X-RAY OF
SPECTRAL
STUDY
AN O U T E R - S P H E R E
STRUCTURE
OF
OF
CATION
A COMPLEX
L . N. M a z a l o v , S. V. Z e m s k o v ,
THE
INFLUENCE
ON T H E
ELECTRONIC
ANION
E. A. Kravtsova, a n d Y u . I. N i k o n o r o v
UDC 546.16;546.91
To investigate the influence of the o u t e r - s p h e r e cation on the electronic s t r u c t u r e of a complex anion [MLG]n-, a study was made of the x - r a y s p e c t r a of fluorine (F Kc~) in the hexafIuoro c o m plexes of Pt, Pd, Ni, Rh, and Sn, and the Kfi s p e c t r a of chlorine in the hexachloro complexes of Pt and Ir. The F K~ s p e c t r a in the alkali metal fluorides were studied as model s y s t e m s . Definite c o r r e l a t i o n s were obtained between the f o r m of the x - r a y F K~ and C1 Kfl s p e c t r a and the nature of the m e t a l - l i g a n d interaction in the s e r i e s of hexafluoro and hexachloro complexes. It was found that the covalent component of the bond i n c r e a s e s with change in the cation of the outer coordination sphere f r o m Li to Cs. In the s e r i e s K2NiF~, K2PdF6, and K2PtF6, the covalent c h a r a c t e r i n c r e a s e s f r o m Ni to Pt.
In [1-7], devoted to the study of the s t r u c t u r e and p r o p e r t i e s of complex compounds of the transition metals, it was shown that the nature of the cation has a definite influence on the nature of the interatomic m e t a l - l i g a n d interaction in a complex anion. Thus with i n c r e a s e in the size of the cation, the c h a r a c t e r i s t i c f r e q u e n c i e s of the vibrations of the [ML~]n- ion in the infrared s p e c t r a a r e displaced r e g u l a r l y towards longer wavelengths, and this was attributed [1-3] to a d e c r e a s e in the f o r c e constant of the bond [M- L]. In addition to the change in the frequencies of the vibrations, splitting of the bands due to the valence vibrations is observed, and this may be due to a d e c r e a s e in the s y m m e t r y of the anion [ML~]n- [1-4]. The study of the dependence of the chemical shifts in the NMR s p e c t r a [6-8J in complex compounds with fluorine-containing ligands on the nature of the cation in many c a s e s shows a monotonic increase in the shielding constant cr with i n c r e a s e in the size of the cation, and this can be interpreted as an increase in the electron density at the fluorine nuclei. In addition to the chemical shift in the NMR spectra, splitting of the absorption band into two components with intensity ratio 1 : 2 is observed, indicating nonequivalence of the two p o l a r and four equatorial fluorine ligands. To investigate the nature of the influence of the outer cation on the electronic s t r u c t u r e of a complex anion [MLG]n-, a study was made of the x - r a y K~ s p e c t r a of fluorine in the hexafluoro complexes of Pt, Pd, Rh, and Sn (Fig. 1) and the K~ s p e c t r a of chlorine in the hexachloro complexes of Pt and Ir (Fig. 2), and also in a s e r i e s of c r y s t a l l i n e alkali metal fluorides, used as model s y s t e m s (Fig. 3). The x - r a y K~ s p e c t r a of fluorine in the o n e - e l e c t r o n approximation in the case of the f r e e fluoride ion a r i s e in the t r a n s f e r of a valence 2p e l e c t r o n to the l s vacant orbital of fluorine. In c h e m i c a l compounds the K~ e m i s s i o n s p e c t r a a r i s e in the t r a n s f e r of electrons f r o m outer valence MO, constructed with the p a r t i c i p a tion of the 2p-AO of fluorine, into a l s vacancy of fluorine, and in c r y s t a l l i n e alkali metal fluorides f r o m the valence 2p zone of the c r y s t a l into a l s vacancy in the fluorine atom. The C1 Kfi x - r a y s p e c t r a in the hexaehloro complexes r e s u l t f r o m a transition f r o m MO having the 3p population of chlorine into a l s vacancy in the chlorine atom. The x - r a y f l u o r e s c e n c e s p e c t r a of fluorine and chlorine w e r e obtained on a Stearat s p e c t r o m e t e r .
