s e in the m e t a l - m e t a l distance c o m p a r e d with that obtained for RhffI) and Ru(IIIqV) e a r b o x y l ares: to 2.20-2.26 A in the Re(Ill) c o m p l e x e s , and to 2.06-2.11 A in the 2r complexes. The d e c r e a s e in the length of the M - M bond with the sharp i n c r e a s e in its multiplicity in the e a s e of r h e n i u m is not so great, s i m p l y because the factor of the "drawing together" of the m e t a l a t o m s by the e a r boxyl groups c e a s e s to be effective: The distance ~ 2.2 A c o r r e s p o n d s to the usual O . . . O distance in the c a r boxyl group. The M o - M o distance is appreciably s h o r t e r than the R e - R e distance; if c o m p l e x e s of the s a m e type are compared, the d e c r e a s e amounts to ~ 0.1-0.14 A. For c o m p l e x e s of equivalent c o m p o s i t i o n , identical e l e e Wontc configuration, and identical bond multiplicity, the question obviously r e I a t e s to the actual nature of the m e t a l A s the c o v a l e n t radii of Mo and Re (for single o--bonds:) are approximately the s a m e , we can apparently speak only of the c h a r a c t e r i s t i c f e a t u r e s of their 7r- a n d / o r 6-interaction. In v a l e n c e - b o n d language this can be f o r m u l a t e d as a d e c r e a s e in the radius of action ("deepening") of the c o r r e s p o n d i n g d-orbitals of the MoII a t o m c o m p a r e d with Re III, which e n s u r e s their optimttm o v e r l a p at s m a l l e r internuclear distances.

383

4.

Dinuclear

Chromium(II)

Complexes

In the s t e r e o c h e m i s t r y of dinuclear chromium(II) complexes with a f o r m a l l y quadruple C r - C r there is one c h a r a c t e r i s t i c f e a t u r e which leads to some confusion.

bond,

In the d i m e r s of Re(HI) and Mo(II), the M - M distance depends little on the nature of the ligands. Thus in the case of Re(III), the p a r t i a l or complete r e p l a c e m e n t of the bridging ligands RCOO by t e r m i n a l halogen atoms produces only a minimum change in the length of the R e - R e bond. In the halides Re~Hals, where the R e - R e bond should be elongated slightly by the repulsion of the halogen atoms of the two halves of the dimer, the R e - R e distances i n c r e a s e by only 0.02-0.03 A c o m p a r e d with Re2(RCOO) 4. The situation is quite different for the f o r m a l l y quadruple C r - C r bond. Here, the m e t a l - m e t a l distance depends much m o r e sharply on the nature of the ligands. All s t r u c t u r a l studies of dinuclear Cr(II) complexes c a r r i e d out up to the p r e s e n t can be divided into three groups. 1) In complexes with noncarbexylate bridging ligands, which for s t e r i c r e a s o n s do not allow the p r e s e n c e of apical ligands, the C r - C r distances lie in the range 1.83-1.89 A. Examples are

Cr--Cr

l,SJO(4)~e]

l, Sz;YO~[g~

,/

ce--ee

Me

c~ce

Ce--Cr

Cr'--Cr

~SSe(/)A~

O~fiO Me

\

/

Ce--Ce

Me Me \

CP~er

2) In the complexes with various h y d r o c a r b o n organic ligands - strong donors - the C r - C r distances i n c r e a s e by m o r e than 0.1 A and lie in the range 1.97-1.98 A. Examples a r e Li4[Crz(Ctta)8] 94C4H80 [104, 105], Li4[Cr2(C4tts)4] "4C4H80 [106], Cr2(C3Hs)4 [107]. 3) In carboxylate complexes with apical ligands or with the ORCOO atoms of neighboring complexes r e p l a c i n g them, the C r - C r distances i n c r e a s e to 2.21-2.54 A. The s t r u c t u r a l data a r e given in Table 8. This m a r k e d dependence of the C r I I - c r l I distance on the nature of the ligands in general and on the c o m position of the carboxylate complexes in p a r t i c u l a r p r o v e s that the e l e c t r o n i c s t r u c t u r e of the dinuclear c a r boxylates of c h r o m i u m differs significantly f r o m the s t r u c t u r e of the complexes of the 4d- and 5d-metals with analogous g e o m e t r y . The Cr-= Cr bond is much m o r e "flexible" than the quadruple bond MoII-Mo II, W I I - w l I , or R e I I I - R e III. In this r e s p e c t , chromAum(II) also differs considerably f r o m rhodium(II). In d i c h r o m i u m and dirhodium t e l ~ a - a c e t a t e dihydrates of analogous composition, the M - M distances a r e a l m o s t identical, but on going f r o m one carboxylate to the other, and on the r e p l a c e m e n t of the axial ligands L, the m e t a l - m e t a l distance in the d i c h r o m i u m carboxylates with a f o r m a l l y quadruple bond changes within a much wider range than that in the r h o d i u m carboxylates. This range c o v e r s 0.3 A in the f i r s t case, and less than 0.1 A in the second. An unexpected f e a t u r e of these data is not only the wide range of C r - C r distances but also the fact that in c h r o m i u m carboxylates, this distance is not s h o r t e r but much longer than that in molybdenum carboxylates of analogous composition. A striking illustxation of the unusualness of the situation is provided by a c o m p a r i son of the a c e t a t e s Mo~(CH~COO) 4, MoCr(CH3COO) 4, and Cr2(CH3COO) 4. In the dimolybdenum acetate, the M - M distance is 2.093(1) A, and in the mixed c h r o m i u m molybdenum acetate, in a c c o r d a n c e with the d e c r e a s e in the size of the atom on going f r o m the fifth to the fourth period within the group, the length of the M - M bond dec r e a s e s to 2.050(1) A. It would be logical to expect a f u r t h e r d e c r e a s e in the M - M distance on going to dic h r o m i u m acetate. In fact, it does not become s h o r t e r , but i n c r e a s e s to 2.288(2) A. A completely analogous effect is o b s e r v e d in group VIII, on going f r o m Rh2(RCOO)4L2 to Co2(RCOO)4L~. In the Rh complexes, as a l r e a d y noted, the M - M distances lie in the r e g i o n of 2.4 A, w h e r e a s in the cobalt complexes of analogous composition they a r e i n c r e a s e d to approximately 2.8 A [112].

384

GO

Unidentate

Terdentate

Bidentate bridging

Denticity of the group RCOO

(Pytt)4lMO2Oa(tt-0)

(NCS)s(iiCO0)~l

1-a

a-3-sa

a-3-sa

Ln(IICOO) a

Th([ICOO) l

a-2a, a -2s

Sc(IICO0) a

Dinuclear complexes

Framework

Framework

Framework

31

Ia

8

Groups

113

23

39

Dinuclear complexes Chains

Mononuclear complexes

s-2- s 32

Pr(I IzO)a(CFaCO0) a

M=-.l[u, Co, Ni

M(IIeO)dCHaCOO)z 1-s

s-2 s 3:

15 Chains s-2 s 3z

Ln(ltzO)2(i-Nic)a

t'(C[ [aC()O)

20, 21 Dimer

17, 24, 25

12

Chains

Chains

literature cited

s-2- s 3:

s.-2-s. 3:

S-2-S

conformatior. characteristic structure of RCO0

Other carboxylates

on the Size of the Substituent

Ln(H~O)._,(Nic)a

Ln([120)(CI laCOO)a

Sc(CI laCOO)a

compound

CarboxyIate

literature cited

of the Rridging

Formates conformation~ characteristic of PC(X3 [ structure . . . . .[ . . .

of the Conformation

compound

TABLE 9. Dependence R in the Groups RCOO

T A B L E i0. G e o m e t r i c C h a r a c t e r i s t i c s of the Adducts of Copper(II) Carbexylates with Urea o~

Distances, ~

~%k ~ ~

tmL)

H CHa PhCIIa CICH~ FCII~

( C u _ O ) a~ ( o . . . O ) a v

' LiteraCu - C u

ACu

2,657 2,624 2,626 2,531 , 2,643 2,665

0,21 0,2t 0,20 0,20~, 0,20~ 0,21

~Z

122,0 i ,272 4,70 125,0 1,250 4,31 125,0 1,256 124,5 1,260 2,80 t23,5 1,277 2,58 126,8 1,255

t ,955 2 005 '1,967 1,970 t,966 1,970

2,2.1t 9. O ..A) '1 ,1') 2,22 2,23 2,2~

ture

Icited 119 120, t21 122 123 t24 125

Note: 1) ( O . . . O)av i s the d i s t a n c e b e t w e e n the p a r a l l e l p l a n e s of t h e c o o r d i n a t i o n s q u a r e s of ORCOO a t o m s a r o u n d two Cu a t o m s . 2) ACu iS the d i s p l a c e m e n t of Cu f r o m the p l a n e of the c o o r d i n a t i o n s q u a r e of O a t o m s .