The
Institute of Inorganic Chemistry, Academy of Sciences of the USSR, Siberian Branch. Novosibirsk State University. Translated f r o m ZhurnaI Strukturnoi Khimii, Vol. 18, No. 3, pp. 565-572, May-June, 1977. Original article submitted August 24, 1976. This material is protected by copyright registered in the name o f Plenum Publishing Corporation, 227 West 1 7th Street, New York, N. Y. 10011. No part ] o f this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, | microfihning, recording or otherwise, without written permission o f the publisher. A copy o f this article is available from the publisher for $ 7.50. i
453
6
!
\
K P t, C"b
670
675
680
Fig.
2810
eV
28t5
2820
eV
Fig. 2
1
Fig. 1. X - r a y F K(~ s p e c t r a in the hexafluoro c o m p l e x e s of rhodium, nickel, palladium, platinum, and tin. Fig. 2. X - r a y C1 Kill,5 s p e c t r a in c o m p l e x e s of platinum and iridium.
829 ', ~
~i ~, ,'
i
' 's~o'
' 'c~4'
' 'e'~s'
' 'e'82'
'.V
!~\
:
/"
/
L% / l/I
L
Fig. 3. F K s s p e c t r a in alkali m e t a l fluorides. Fig. 4. D i a g r a m of the m o l e c u l a r o r b i t a l s in o c t a h e d r a l c o m plexes of t r a n s i t i o n m e t a l s . c r y s t a l - a n a l y z e r s used w e r e : m i c a (2d = 19.9 .~) f o r the r e c o r d i n g of the F Ks s p e c t r a , and quartz (2d = 6.67 ]k) f o r the r e c o r d i n g of the C1 Kp s p e c t r a . The F K s and CI Kfl s p e c t r a w e r e studied in the f i r s t o r d e r of r e flection. The conditions of o p e r a t i o n of the x - r a y tube w e r e as follows: c u r r e n t 0.8 A, voltage 8 kV. A c h a r a c t e r i s t i c f e a t u r e of the s e r i e s of complex compounds studied is the p r e s e n c e of a complex anion [M~L6]n-, in which the p o s i t i v e m e t a l ion (M1) is s u r r o u n d e d by an octahedron of six fluoride (or chloride) anions, f o r m i n g the inner s p h e r e of the complex, while the positive alkali m e t a l cations (M2 = Li, Na, K, l~b, o r Cs) f o r m the o u t e r s p h e r e . According to the usual ideas, the i n t e r a c t i o n between the cations of the o u t e r s p h e r e of the complex and the complex anion of the t r a n s i t i o n m e t a l is purely e l e c t r o s t a t i c . Thus the e l e c t r o n i c s t r u c t u r e of the anion is d e t e r m i n e d chiefly by the nature of the e l e c t r o n i c interaction between the fluorine a t o m s and the t r a n s i t i o n m e t a l atom. F o r o c t a h e d r a l m o l e c u l e s , the nature of the e l e c t r o n i c s p e c t r u m can be d e s c r i b e d by the following s c h e m e of m o l e c u l a r l e v e l s (Fig. 4) [9]. The lowest-lying is the g r o u p of m e t a l s a~g, eg, tlu (these levels a r e not shown in the figure), the MO of which a r e c o n s t r u c t e d with the p a r t i c i p a t i o n of chi6fly the 2 s - A O of fluorine. This is followed by a group of levels which c o r r e s p o n d to the ~ bonds in the complex (alg , tlu , eg). The third group is f o r m e d by the levels r e s p o n s i b l e f o r the 7r bonds (t2g). These involve chiefly the 2p e l e c t r o n s of fluorine and the nd e l e c t r o n s of the metal. The fourth group is f o r m e d by levels c o n s t r u c t e d chiefly f r o m the 2p-AO of fluorine
454
TABLE 1. Ratio of the Integral Intensities I B / I A in Hexafluoro Complexes Cation~tSnF~-- I Pt F~i-- PdF~-- tlh[:~-Li + K+ l~b + Cs +
5,.~ 6,3 5.(~ ~,1
~ .6 3.{ 3,3 " "
'1.8
5.cl
2.I
3.5
and exhibiting nonbonding c h a r a c t e r (t2u, tlg , tlu ). In all the hexafluoro complexes of the transition metals studied, the upper filled level will be the antibonding level t*g. In the case of the anion [SnF6] 2- this level will be empty. This qualitative s c h e m e of levels can be made the basis of an interpretation 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 the fine s t r u c t u r e of the Ka s p e c t r u m of fluorine in the hexafluoro complexes. The m o s t intense peak B in the Ka s p e c t r u m of fluorine is due to x - r a y transitions f r o m levels for which the M e have nonbonding c h a r a c t e r and a r e constructed chiefly f r o m the 2p-AO of fluorine. The shoulder on the long-wave side of the K a line A is a s s o c i a t e d with the electrons of the second and third groups, that is with the levels which f o r m the bond M 1 - F (g o r ~r bond). (The maximum C in the s p e c t r a of fluorine is not considered, since it is a satellite c h a r a c t e r i s t i c of all fluorine spectra.) X - r a y transitions f r o m the lowest-lying levels a r e forbidden by the s e lection r u l e s (the l s - 2 s transition is forbidden). It is obvious that if the M1--F bond is purely ionic in c h a r a c ter, there will be no c o r r e s p o n d i n g interactions leading to the f o r m a t i o n of the levels of the cr and v bonding, and the long-wave m a x i m u m A will not be p r e s e n t in the spectrum. In fact, this shoulder is not o b s e r v e d in the s p e c t r a of the ionic c r y s t a l s NaF, KF, RbF, o r CsF (Fig. 3). In the c a s e of the LiF c r y s t a l , f o r which the bonding might be expected to have a strong covalent component, however, the fluorine s p e c t r u m shows a distinct shoulder A. Thus f r o m this qualitative analysis of the s p e c t r a of fluorine in the hexafluoro complexes of transition metals and alkali metal halide c r y s t a l s , it can be seen that the nature of the bond between the metal ion and fluorine should influence the intensity of the maximum A. Thus i n c r e a s e in the degree of interaction M l - L leads to an i n c r e a s e in the participation of the 2p-AO of fluorine in the bonding M e of the second and third groups and hence to an i n c r e a s e in the intensity of the shoulder A in the Ka s p e c t r u m . The F Ka x - r a y s p e c t r a w e r e broken down into individual components of d i s p e r s i o n f o r m with width at half the ordinate of the m a x i m u m ~ 1 eV (the width includes the width of the K level of fluorine plus distortions due to the apparatus). The n u m b e r of lines in the breakdown c o r r e s p o n d e d to the n u m b e r of M e which contain 2p electrons of fluorine. The r e s u l t of the breakdown made it possible to estimate the ratios of the integral intensities of the lines c o r r e s p o n d i n g to the nonbonding orbitals I B to the intensities of the bonding groups of levels IA. These ratios f o r the s e r i e s of compounds studied a r e given in Table 1. It can be seen f r o m Table 1 that with i n c r e a s e in the size of the cation, the ratio I B / I A d e c r e a s e s , and this may indicate an i n c r e a s e in the extent of the participation of the 2p-AO of fluorine in the m o l e c u l a r orbitals c o r r e s p o n d i n g to the m e t a l - l i g a n d bond. The o b s e r v e d dependence of the nature of the M--Lig interaction on the nature of the cation may be due to covalent interaction of the complex anion [MIL6] n- with the ions of the outer coordination sphere. The unique position in the series of complexes studied occupied by the hexafluoro complexes of tin [SnF6] zshould be noted. The 4d level of tin is full, and does not take an active part in the formation of the metal-ligand chemical bond. Thus the valence molecular orbitals t2g and eg are practically nonbonding orbitals of fluorine. The metal-ligand bond is formed