T A B L E 11. D e p e n d e n c e of the C o o r d i n a t i o n C a p a c i t y ( d e n t i e i t y ) of the C a r b o x y l G r o u p on t h e N u m b e r of A c c o m p a n y i n g L i g a n d s of Number of ]LiteraCN Denticity Compound No. Of M the group accompany-ltuxe ing ligands" leited RCOO I

2 3 4 5 6

7 8 9

t0 I1 '12 t3 14 15 t6 t7 18

Cu(HCO0)2 Cua(CltaC00)dlt~O)z Zn(l{zO)~(CHaCOO)z Co(tI~O)~(Ctt3COOh

4+: 6 6

Zn(H,0h(CILGO0h Co(etu)2(CHsCOO),a Zn(tu).a(CHaCOOh Zu(NaH4),,(CHaCOO)a

11 4 4 6

Cu(HCO0)~ Cu(H~O)~(HCO0)~ Cd(tu)o(HCO0).,

4,4+ 6

Cu(NH3)~(CH3CO0)~ [Co(NH.~)4(C[I.~COO).2]CIO~ [Co(N[I.~)~(CIIaCO0)]CICIO~ [Gu(~t-L)(CHsCOO)(H~O)]~t L=:Cu(I~-C~NH4CH.~O)dCH.~CO0)4.~H~.O (CO}(PPh3)Ir(~t-CH3NaCH~). 9AgMe~CHCOO))

4+ 6 6

Gd(HCOOh Eu(Phen)(IICOO)s

3.~ 2~ 21 l

21 t t

`1

2 4

t30 i3 t31 32

2(33% ) 2(50 ,~ ) 2(50%) 4(66 %)

13'1 t 32 '133 t34

-l

--

2~ 1

21 1 t

O, 4 4 2 4 5

`1

t30 t35 136 137,t38 t39 t40 t4t t42

% 2~., t

--

t9

3

t43

The b a s i s of t h i s e f f e c t i s n o t c o m p l e t e l y c l e a r . I t i s e v i d e n t l y due to s o m e a p p r e c i a b l e r e a r r a n g e m e n t of the s y s t e m of MO on g o i n g f r o m the f o u r t h to the f i f t h a n d s i x t h p e r i o d s . One p o s s i b l e r e a s o n f o r t h i s r e a r r a n g e m e n t m a y be t h a t on m o v i n g w i t h i n a g r o u p o f the p e r i o d i c t a b l e , the o c c u p i e d d - l e v e l s of the m e t a l i n t e r a c t ( " m i x " ) to d i f f e r e n t e x t e n t s w i t h the o r b i t a l s of c o r r e s p o n d i n g s y m m e t r y on the e q u a t o r i a l l i g a n d s , l e a d i n g to a d d i t i o n a l s t a b i l i z a t i o n (for the m e t a l s of p e r i o d s 5 a n d 6) o r d e s t a b i l i z a t i o n (for the m e t a l s of p e r i o d 4) of the b2g(6)- a n d e u ( T r ) - o r b i t a l s b a s e d on t h e m . A l l t h i s i n d i c a t e s a g a i n the a r b i t r a r y c h a r a c t e r of the c o n c e p t of m e t a l - m e t a l b o n d m u l t i p l i c i t y and t h e a b s e n c e of a s i m p l e c o r r e l a t i o n b e t w e e n t h i s c h a r a c t e r i s t i c , the i n t e r n u c l e a r d i s t a n c e , and the a c t u a l s t r e n g t h of the bond on g o i n g f r o m one m e t a l to a n o t h e r .

386

IV.

THE OF

ROLE

THE

OF

THE

SUBSTITUENTS

CARBOXYLATE

R

LIGANDS

The r o l e of the size of the substituent R is quite obvious: it d e t e r m i n e s the choice of conformation of the carboxyl ligand. F o r m a t e s p r e f e r the a n t i - a n t i o r a n t i - s y n c o n f o r m a t i o n , and hence layer or f r a m e w o r k s t r u c t u r e s ; acetates and other c a r b o x y l a t e s with l a r g e r R groups p r e f e r the s y n - s y n conformation and hence chain units or a cluster s t r u c t u r e . Table 9 gives the c o r r e s p o n d i n g data for compounds of analogous c o m p o s i tion. The reason for this difference in conformation is understood. With the anti-structure of the coordinated carboxyl group, the metal IVi and the group R approach one another (Fig. 7a). If R is the small hydrogen atom, it does not create sterie hindrance. On going to the methyl group, the CH 3. .. M distance of N3.1 A is smallish for a nonvalenee interaction. The sterichindrance increases on going to larger R. On the other hand, if there are no metal-metal bonds, then for the syn-syn structure of the ligand, the metal atoms are extremely close to one another (see Fig. 7b). It is not necessary, however, that the valence angle between the bonds of the electronegative oxygen atom be equal to 120 ~ With increase in the angle, the distance R... iV[ in the anti-conformer increases slowly (see Fig. 7a), whereas the M... M distance in the syn-synconformer increases rapidly, both because of the favorable direction of the displacement and because of its splitting (Fig. 7b). Thus the stcric hindrance in the contacts M... M forthesyn -syn structure is overcome more easily than that in the R.., M contacts for the anticonfiguration. A general conclusion is that ff the metal atoms tend to form bonds with one another, the syn-syn confo rmation is established in all carboxylates, including formates. If this tendency does not exist, the formates prefer the anti-anti or anti-syn structure, and the other earboxylates the syn-syn conformation. This naturally represents only a tendency, and not an imperative rule. The following may be added. The generally accepted view is that the widely encountered character of the syn-syn structureof carboxylate bridges is due to the tendency of certain metals to form direct bonds with one another. This is undoubtedly correct. It is possible, however, that the opposite relationship exists: The advantage of the syn-arrangement of the O-M bonds in relation to the Ni... R contacts in the crystallization process leads to an approach (initially forced) of the metal atoms which creates conditions for an increase in the strength of the bonds between them. It is possible for this r e a s o n that c l u s t e r s with m e t a l - m e t a l bonds a r e m o s t frequently encountered among carboxyl compounds and their s t e r e o c h e m i c a l analogs. The r o l e of the e l e c t r o n i c ( d o n o r - a c c e p t o r ) p r o p e r t i e s of the substituents R is much less obvious and m o r e complex. Here it is possible to speak only of individual aspects of the influence of these p r o p e r t i e s and individual curious facts. F i r s t l y , it follows f r o m general chemical data that with i n c r e a s e in the aceeptor power of the substituent there is a d e c r e a s e in the stability of both the c o r r e s p o n d i n g carboxylic acids and their salts. This applies, in p a r t i c u l a r , to the s e r i e s CHa > CH.zC!> CIt,zF CtI~ > CHeF > CHF.., > CFs CHa > Nic > i-Nic it is not excluded that in the last s e r i e s , on going f r o m the acetate to the nicotinate and isonicotinate, there is a trend towards a d e c r e a s e in the denticity of RCOO and in its tendency to adopt a chelate function. This conclusion can be r e a c h e d f r o m data on the carboxylates of the lanthanides, given in Table 3. These data a r e not v e r y significant, however, and apply to only a v e r y n a r r o w r a n g e of compounds. On the other hand, it is possible to point to a number of significant c a s e s of a complete change in the s t r u c t u r e of complexes when one substituent is r e p l a c e d by another. Thus in the t e t r a m e r of copper(I) b e n z o ate, all the metal atoms are joined to one another by single carboxyl groups (along the perimeter of a Cu 4 parallelogram) [9], whereas in the tetramer of copper(1) acetate, two pairs of Cu atoms joined by double bridges are formed, and the pairs are joined to one another by additional Cu~-O bonds [115, 116]. A curious change is undergone by the structures of the nieotinates of the rare earth elements on going from the normal to the acid salts. The normal nieotinate is a dimer with a quadruple bridge and a pair of closing chelate car boxy[ groups on the "outer" sides of the dimer [20, 21]. In holmium hydrogen nieotinate,

387

HzO

a

M...

Fig. 9. S t r u c t u r a l units in the c r y s t a l s of the e a r b o x y l a t e s of copper(II) containing s a t u r a t e d a m i n e s : a) Cu(p-Ntt2C6II4Me)2(CH3COO)2"3H2 O, b) Cu(NH3)2(CH3COO)2, c) Cu(p-NHzC6tt4X)2(C2HsCOO)2, X = Me, CI, Br.

o~

~/0C~ ~~~,/0C=L

/•.•0Rcoo

C a

b

...~lO~coo Cd. c

Cd. ( d

Fig. 10. V e r s i o n s of the s t r u c t u r e of c a r b o x y l a t e c o m p l e x e s of m e t a l s of the fourth p e r i o d (MnII, . . . ZnII): a) t r a n s - o c t a h e d r a l c o m p l e x with bidentatecyclic RCOO-, b) c i s - o c t a h e d r a l complex with bidentate-cyclic RCOO-, c) tetra~edral c o m p l e x with unidentate RCOO-, and d) dinucIear complex with bridging RCOO-. these m e t a l - c o n t a i n i n g r i n g s a r e opened, and the d i m e r i c f r a g m e n t s a r e joined in a chain by double c a r b o x y l c r o s s - l i n k a g e s [30]. The c a r b o x y l a t e s of the divalent m e t a l s of the iron group M_n, Fe, Co, and Ni g e n e r a l l y have the c o m p o s i tion MLRCOO) 2 9 2L or M(RCOO) 2 94L, and a m o n o n u c l e a r c o m p l e x s t r u c t u r e . If however the group R is a powerful e l e c t r o n donor, for e x a m p l e CMe 3, t h e r e is the possibility of a change to a dinuclear s t r u c t u r e in the f o r m of lantern c o m p l e x e s and a c o r r e s p o n d i n g change in composition to M(RCOO) 2 9 L [54, 117]. This r e a r r a n g e m e n t is m o s t c h a r a c t e r i s t i c of Mn, Fe, and Co, and in individual f a v o r a b l e c a s e s also takes place in nickel c a r b o x y l a t e s . E x a m p l e s a r e Co(PhCOO)2Quin [94] and Ni(CMe3COO) 2" MeQuin (MeQuin=quinaldine NCgH6CH3) [l18a]. The "pumping" of e l e c t r o n s f r o m the substituent to the oxygen a t o m s a p p a r e n t l y i n c r e a s e s c o n s i d e r a b l y the covalent c h a r a c t e r of the M - O bonds, and this, as shown above, i n c r e a s e s the tendency towards a bridging function for the c a r b o x y l a t e ligands. M o r e o v e r , the d e c r e a s e in the effective positive c h a r g e s on the m e t a l a t o m s d e c r e a s e s the component of the e l e c t r o s t a t i c r e p u l s i o n between them. The d o n o r - a c c e p t o r p r o p e r t i e s of the c a r b o x y l ligands naturally have a definite influence on the i n t e r a t o m i c d i s t a n c e s and v a l e n c e angles in the c o m p l e x e s . We can c o n s i d e r as an e x a m p l e the influence of the nature of the group R on the m e t a l - m e t a l distance in lantern c o m p l e x e s of divalent copper. The c o r r e s p o n d i n g data for u r e a adducts of c o p p e r c a r b o x y l a t e s a r e given in Table 10. The groups a r e a r r a n g e d in o r d e r of d e c r e a s i n g donor p r o p e r t i e s . The C u - C u distance i n c r e a s e s slightly but s y s t e m a t i c a l l y . T r i m e t h y l a c e t a t e should a p p a r e n t l y lie at the s t a r t of this s e r i e s , before acetate. Judging f r o m the O . . . O and C u . . . Cu d i s t a n c e s in the adduct of copper f o r m a t e , the latter occupies in the o v e r a l l s e r i e s a position s o m e w h e r e between m o n o e h l o r o a c e t a t e and monofluoroacetate. That this con-

388

NCsH 5~

o(-NC~H,X ~ X ~L"

,

I

I

I '

2,g-~

\

7I

o-I'

/I\

o

Fig. ii. Intramolecular steric hindrance O... L in dinuclear carboxylates M2(RCOO)4L2: a) accompanying ligand pyridine, b) accompanying ligand c~-substituted pyridine. clusion is not based on an accidental fact is confirmed by data on the isostructural crystals of the formate and the acetate with the composition (Me4N)2Cu(RCOO)4(NCS) 2 [126]. The change from the acetate to the formate in this case also leads to an increase in the C u . . . Cu distance. The tom[ range of change in the Cu-Cu distance from the acetate to the monofluoroacetate is 0.04 A. The distance between the oxygen atoms increases by the same amount. This means that the Cu-Cu distance increases not because of the greater displacement of the copper atoms from the planes of the oxygensquares but because these planes themselves move apart. In other words, the withdrawal of electron density onto the substituent influences chiefly the geometric characteristics of the carboxyl groups themselves, by increasing the valence angle OCO and/or the length of the C-O bonds. As a result of the increase in the positive charge on the copper atoms for this withdrawal, and the low strength of the direct metal-metal bonding, the copper

atoms passively follow the oxygen atoms. The definite relationship between the donor properties of the substituents R and themetal-metal distance is also revealed in the lantern complexes of divalent chromium. Although the available structural data on the complexes Cr2(RCOO)tL2 (see Table 8) are few and relate to axial Hgands L of varied character, some conclusions regarding the dependence of the length of the Cr-Cr bond on the nature of the substituent R can nevertheless be reached. If all the compounds with oxygen-containing axial ligands of similar donor power (L=CK3COOH , PhCOOH, Cr(H20)2(HCOO)2,* Et20) are grouped together, it can be seen that here, as in the copper complexes, the metal-metal distance increases with decrease in the donor power (increase in the aceeptor power) of the group R: 2.300(1) in the acetate, 2.352(3) in the benzoate, 2.451(1) in the formate, and 2.541(1) A in the trifluoroacetate. In the anthracene-9-carboxylate complex, the Cr-Cr distance, 2.283(2) A, was found to be shorter than that in the related benzoate complex. This can be attributed to the fact that the characteristic plane of the anthracene group, unlike the phenyl group in the benzoate, is rotated through a

I large angle relative to the plane of the

~///~__~ group, leading to a loss of the ~r-aceeptor power of the ligand. ' u

The anthracene-9-carboxylate

is therefore an analog of the acetate, rather than the benzoate.