only by the levels with symmetry alg and tlu (see Fig. 4). In Table I, the value of the ratio of the intensities IB/I A for the tin complexes is greater than the values for the transition metal complexes, indicating that the Sn-F bond is more ionic than the bond in the transition metal complexes. Examination of the values of the ratio IB/I A in the series of transition metal compounds K2NiF o K 2pdF6, and K2PtF 6 (IB/I A has the values 5.6, 4.8, and 3.4 respectively) indicates that the covalent character of the bond in the hexafluoro complexes increases from nickel to platinum. The x-ray spectra of fluorine and chlorine in K2PtF 6 and K2PtCI 6 are compared in Fig. 5. The ratio of the intensities of the lines IB/I A will be -~2.0 for K2PtCl 6 and ~-3.0 for K2PtF G. (The possible inaccuracy observed in the determination of the ratio IB/I A in fluorine complexes is associated with the inaccuracy of the isolation of the range corresponding to the satellite C.) According to what has been said above, the covalent component of the bonding is greater in the case of the chloro complexes. The observed experimental dependence of the extent of the participation of the 2p electrons of fluorine in the metal-ligand chemical bond on the nature of the outer-sphere cation in the case of the fluoro compounds of
455
z~E,, r ~810
2815
92 8 2 0
A8 7Ka
/f'~
/l
e~V
K,2P',','bF6
"--+--:7
~\
1 ! ""4.. i '-
670
675
680
hi,
~V
i
Ndlo
"1" :<:
Rb
Fig. 5
C$
I
R
2A
IA
Fig. 6
Fig. 5. B r o k e n - d o w n F K~ and C1 Kfli,s s p e c t r a in platinum c o m p l e x e s . Fig. 6. Dependence of the width of the K(~ line of fluorine on the s i z e of the cation in the s e r i e s of o c t a h e d r a l c o m p l e x e s of Pd, Pt, and Sn. the noble m e t a l s m a y be due to covalent i n t e r a c t i o n between the complex anion [M1L6]n- and the ions of the o u t e r coordination sphere. In fact, with allowance f o r the i n t e r a c t i o n of the anion [M1L6]n- with the ions of the o u t e r coordination s p h e r e f o r m e d by the alkali m e t a l ions, the bonding m o l e c u l a r o r b i t a l can be w r i t t e n in the f o r m
Here, ~A..M2 is the m o l e c u l a r o r b i t a l of the c o m p l e x anion (A) with allowance f o r the o u t e r coordination s p h e r e of positiv~ alkali m e t a l ions, ~I,A. is the m o l e c u l a r o r b i t a l of the complex anion, and ~M2 is the group m o l e c u l a r I o r b i t a l c o n s t r u c t e d f r o m the AO of the p o s i t i v e m e t a l ions M2, f o r m i n g the o u t e r coordination s p h e r e of the complex. The MO of the c o m p l e x anion (~Ai) in turn can be written in the f o r m
Here, ~2M1 is the AO in the c e n t r a l a t o m of the c o m p l e x anion (the nd-AO of the c o r r e s p o n d i n g m e t a l M 1 is taken as q~Ml), and q~Li is the g r o u p o r b i t a l of the ligands (L), c o n s t r u c t e d f r o m the AO of fluorine in such a way that q'.Li =
l':'i
CiU Wr r
The intensity of the x - r a y e m i s s i o n t r a n s i t i o n is p r o p o r t i o n a l to the s q u a r e of the o r b i t a l coefficient I C [2 for the AO of the a t o m whose s p e c t r u m is being studied and whose s y m m e t r y s a t i s f i e s the atomic dipole s e l e c t i o n r u l e s [1.0, 11]. In the c a s e of the Kvz s p e c t r a of fluorine studied in the p r e s e n t work, the intensity of the x - r a y t r a n s i t i o n s in a given MO of the c o m p l e x w i l l be p r o p o r t i o n a l to the s q u a r e of the coefficient C F [2, With which the 2p-AO of one of the fluorine a t o m s is p r e s e n t in the c o r r e s p o n d i n g MO. S ~ I CrJ~ =
],VA,~,..,,%,:r,~A,.,,,C,. ['-= .VS,,,.,,I CA?I ~'.
(3)
Here, CAi F = [ NAiM1TAiMICiF i [ is the coefficient of the l i n e a r approximation, with which the AO of fluorine a p p e a r s in the MO of the c o m p l e x anion [ML6] n- (see Eq. {2)). The value of the coefficient is defined by the equation: :~
w h e r e 7A:~ =: --
iM." = :
1 l - r ~'Ai~: 27A:~-fiAiM: '
(4)
lrAv~ - ' HA~A{;A?~, H~,M _ HA~A~ , HM2M2 is the diagonal m a t r i x e l e m e n t of the Hamiltonian, defined in the
AO of the alkali m e t a l (M~) situated in the o u t e r coordination s p h e r e of the complex, and HAiAi is the analogous m a t r i x e l e m e n t defined in the MO of the c o m p l e x anion.
456
is the group o v e r l a p integral between CA. - the MO of the c o m p l e x a n i o n - and q~M~- the l i n e a r combination of 1 AO of the m e t a l ions in the o u t e r s p h e r e of the complex. This integral can be e x p r e s s e d in t e r m s of the o v e r lap i n t e g r a l s of the s e p a r a t e AO of the s y s t e m being considered.
Since the s t r u c t u r e of the c o m p l e x anion [MIF6] n- does not change in the s e r i e s of hexafluoro c o m p l e x e s studied, the coefficient [ CAi F Iz a p p e a r i n g in Eq. (3) should r e m a i n unchanged, so that the o b s e r v e d change in the intensities I B / I A can be a s s o c i a t e d with the change in the f a c t o r [ NAiM2 [2. The e x p e r i m e n t a l data given in Table 1 show that the r a t i o of the intensities of the two m a x i m a I B / I A d e c r e a s e s in the s e r i e s of compounds studied, that is the intensity of the x - r a y t r a n s i t i o n s f r o m the MO r e s p o n s i b l e f o r the c h e m i c a l bonding in the complex i n c r e a s e s r e l a t i v e to the t r a n s i t i o n s f r o m the nonbonding o r b i t a l s . If the wavefunction of the bonding MO is w r i t t e n in the f o r m (1), the ratio of the intensities of the two m a x i m a can be written: i
-~B= ~ I, (nonbonding)-- ~f[Cij ~.~ I2 1\/i s l r l 2 p ) l "~
~lCil
r"
"
(6)
This l a s t e x p r e s s i o n can be w r i t t e n in a s i m p l e r f o r m by taking account of the f a c t that the s u m of the coefficients Cij in the d e n o m i n a t o r depends only on the fluorine o r b i t a l s ~ L i ' that is it is a constant* I4 IB
Z NA~z~ (C~F)'.