The same relationship is revealed in the comparison of the pair of carboxylates studied with N-donor axial ligands (NCsIIil and oNCsH4); on going from the acetate to the formate, the Cr-Cr distance increases from 2.340(2) to 2.408(1) A. The results for the hydrates are less significant: 2.362(1) in the acetate, and 2.373(2) A in the formate. The pair of "axial-vacant" carboxylates Cr2(CH3COO) 4 and Cr2(CMe3COO) 4 clearly do not fit into the general scheme. The change from the acetate to the trimethylacetate is accompanied not by a decrease but by a sharp increase in the Cr-Cr distance: from 2.288(2) to 2.388(4) A. This anomaly is readily explained by the steric conditions of packing of the complexes. In both structures, the complexes are joined to form polymeric chains, and the role of axial ligands in each of them is played by the ORCOO atoms of neighboring complexes. In the acetate, this takes place without particular difficulty, but in the trimethylacetate the branched CMe 3 groups interfere in the conjugation of the complexes in the chain (Fig. 8), and this makes necessary an increase in the displacement of the metal from the plane of the oxygen square, that is, an increase in the Cr-Cr distance.

*In the s t r u c t u r e of Crs(HCOO) 6" 2H20 , the dinuclear complexes Cr2(HCOO) 4 are joined through the axial positions by the "carbonyl" O atoms of the mononuclear fragments Cr(H20)2(HCOO) 2.

389

The general factors responsible for the approach of the metal atoms with decrease in the accepter p ro perties and increase in the donor properties of substituents R are evidently those which in general increase the tendency towards the formation of lantern complexes by metals of the fourth period: an increase in the covalent character of the O - M bond and a decrease in the component of the electrostatic repulsion between the metal atoms. The role of this inductive factor should naturally decrease on going to metals which fo rm strong bonds with one another: The stronger this bond, the more difficult it is for "secondary" effects to change the equilibrium M - M distance. An illustration is provided by tile data on dinuclear complexes of Rh(II) (see Table 6) and Mo(II) (see Table 7). Judging f r o m the XES for rhodium complexes [127] and the PES for molybdenum complexes [128], the change from acetate to trifluoroacetate is accompanied by a marked increase in the positive charge on the metal atoms (the chemical shifts exceed 1.0 eV). At the same time, the length of the "strengthened" single bond R h - R h , on the replacement of CH3 by CF3, increases by only 0.017 A, and the length of the quadruple bond M e - M e r emain s unchanged, within the limits of e r r o r , that is, to within 0.003-0.004 A.* V.

THE ROLE

COMPOSITION

OF

THE STOICHIOMETRIC

AND THE A C C O M P A N Y I N G LIGANDS L

A.

Composition

and General

Structure

of the

Complexes

The structural function of the carboxylate ligand must depend on the overall composition of the compound, and primarily on the stoichiometric ratio n L : nRCOO : n M. It is fairly obvious that with increase in the number of groups RCOO corresponding to one metal atom, their coordination capacity (denticity) should gradually (on the average) decrease. Unfortunately, there are very few data to illustrate this tendency. Essentially the only s e r i e s relating to the same central atom and the same substituent R is the s e r i es formed by iris(acetate)cerium Ce(CH3COO)3H20 [17], guanidinium tetrakis(acetatoIcerate (CN3H6)[Ce(CHsCOO)aH20] [22], and potassium pentakis(acetato)cerate K2[Ce(CH 3" COO)s] "tt20 [129]. In the f i r s t of these compounds, two of the three groups are terdentate 32, and one is bidentate-bridging Z2; in the second, one is terdentate 32, and the others are cyclic 21; and in the third, there are no terdentate groups, one group is bridging 22, three ar e chelate 21, and one, replacing the inner-sphere water molecule, is tmidentate. The influence of the accompanying ligands on the structural function of the carboxyl groups is extremely marked and varied, beginning f r o m simple competition for a position in the complex and ending in the fine nuances of the mutual influence of the ligands. The competition of accompanying ligands for the formation of strong bonds with the metal is revealed quite naturally - it also leads to a decrease in the denticity of the carboxylate anion. The data on the hydrates of the acetates and formates of Co, Cu, and Zn are significant in this connection: With increase in the number of water molecules in the complex to four, the denticity of the anion decreases from three to one. In the octahedral complexes with four or five accompanying ligands, however, the carboxyl group almost always plays the role of a unidentate ligand. Some examples are given in Table 11. The competition for the formation of strong m e t a l - l i g a n d bonds is revealed in very characteristic fashion in the compounds of divalent copper, where the Jab.n-Teller pseudo-effect operates, and the bonds between the metal and the ligands become disproportionate to give strong bonds in the equatorial plane (apices A) and weak bonds at the apices of the bipyramid (apices B). In complexes with donor ligands L, the distribution of the ligands among the apices of types A and B depends on the electron-donor properties of these Iigands. If L is an oxygen ligand or an unsaturated amine, lantern complexes are always formed with L in the axial positions (the B apices). This applies, in particular, to pyridine, its derivatives quinoline and quinatdine, and other analogous ligands. If however L is a saturated amine, for example ammonia, aniline, and its derivatives, then this ligand wins over the position of a c a r boxyi group in the equatorial plane of the bipyramid and occupies an A position. As a result, a complete r e arrangement of the complex takes place, which is different for different L and depends on other secondary fac* This feature by itself confirms that the formally single (or possibly triple) R h - R h bond is much stronger than the formally quadruple C r - C r bond. 390

tors. Thus in the para-toluidine adduct of copper acetate, the acetate groups a r e unidentate, and the B positions a r e occupied by two water m o l e c u l e s (Fig. 9a), w h e r e a s in the ammonia adduct the acetate groups a r e pseudo-chelate, and both the A and B positions a r e used (Fig. 9b). In the propionates with para-tohiidine, p a r a chloroaniline, and p a r a - b r o m o a n i l i n e as donor ligands, one carboxyl group r e t a i n s the bridging function, and the second becomes unidentate, liberating one A position for the donor ligand (Fig. 9c). In the examples considered, the principal r o l e is evidently played by the e l e c t r o n i c c h a r a c t e r i s t i c s of the accompanying ligands L. Their size p a r a m e t e r s , however, may influence significantly not only the composition but also the s t r u c t a r e of the complex. In this connection, the s t r u c t u r a l r e a r r a n g e m e n t s in the carboxylates of the divalent metals of the iron group (from 1Vinto Zn) a r e of considerable interest. We shall take as a basis the t r a n s o c t a h e d r a l complex M(RCOO)2L2, without as y e t making specific the mode of coordination of the carboxyiate groups. We shall a s s u m e that the donor ligands occupying the apical positions have a configuration which c r e a t e s s t e r i c hindrance in the contacts with the equatorial oxygen atoms of the carboxyl groups. Following Pasynskii [54, 117], we shall take a - s u b s t i t u t e d pyridine as a specific model of a ligand of this kind. For r e g u l a r octah e d r a l coordination, the distances C a . . . O a r e too short, of the o r d e r of 2.7-2.8 A. Steric hindrance a r i s e s . How is it avoided? T h e r e exist at least four possibilities. The f i r s t possibility is chelate coordination of the carboxyl groups (front and back in Fig. 10a), leading to displacement of the M - O bonds in the equatorial plane and an i n c r e a s e in the angle O - M - O , w h e r e the a - s u b s t i t u e n t of pyridine lies above t h e bisector of this angle. The distance C a . . . O i n c r e a s e s to 3.0-3.1 A. This s t r u c t u r e is shown, in p a r t i c u l a r , by nickel benzoate containing quinoline as donor ligands [l18a]. Judging f r o m i n d i r e c t physicochemical data, an analogous s t r u c t u r e is c h a r a c t e r i s t i c of many other nickel compounds with the composition Ni(RCOO)2L 2 [56, 57]. The second possibility is a change to the c i s - o c t a h e d r a l s t r u c t u r e with p r e s e r v a t i o n of the cyclic function of the carboxyl groups. In this case the a - s u b s t i t u e n t is situated not above the bisector of the external angle O - M - O but above the bisector of the angle O - M - N , f o r m e d by the M - O R c o o bond and the M - N bond with the second ligand L a . This coordination can apparently take place only if the ligands L~ are not e x t r e m e l y bulky. It is found, for example, in the case of nickel benzoate when quinoline is r e p l a c e d by a - p i c o l i n e (see Fig. lOb). A s i m i l a r but n e v e r t h e l e s s slightly different s t r u c t u r e is shown by zinc acetate dihydrate [131]. It is m o r e convenient to d e s c r i b e the Zn polyhedron as a t e t r a h e d r o n with two split apices, occupied by chelate acetate groups. Judging f r o m the p h y s i c o c h e m i c a l c h a r a c t e r i s t i c s , the same s t r u c t u r e is shown by a number of other zinc carboxylates Zn(RCOO)2L 2 with l a r g e r donor ligands. The third possibility is t e t r a h e d r a l coordination with unidentate carboxy[ groups (see Fig. 10c). Structural data a r e available for Co(SCNzC2H6)z(CI-I3COO) 2 [132], Co(C3N2tt4)2(CH3COO)2 [144] and Zn(SCN2H4) 2" (CH3COO) 2 [133], but judging f r o m the analogy in the magnetic and s p e c t r a l c h a r a c t e r i s t i c s , the same s t r u c t u r e is shown by many other carboxylates of these metals, including those with a - s u b s t i t u t e d pyridine as donor [igand [54, 117]. The fourth possibility is a change to a dinuclear s t r u c t u r e with displacement of the metal above the plane of the oxygen square, which i n c r e a s e s the valence angles O - M - L and hence the distances C a . . . O (see Fig. 10d). This s t r u c t u r e is c h a r a c t e r i s t i c p r i m a r i l y of copper carboxylates, and is also typical of many carboxylates of manganese and cobalt with a - p i c o l i n e , quinoline, and quinaldine, and also various nickel c a r boxyiates [54-56, 117]. In p a r t i c u l a r , it is o b s e r v e d in the previously mentioned compound of nickel t r i m e t h y l acetate with quinaidine Ni(CMeaCOO)2NCgH6CH 3 [118a]. Thus nature s e l e c t s different ways of avoiding u n n e c e s s a r y s t e r i c hindrance in the contacts between c a r boxylate and accompanying ligands. The choice of way depends on other p a r a m e t e r s , and p r i m a r i l y on the nature of the m e t a l s - the differences in their e n e r g i e s and stabilization in the ligand field for different coordination polyhedra. Manganese, cobalt, and zinc in mononuclear complexes show not only octahedral but also t e t r a h e d r a l coordination, nickel p r e f e r s octahedral coordination, and copper a dinuclear s t r u c t u r e . This brings us back to the f i r s t of the f a c t o r s examined above, namely the r o l e of the e l e c t r o n i c s t r u c t u r e of the metal. The dinuclear s t r u c t u r e also becomes favorable for other m e t a l s of the fourth period if a large (branched) accompanying ligand L is combined with strong donor p r o p e r t i e s in the substituent R. An i n c r e a s e in the covalent c h a r a c t e r of the M - O R c o o bonds m a k e s the s q u a r e - p y r a m i d a l coordination competitive with oeta39l

GO bO

a

3

H2C/

H2C\ acetate

2,6i0 [t56] 2,t02

2,09

2,64 [t551

2,6t5 [591 2,156

chloroacetate

2,20 [t20, 2,624 t2i I 2,t3 2,22 2,626 [i22] 235 2,21 2,631 [123] 2,t2 2,23 2,643 [i24] 2,t0 2,24 2,657 [it9] 2,114 2,24 2,665 av [t251 2,it

Urea

2,20 2,645 [t49] 2,186

rhombic)

Py (ortho-

2,23 2,685 [t52] 2,169

[~-pir

LCu(CHaCOO)2(CK2CICOO)2CuL.

2,i9 2,630 [i481 2,125

~y (monoclinic)

2,716 [t261 2,093

2,6~3 [i25) 2,08

NCS

2,23 2,747 [i53] 2,461

2,67 [t501

o:-pic

2,20

2,25 2,886 [t54] 2,t07

2,23 2,724 [t5t] 2,2tl

2,224

2,652 [151l

Quill

2,22 2,702 [1t8b] 2,371

Act

? Copper sueeinate C u ( O O C C H a C K a C O 0 ) 9 2KaO , in which each of the two carboxylate branches of the succinate ligand are involved in "its own" dinuclear fragment.

* Mixed copper

2

I

3

2

t 2 3 1 2 3 1 2 3 1 2 3 t 2 3 1 2 3 1 2 3 t

HOC~H4

CF3

CH,F

H

CHIC1

*

CH~Ph

CH8

CM%

/-I~O

TABLE 12. G e o m e t r i c P a r a m e t e r s of the C o m p l e x e s LCu(RCOO)4CuL , C h a r a c t e r i z i n g the D i s p l a c e m e n t of the A t o m s Along the P r i n c i p a l A x i s of the C o m p l e x e s (1 - d i s t a n c e between p l a n e s O . . . O, 2 - C a - C u D i s t a n c e , 3 -C u - L bond length)

h e d r a l coordination, and the displacement of the metal f r o m the plane of the oxygen s q u a r e r e m o v e s the s t e r i c hindrance in the contacts O . . . L. H e r e again, a combination of two f a c t o r s is operating: the e l e c t r o n i c p r o p e r t i e s of the substituent R and the size c h a r a c t e r i s t i c s of the ligand L. The size p a r a m e t e r s of the accompanying ligands also f o r m the basis of the tendency noted by Koz'min and c o - w o r k e r s [11], according to which, in Re(III) d i m e r s , the complex generally has the c i s - s t r u c t u r e when apical ligands are p r e s e n t , and the t r a n s - s t r u c t u r e when they a r e absent (see Fig. 4a and b). This r u l e is apparently also applicable to dinuclear complexes of other metals. Thus in the Mo(II) complex Mo2Br2(PBu~)2(PhCOO) 2 with equatorial unidentate ligands of two kinds and without axial supplementary ligands, the benzoate groups a r e in the t r a n s - p o s i t i o n to one another [145], w h e r e a s the Ru(I) complex Ru(CO) 4" (C3H7COO)2 (PBut3)2, with equatorial carboxyl ligands and axial phosphine ligands, has the c i s - s t r u c t u r e [146]. Other examples for Re, Mo, Ru, and other m e t a l s could be given.* The r e a s o n for the existence of this r u l e is obvious. The ligands X are r e p e l l e d f r o m one another and displaced towards the apical positions. For the t r a n s - s t r u c t u r e , this c r e a t e s s t e r i c hindrance in the contacts X . . . L, and for the ligands " t h e r e is nowhere to go." With the c i s - s t r u c t u r e , they can be displaced to the bisector of the angle between two carboxyl ligands. Thus for complexes with axial ligands, the c i s - s t r u c t u r e is p r e f e r r e d . It is apparently possible to f o r m u l a t e a wider " i n v e r s e t h e o r e m " as follows: If the solution contains donor ligands which tend to f o r m strong bonds with the metal, the c i s - i s o m e r c r y s t a l l i z e s . If the L ~ M bonds a r e weak (such donor molecules, for example water, a r e almost always p r e s e n t in solution), then the L-~M bonds a r e broken on crystallization, the complex undergoes r e a r r a n g e m e n t , and the t r a n s - i s o m e r c r y s t a l l i z e s , since the packing of the m o r e compact and s y m m e t r i c a l t r a n s - c o m p l e x e s without apical ligands gives a large gain of crystallization energy. B.