(7)
i
Since the s t r u c t u r e of the complex anion [1V[1L6]n- in the s e r i e s of fluoro compounds studied r e m a i n s unchanged, the coefficients a p p e a r i n g in the s u m in Eq. (7) should r e m a i n unchanged, so that the o b s e r v e d change in intensity can be r e l a t e d to the change in the f a c t o r s [ NAiM. [2. The e x p e r i m e n t a l data show that with change in the z 2 s i z e of the cation (M2) in the s e r i e s L i - C s , the coefficients [ NAiM2 ] should i n c r e a s e . The i n c r e a s e in the N~iM2 in a c c o r d a n c e with Eq. (4) can be a s s o c i a t e d with d e c r e a s e in the coefficient YAiM2; that is the c o n t r i bution of the AO of the cations should d e c r e a s e with i n c r e a s e in the s i z e of the cation in the MO ~I,AiM2, so that the bond between the c o m p l e x anion and the o u t e r coordination s p h e r e (M2) b e c o m e s m o r e ionic. D e c r e a s e in the value of YAiM2 (see Eq. (9)) m a y be due to a d e c r e a s e inG A ~a HA ~ and H ~ M in the i~'~2 ~i• ~2 2 s e r i e s of compounds studied. In fact, with i n c r e a s e in the size of the cation, the group o v e r l a p i n t e g r a l between the MO of the c o m p l e x anion and the c o r r e s p o n d i n g group o r b i t a l of the cations d e c r e a s e s
Since GAiM2 is s m a l l (~ 0.1), we obtain f o r TAiM2 the a p p r o x i m a t e e x p r e s s i o n H.~Iz~[~GA
iM~
GAiM, H~(s 2]
To e s t i m a t e the quantities H~,~,~ and H^.^.,,~l~l it is p o s s i b l e to use e x p e r i m e n t a l data. The value of HAiAi f o r all the c o m p l e x e s studied rem~iffs constant and can be e s t i m a t e d d i r e c t l y f r o m e x p e r i m e n t f r o m the position of the m a x i m u m A (IA) r e l a t i v e to the level of a v a c u u m I i : E i - - I t s ~ 10 ev,
w h e r e E A is the e n e r g y of the x - r a y t r a n s i t i o n (~ 675 eV), and Ils is the ionization potential of the inner l s level of fluorine in the c r y s t a l (Ils ~ 685 eV). The nature of the i n t e r a c t i o n in the s e r i e s L i - C s can be e s t i m a t e d as follows: * If however the nature of the L - - M 2 i n t e r a c t i o n in the s e r i e s being c o n s i d e r e d d e c r e a s e s , the change in the intensity of the p e a k B should be s i m i l a r to the change in the peak A, since d e c r e a s e in the M 2 - L i n t e r a c t i o n should also lead to an i n c r e a s e in the intensity I B. Since however the e x p e r i m e n t a l value of the r a t i o I B / I A d e c r e a s e s in the s e r i e s of compounds considered, it m a y be a s s u m e d that the intensity of the p e a k B i n c r e a s e s m u c h m o r e rapidly, indicating that t h e r e is little p a r t i c i p a t i o n of the lr-nonbonding o r b i t a l s of the complex anion in the bonding with the o u t e r coordination s p h e r e .
457
~er e, /'/l~,M, ~ -- IM,M,, 9 [M,M, = IMAM, -- q) "~-,
whore IMzMI is the Ionization potential of the outer np-AO of the cations l n t h e crystal, a n d i ~ ) is the ionization potential of the free ion, The expression ~(e2/R) takes account of the influence of the crystalline e n v i r o n m e n t . In the series Na-Cs, the value of I ~ r ~ ) varies In the r ~ e 50-25 eV, that is it decreases by a factor of - 2 , w h e r e a s the quantity ~(eZ/R) d e c r e a s e s 2 by a f a c t o r of ~ 1 . 3 , so that the Values of HM2M2 a r e s m a l l . N Similarly, for 7AIM2 the ratio 7 ~free) i M /Y~leM for the two extreme terms of the series will be: -
GAjM,
GA'
O)
Since the ratio GAIM/GA is much less than 1, it can be seen that YAIMI decreases in the series of compounds considered, in agreement with experiment. Examination of the Ka specWa of fluorine in the series of octahedral complexes of various metals studied shows that the width of the K~ l i n e of fluorine clearly depends on thenature of the cation. The nature of this dependence is analogous to the observed relationship for the width of the valence zone in the series of alkali halide crystals. In [12] it was shown that increase in the size of the cation in the s e r i e s of crystalline alkali metal chlorides leads to a decrease in the width of the Kfl 1,5 line of chlorine. The observed experimental features were attributed to the change in thewidth of the valence zone in the crystals of the alkali metal chlorides with inc r e a s e in the lattice constant of the crystal. Figure 3 gives the observed K~ spectra of fluorine in crystalline alkali metal fluorides. As in the case of the chlorides and bromides [12], the valence band of fluorine decreases with increase in the lattice constant. Analysis of the dependence of the width of the valence absorption band, associated chiefly with the valence band in these crystals, on the size of the cation shows that Inthe series considered, the crystals containlng the lithium cation occupy a unique position. In fact, according to Pauling, the standard ionic crystals with the NaCl structure are those for which the ratio of the radii of the cation and the anion amounts to 0.75. In this case, according to crystal-chemical theories, the anion Is in contact with the cation. If however the ratio of the radii p = R + / R - becomes less than 0.414, the anions are in contact with not only the cations but also anions. In the case of crystalline lithium chlorides and fluorides, the chloride and fluoride ions are in contact with one another, so that we should expect much g r e a t e r overlap of the wavefuncUons of the neighboring chloride or fluoride ions, and hence an appreciable width for the valence zone in these crystals, compared with other crystalline alkali metal chlorides and fluorides. The dependence of the width of the K~ line of fluorine on the size of the cation in the series of octahedral complexes of Pd, Pt, and Sn is shown in Fig. 6. It can be seen from Fig. 6 that the width of the K~ line decreases regularly with increase in the size of the outer-sphere cation of the complex. The observed relationship for the width of the K~ line of fluorine in the series of complexes studied can be explained in the same way as in the case of the simple alkali metal halides studied, that is the width of the valence band in the crystalline complexes is determined by the degree of overlap of the wavefunctions of fluorine, so that if we regard the complexes as solids, we can assume that allowance for the F - F (or C1-CI) interaction between neighboring octahedra leads to broadening of the corresponding Me to form an energy band of finite width. LITERATURE 1. 2. 3.
4. 5. 6. 7. 8.
458
CITED
D.M. Adams and D. M. Morris, J. Chem. Soc., A, 1967, 1666. D . M . Adams and D. M. Morris, J. Chem. Soc., 1967, 1669. A. Finch, P. N. Gates, K. Radcliffe, F. N. Dickson, and F. F. Bentley, Chemical Applications of F a r Inf r a r e d Spectroscopy,Academic P r e s s , New y o r k (1970) [Russian translation: Izd. Mir, Moscow (1973), p. 132]. D.M. Adams, Metal--Ligand and Related Vibrations, Edward Arnold, London (1967). M. Roland Bougon, C.R. Acad. ScL Paris, Ser. C, 267, 681 (1968). S.V. Zemskov, S. P. Gabuda, Yu. L Nlkonorov, E. D. Pastukhova, and V. A. Selezneva, DokL Akad. Nauk SSSR, 216, 123 (1974). S.V. Zemskov, S. P. Gabuda, Yu. L Nikonorov, and V. A. Seleznev, Zh. Strukt. Khim., 15, 933 (1974). S.V. Zemskov, S. P. Gabuda, Yu. I. Nikonorov, V. A. Shchipachev, and G. I. Zharkova, Izv. Sib. Otd. Akad. Nauk SSSR, Ser. Khhn., P a r t 3, No. 7, 92 (1976).
9. 10. 11. 12.
I . B . B e r s u k e r , The Electronic Structure and P r o p e r t i e s of Coordination Compounds [in Russian], Khimiya, Leningrad (1976). V . I . Nefedov, A. P. Sadovskii, L. N. Mazalov, E. S. Gluskin, and L. !. Chernyavskii, Zh. Strukt. Khim., 12, 681 (1971). L . N . Mazalov, Doctoral Dissertation, Institute of Inorganic C h e m i s t r y , Siberian Branch, Academy of Sciences of the USSR (1972). L . N . Mazalov, Summary of Candidate's Dissertation, Institute of Inorganic C h e m i s t r y , Siberian Branch, Academy of Sciences of the USSR (1967).