Interatomic

Distances

in

Dinuclear

Lantern

Complexes

i. Dinuclear Copper(If) Carboxylates Like the substituents R, the accompanying ligands L have a definite influence on the interatomic distances in dinuclear complexes with quadruple carboxylate bridges. This influence may have an electronic or a purely steric basis. In copper(If) carboxylates, the steric factor predominates. Table 12 gives data on the distances O. 9 O, Cu... Cu, and Cu-L in complexes LCu(RCOO)4CuL with different ligands L and different substituents R. It can first of all be clearly seen that the nature of the ligand L influences the Cu-Cu distance much more sharply than the individuality of the group R, and the chief role is here played not by the characteristic features of the electronic structure of the ligand L but by its size characteristics. The latter determine the presence or absence of steric hindrance in the contacts between the apical ligands L and the equatorial oxygen atoms. We shall take as typical models pyridine (Ca 9 9 9 O contacts extremely in Fig. ii.)

the complexes with pyridine (no steric hindrance) and with a-substituted short). (The upper parts of the complexes are represented schematically

To avoid the extremely short contacts Ca 9 9 9 O, it is necessary to increase the valence angle O-Cu-L, that is, to increase the displacement of copper from the oxygen square and hence the Cu-Cu distance. Table 12 on the left gives data for c o m p l e x e s without s t e r i c hindrance. H e r e , the C u - C u distances lie in a c o m p a r a tively n a r r o w r a n g e with an a v e r a g e value of 2.64 A. The data for c o m p l e x e s with s t e r i c hindrance a r e given on the right. The distances have i n c r e a s e d : T h e y v a r y o v e r a wider range, and the a v e r a g e value is 2.73 A.

The influence of the size c h a r a c t e r i s t i c s of the ligands L on the C u - C u distance is quite obvious f r o m these data. This a s p e c t of the s t e r e o c h e m i s t r y of dinuclear complexes of copper was f i r s t pointed out by Yu. A. Simonov [151]. A m o r e detailed analysis of the s a m e Table, however, makes it possible, against the background of this g e n e r a l relationship, to distinguish various additional details - the dependence of the C u - C u and C u - L distances on the e l e c t r o n i c c h a r a c t e r i s t i c s of both the ligands L and the substituents R. On the left hand side, the s u m of the distances C u - C u and C u - L a x lies in the r a n g e 4.65-4.87 A, w h e r e a s on the r i g h t hand side it lies in the range 4.86-5.07 A. The a v e r a g e values a r e 4.765 and 4.938 /~, r e s p e c t i v e l y . The diff e r e n c e r e a c h e s an a v e r a g e of 0.17 A: * T h e r e a r e also exceptions. Thus in Mo(I) (t-BuO)4(t-BuOCOO)2 , the bridging butylcarbonate groups occupy the cis-position, although t h e r e a r e no axial ligands in the complex [147].

393

1) We can consider f i r s t the left hand side of the Table and take as s t a n d a r d the d i s t a n c e s C u - C u along the horizontal in the a c e t a t e s (2.62-2.64 A) and along the v e r t i c a l in the adducts with u r e a (2.62-2.66 A). Judging f r o m the acetate s e r i e s , it m a y be a s s u m e d that an i n c r e a s e in the donor p r o p e r t i e s of the axial ligand L i n c r e a s e s slightly the C u - C u distance. This is natural, since for the d 9 configuration of the metal, the e l e c t r o n s of the axial ligand enter the cr-antibonding o r b i t a l of the f r a g m e n t L ~ C u - C u * - L . In the a c e t a t e s , the C u - C u distances lie in a n a r r o w range, but on going f r o m the a c e t a t e to the m o n o c h l o r o a c e t a t e t h e r e is an i n c r e a s e in the dependence of this distance on the nature of the ligand L: Here, the r e p l a c e m e n t of u r e a by fi-picoline leads to an i n c r e a s e in the C u - C u distance to 2.68 A. The situation is analogous on going f r o m a c e t a t e to f o r m a t e . In the acetate, the r e p l a c e m e n t of u r e a by the thiocyanate group (anionic complex) i n c r e a s e s the C u - C u distance by only 0.02 A, w h e r e a s in the f o r m a t e it i n c r e a s e s it by 0.06

s

It m a y be concluded that with i n c r e a s e in the a c c e p t o r p r o p e r t i e s of the substituent R, the sensitivity of the C u - C u distance to the individuality of the a c c o m p a n y i n g ligand L and to its donor p r o p e r t i e s i n c r e a s e s . 2) We can now consider the r i g h t hand side of the Table, c o r r e s p o n d i n g to the c o m p l e x e s in which Cc~... O s t e r i c hindrance has been o v e r c o m e . The data for the quinoline adducts show that in this c a s e , with i n c r e a s e in the a c c e p t o r power of the substituent R, the C u - C u d i s t a n c e s i n c r e a s e much m o r e rapidly than in the a b s e n c e of s t e r i c hindrance: not by 0.04-0.06 A but by a whole 0.23 A on going f r o m a c e t a t e to f l u o r o acetate. It a p p e a r s that the s t r o n g e r the withdrawal of e l e c t r o n density to the substituent, the m o r e polar the bond between oxygen and copper, and the e a s i e r the change in the valence angles at the oxygen a t o m s , if the s t e r i c conditions r e q u i r e this. 3) Examination of the s a m e v e r t i c a l s e r i e s of quinoline adducts f r o m below upwards shows that in the t r i m e t h y l a c e t a t e complex, the C u - C u distance should be of the o r d e r of 2.60 A. The f a c t that in the a c r i d i n e adduct of t r i m e t h y l a c e t a t e it is much l a r g e r (2.70 A) should not be s u r p r i s i n g . Quinoline c r e a t e s C a . . 9O s t e r i c hindrance only in one direction away f r o m the v e r t i c a l axis of the d i m e r , and this s t r a i n is p a r t l y dec r e a s e d by the deviation of the C u - L bond f r o m this axis. Acridine c r e a t e s s t e r i c hindrance in both d i r e c tions away f r o m the axis, and this a u x i l i a r y m e a n s of r e m o v i n g the s t r a i n s is not available in this case. 4) The d e c r e a s e in the C a . 9 9O s t e r i c hindrance involves not only the v a l e n c e angle at the copper (and oxygen) a t o m but also the length of the C u - L bond. In m o s t compounds on the left-hand side of the table a l r e a d y considered, the C u - C u d i s t a n c e s lie in the r a n g e 2.72-2.75 A, and the C u - L d i s t a n c e s in the r a n g e 2.17-2.22 A. T h e r e a r e of c o u r s e two s h a r p deviations: one in the complex with the limiting a e c e p t o r substituent, and the other in the complex with the limiting donor substituent in the c a r b o x y l group. In the t r i f l u o r o a c e t a t e , the C u - C u distance is i n c r e a s e d to 2.89 A, and C u - L is s h o r t e n e d to 2.11 A, w h e r e a s in the t r i m e t h y l a c e t a t e , the f i r s t distance, on the c o n t r a r y , is s h o r t e n e d to 2.70 A (this is in the c a s e of acridine, and in the e a s e of quinoline, 2.60 A m i g h t be expected), and the second is i n c r e a s e d to 2.37 A. In other w o r d s , in the c a s e of a c c e p t o r c a r b o x y l a t e s the s t e r i c hindrance is lowered chiefly by a change in the v a l e n c e angles (the copper a t o m s m o v e apart), and in the c a s e of donor c a r b o x y l a t e s it is lowered by an i n c r e a s e in the length ( d e c r e a s e in the strength) of the bond between copper and the apical ligand. This r e l a tionship c o n f i r m s that the d o n o r - a c c e p t o r p r o p e r t i e s of the substituent R have a significant influence on the p o l a r i t y of the C u - O bonds and hence on the "rigidity" of the v a l e n c e angles at the oxygen a t o m s . All that h a s been said above shows that the r o l e of the a c c o m p a n y i n g ligands in d e t e r m i n i n g the individual f e a t u r e s of the s t r u c t u r e of c a r b o x y l a t e c o m p l e x e s of m e t a l s of the fourth period, in p a r t i c u l a r copper, is e x t r e m e l y i m p o r t a n t and is d e t e r m i n e d both by their s i z e and by their e l e c t r o n i c c h a r a c t e r i s t i c s . Together with the influence of the nature of the substituent R and the individuality of the m e t a l M, it leads to a wide v a r i e t y of s t r u c t u r e s in the c o m p l e x e s , both in their g e n e r a l c o m p o s i t i o n and in individual details of i n t e r a t o m i c d i s fences and v a l e n c e angles. 2. Dinuelear Re(III) C a r b o x y l a t e s In the c o m p l e x e s of copper(II) with v e r y weak d i r e c t C u . . . Cu i n t e r a c t i o n , the dependence of the M - M distance on the e l e c t r o n i c p r o p e r t i e s of the axial ligands is m a s k e d by the influence of the s t e r i c conditions, but it is n e v e r t h e l e s s p o s s i b l e to p e r c e i v e a tendency t o w a r d s a d e c r e a s e in this distance with d e c r e a s e in the donor p r o p e r t i e s of the ligand Lax. In Re(III) c o m p l e x e s with a quadruple R e - R e bond, the s a m e r e l a t i o n s h i p is o b s e r v e d m o r e c l e a r l y . The data given in Table 7 show that the R e - R e distance d e c r e a s e s s u c c e s s i v e l y in the following s e r i e s : the anionic c o m p l e x [Re2CI4(HCOO) 2 9 Cl2] 2-, the n e u t r a l c o m p l e x e s Re2CI4(CH3COO)2L2

394

with L =OCHNMe2, OSMe2, and OPPh3, the neutral aquo complex Re2CI4(CH3COO)z(H20)2, the "axial-vacant" complex Re2C14(CH3COO)2, and the a p i c a l - v a c a n t complex Re214(CH3COO) 2. In the f i r s t of these complexes, c a r r y i n g a double negative charge, the e l e c t r o n density on the Re atoms is undoubtedly a m a x i m u m (the C1 anion is a better donor than the neutral ligand L). The donor p r o p e r t i e s of water are weaker than those of the other three oxygen-containing ligands L. In the RezCI4(CH~COO) 2 crystal, the complexes form chains in such a way that the role of apical ligands is played by the chlorine atoms of neighboring complexes. Under these conditions they play the role of weak donors. In the Re214(CH3COO)2 crystal, the analogous linking of the complexes does not take place; there are no apical ligands. The total range of change in the Re-Re distance is 0.06 A. This influence of the axial ligands length of the M-M bond is readily explained. It is related to the effective participation of the L-~M bonds in the samc~-type ~O (and also in 7r-bonds, if the ligands also exhibit ~r-donor power). Here dealing essentially with the trans-influence, in this case with a decrease in the strength of the M-M increase in the strength (increase in the donor strength) of the bond L-~M.

on the and M-M we are bond with

The reverse relationship is also applicable. K we compare, for example, the complex [Re2CI4(HCOO)~CI~] 2- with Re2CIa(CH3COO)2, the shortening of the Re-Re bond by 0.05 A corresponds to an increase in the length of the Re-Clax bond by 0.17 A.* On going from ReCI4(CH3COoO)2L 2 with L=OCHNMe~, OSMe 2 and OPPh 3 too L=H20, the Re-Re bond becomes shorter, by 0.012-0.015 A, and the Re-O L bond becomes longer, by 0.15 A.

Overall, the c o r r e l a t i o n between the length of the quadruple R e - R e bond and the donor bond R e - L a x can be r e g a r d e d as the effect of the mutual trans-influence; the t r a n s - l a b i t i z i n g action of the strong quadruple R e Re bond is r e v e a l e d much m o r e sharply than that of the cr-donor Re'--L; the R e - L bond is Ionger by 0.2-0.4 than the usual bonds, and the entire range of change in the Re--Re distance c o r r e s p o n d s to 0.6 A. 3. Dinuelear Rh(ll) Carboxylates An almost analogous picture of the relationship between the distances M-M and M-Lax can be observed in the complexes Rhi(CH~COO)4L 2 (see Table 6). In the series PI20 , Py, CI-, NHEt2, PR 3 (compounds i, 4, 5, 6, and 8-11 in Table 6), the length of the Rh-Rh bond increases successively, but the increase in the length of the Rh-Lax bond compared with the usual values becomes less clearly defined: It amounts to ~ 0.25 A for Rh-H20 , an average of 0.20 A for Rh-NL, and only 0.05-0.10 A for Rh-PL.? The stronger the donor properties of the ligand, the longer the Rh-Rh distance, and conversely, the shorter the Rh-Rh distance, the more marked the decrease in the strength of the bond with the axial Kgands. The total range of change in the Rh-Rh distances is 0.07 A, and the decrease in the length of the RL-L bond is N0.2 /~. There is one result, however, which destroys the general harmony. In the carboxylate complex Rh2(CK3COO)4(CO)2, the axial ligands are very weak if-donors, and also strong v-acceptors. If the Rh-Rh bond is a single bond (see schemes I and III in Fig. 6), the v-accepting process should have involved the antibonding orbital 9 *(M-M) 4, and the Rh-Rh distance in this compound should have been the shortest of all. Moreover, the authors of most structural studies of the compounds in Table 6, Christoph and Koh [66], considered that the PR 3 ligands are not so much good or-donors as ~-acceptors, and this should also have led to a decrease in the length of the Rh-Rh bond when they are present. In this respect, the overall picture of the change in the Rh-Rh distances show better agreement with the model based on the triple M-M bond (scheme If in Fig. 6).

Christoph and Koh w e r e able to " s a v e " the R h - R h single bond by a detailed model analysis of the i n t e r action of the atomic orbitals of the f r a g m e n t Rh--Rh and the equatorial ligands CHaCOO. Their final r e s u l t is r e p r e s e n t e d in s c h e m e IV in Fig. 6. This shows only those MO of the complex Rh2(CH3COO) 4 (without allowance for the axial ligands) which a r i s e f r o m the initially occupied MO of the c e n t r a l R h - R h f r a g m e n t on mixing with the MO of the set of four acetate groups belonging to the same i r r e d u c i b l e r e p r e s e n t a t i o n s and of c o m p a r a b l e energy. Only one significant f e a t u r e is postulated: The MOwith s y m m e t r y eu(~r) of the c e n t r a l f r a g m e n t and e u o f the ligands have s i m i l a r e n e r g y positions, which leads to sharp splitting of the two e u or bitals produced on 9 This example is not completely a c c u r a t e : T h e d o n o r p o w e r is different for the t e r m i n a l and bridging C1 atoms. For the standard values of the distances Rh-L see [157, 158].

:~95

mixing. As a result, the energy of the highest occupied level eu(V) is higher than than of eg(V*) (here the symbols v and v* r e l at e to v bonding and antibonding in the R h - R h r e g i o n ) . In other words, the effect of the equatorial ligands on the MO of the central fragment is so strong that it leads to the interchange of the order of the eu(V) and eg(V*) orbitals with preservation of the formally single bond R h - R h . With this o r de r of orbitals, the r-acceptance into the axial ligand should lower the electron density in the R h - R h region in the bonding orbital Cu(r), that is, it should lead to additional weakening of this bond, as observed in the complex with carboxyl axial ligands. The model interpretation of the mixing of the orbitals of the central fragment and the equatorial acetate ligands, proposed by Christoph and Koh, naturally does not agree with the calculated data of Norman and Kolari for rhodium formate [75]. However, as pointed out by Christoph and Koh, the results obtained in [75] can be readily modified as required by changing the original p a r a m e t e r s of the calculation so that the levels e u of the central fragment and the ligands have similar energies. It is not excluded that this takes place when the f o r mate groups are replaced by acetate. 4. Dinuclear Chromium(II) Complexes As already noted, the structural data on dinuclear chromium(II) carboxyLates are fairly limited, and the compounds have fairly varied compositions. Thus the influence of the nature of the ligands Lax on the C r - C r distance can be characterized only in the most general way. 1. It can readily be seen f r o m Table 1 that there is no direct correlation between the donor power of the axial ligand and the distances C r - C r and C r - L a x in the carboxyLates studied. The correlation between the C r - C r and Cr--Lax distances also cannot be represent ed by a smooth curve. It is possible to speak only of a general and not v er y clearly defined tendency for the C r - C r distance to increase with decrease in the C r - O L or C o - N L distance. The data for chromium trimethylacetate clearly do not fit into the general relationship, as noted above: Here, the C r - C r bond is much longer than expected for R=Me3C and the weak donor bond Cr ~ O R c o o in the polymeric chain of the complexes. Comparison with the data for the polymeric acetate shows that the steric hindrance i nc r eas e s the length of not only the C r - C r bond but also the Cr-ORCOO bond. This example again emphasizes that the curve giving the dependence of the potential energy of the interaction on the C r - C r distance is very flat, and "external" factors readily change this distance. 2. As already noted, in both the ,axial-vacant" chromium carboxylates studied, Cr2(CH3COO) 4 and Cr~(CMe3COO) 4, the apical positions a r e occupied by ORCOO from neighboring complexes. The Cr--Ol~CO O distances ar e much shorter than those in the Mo complexes of analogous composition (see Table 7). A polymeric structure is also shown by Cr2(anthracene-9-carboxylate)4(MeOCH2CHzOMe), where the role of bridges joining the lantern complexes in the chain is played by the MeOCH2CH2OMe molecules, which have the tra n sstructure. In the polymeric structure of Cr3(HCOO)s(H20) 2, the dinuclear complexes Cr2(HCOO) 4 are connected in a chain via the "carbonyl" atoms of the formate groups of the mononuclear Cr(HzO)z(HCOO) 2 units. On the whole, the impression is gained that chromium(II), unlike Re(IH), Mo(H), and W(ID, is much more active in striving to complete its coordination by apical ligands. This valence unsaturation of the fragment Cr2(RCOO)4, together with the much greater M - M distances, leads to the conclusion that the actual multiplicity of the C r - C r bond is much lower than four. In other words, it may be assumed that under the influence of the carboxyl ligands, the energy spectrum of the fragment C r ~ C r is r e a r r a n g e d in some sense in a way directly opposite to that in the spectrum of the fragment R h - R h ; the r e a r r a n g e m e n t leads to not an increase but a dec r e a s e in the strength of the m e t a l - m e t a l bond. By comparing the increased C r - C r distances in the carboxylates with the s u p e r - s h o r t (1.83-1.89 JA bonds in dinuclear complexes of chromium(ID with other bridging ligands (see above), Cotton and co-workers [101] suggestedvery carefully that the difference in the distances is determined not by the difference in the nature of the ligands but by the fact that in the complexes with s u p e r - s h o r t distances studied, the apical positions r emain vacant.* They cannot be occupied by accompanying ligands or by the carboxy[ oxygen atoms of neighbors, for purely s t e r i c reasons; the bridging ligands themselves are too bulky. This suggestion leads to a somewhat paradoxical situation. On the one hand, change in the donor power of the axial ligand over a wide range floes not lead to fundamental changes in the electronic structure of the fragment C r - C r ; in particular, it r e m a i n s unchanged in the polymer Cr2(Me3CCOO) 4 with relatively weak Cr ~ ORCOO interaction along the *As c o r r e c t l y noted by the authors, to verify this suggestion it would be necessary either to c a r r y out an electron diffraction study of an axial-vacant chromium carboxylate in the gaseous phase or to synthesize a carboxylate with substituents R which would effectively shield the apical positions.

396

axis of the c o m p l e x ( C r - O d i s t a n c e s 2.44(1) and 2.47(1) A). On the other hand, if this i n t e r a c t i o n is e l i m i n a t e d c o m p l e t e l y , t h e r e is a qualitative change in the e l e c t r o n i c s t r u c t u r e with a s h a r p i n c r e a s e in the s t r e n g t h of the C r - C r bond. O v e r a l l , the s t e r e o c h e m i s t r y of the dinuclear c o m p l e x e s of divalent c h r o m i u m in the f o r m in which it is r e p r e s e n t e d by s t r u c t u r a l data obtained up to 1979 c a u s e s s o m e s u r p r i s e . T h e r e is as y e t no r e a l r a t i o n a l explanation for all its c h a r a c t e r i s t i c f e a t u r e s . This applies to the v e r y large d i f f e r e n c e in the C r - C r d i s tances in dinucIear c o m p l e x e s with ligands of different c h a r a c t e r , to the s h a r p i n c r e a s e in this distance in the c a r b o x y l a t e s on going f r o m Mo(II) and W(II) to Cr(II), to the wide r a n g e of C r - C r d i s t a n c e s in c a r b o x y l a t e s with different R and L, and to the a b s e n c e of a distinct c o r r e l a t i o n between these d i s t a n c e s and the donor power a n d / o r the size c h a r a c t e r i s t i c s of the a c c o m p a n y i n g [igands. It is to be expected that this confused situation with dinuclear c l u s t e r compounds of c h r o m i u m will in the n e a r future lead to a new flow of w o r k on the s y n t h e s i s and s t r u c t u r a l study of these c l u s t e r s and to a t t e m p t s to develop a deeper t h e o r e t i c a l t r e a t m e n t .

CONCLUSION In the present review, an attempt has been made to outline only the general features of the stereochemistry of earboxylates, chiefly the carboxylate complexes of transition metals. The author has seen his main task as the analysis of the principal factors determining the structural function of the carboxy[ ligands in coordination compounds. The separate examination of the role of the metal, the substituents, the carboxyl ligands, and the accompanying ligands is of course a somewhat artificial operation. All three factors are in fact interrelated. An understanding of the essential features of their synthesis would probably be extremely difficult or even impossible, however, without a preliminary study of the separate contributions of each factor. It is of course true that with the extension of the experimental base and the further development of theoretical ideas, the general picture of the stereochemistry of this field will gradually be modified: Some things will be added, others removed, new details and interrelationships will appear, and broader and deeper generalizations will become necessary. In particular, there is now arising the idea of comparing the stereochemistry of the carboxylates and complexes with other related ligands, for example monothiocarboxylate and dithiocarboxylate compounds. Another specific theme is the comparison of the geometric and structural chemicai characteristics of dinuclear carboxylates and dinuclear complexes of analogous structure with other ligands. Another important question which has not been dealt relationship between the crystal structure of carboxylates the magnetic and spectral-resonance properties. Work in in the Soviet Union (in Moscow, Kazan', Saratov, Kishinev, ticular Poland.

with in the preceding pages is the problem of the and their physicochemical properties, primarily this field is being carried out on a fairly broad front and other centers) and in other countries, in par-

Data being published regularly on polybasic (particularly dibasic) carboxylates and especially R-active polybasic carboxylates (complexonates) also await generalization. The list of various aspects of the stereochemistry and crystal chemistry of carboxylates and the possible extension of the theme could naturally be continued. All this may however be the subject of independent analytical work by many specialists concerned with the problems of the crystal chemistry and stereochemistry of coordination compounds. LITERATURE 1. 2.

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