CRYSTAL OXIDE
STRUCTURE
OF
PRASEODYMIUM
IODIDE O. G. P o t a p o v a , I. G . a n d S. V . B o r i s o v
Vasil'eva,
UDC 548.736
The s t r u c t u r e of P r O I was determined f r o m ]6 single reflections m e a s u r e d on a d i f f r a c t o m e t e r (space group P 4 / n m m , a = 4.086 (2), c = 9.162 (2) .~, Z = 2, dmeas = 5.89 g / c m 3, s t r u c t u r a l type PbFC1). The coordinate p a r a m e t e r s of the s t r u c t u r e , refined by the method of least squares, a r e : z (Pr) = 0.131 (4), z (I) = 0.672 (4). The coordination polyhedron of the P r atom is a square antiprism, with 4 P r - O to one s q u a r e face 2;37 .~, 4 P r - - I to the other 3.41 ~. The a n t i p r i s m s a r e joined by s h a r e d oxygen edges ( P r - P r 3.76 ~) and I - O - I faces. The period a is determined by the double l a y e r of cations, s e p a r a t e d by a l a y e r of 02-, and the period c is d e t e r mined by the halide ions.
It is known that the oxide halides of the r a r e - e a r t h elements (REE) MOX c r y s t a l l i z e chiefly with the PbFC1 s t r u c t u r e (tetragonal space group P 4 / n m m , two coordinate p a r a m e t e r s ) . F o r the i s o s t r u c t u r a l s e r i e s of oxide chlorides [1] and oxide b r o m i d e s [2, 3] of the REE, complete data are available on the s t r u c t u r a l p a r a m e t e r s and unit cell dimensions, obtained by the x - r a y diffraction study of powdered specimens. Only a few oxide iodides have been studied, however (Table 1). The absence of x - r a y diffraction data f o r the other m e m b e r s of the s e r i e s ks apparently due to the difficulties of obtaining them in pure and well c r y s t a l l i z e d form. The r e c o m m e n d e d g e n e r a l methods for the synthesis of the REE oxide halides by the t h e r m a l decomposition of the hydrated halides MX3 9 xH20 o r by hydrolysis of the anhydrous trihalides a r e not reliable for the p r e p a r a t i o n of M(III) oxide iodides. We have been unable to p r e p a r e p r a s e o d y m i u m oxide iodide by different v e r s i o n s of these methods. Under the conditions in which the oxide chlorides and oxide bromides are formed, we obtained instead of the expected oxide iodide only a m o r phous p r a s e o d y m i u m hydroxide iodides and oxide hydroxides, unstable in air. The apparent r e a s o n is that as a consequence of the l a r g e r size of the iodide ion and the lower strength of the M--I bond, the coordination of w a t e r to the REE ions is p r e f e r r e d to the coordination of iodide. Thus in the case of the oxide iodides it is n e c e s s a r y to r e c o m m e n d methods of synthesis which completely exclude the possibility of contact with w a t e r vapor: halogenation of the I~EE oxides o r mild oxidation of the halides. The method involving the h i g h - t e m p e r a t u r e sintering of the oxide and the trihalide, used in [8], did not give a s i n g l e - p h a s e product, stable in air. Single-phase p r a s e o d y m i u m oxide iodide was obtained by the r e a c tion PrI3(liquid ) + 1/2 0 2 -* PrOI(solid) + I2 (gas) at a p a r t i a l p r e s s u r e of oxygen in the s y s t e m .~ 10 -3 m m Hg. A graphite container p r o t e c t e d the iodide f r o m r e a c t i o n with quartz, and eliminated side reactions, while the iodine liberated and the excess of iodide w e r e sublimed into the cold end of the ampul. P r a s e o d y m i u m oxide
Institute of Inorganic Chemistry, Academyof Sciences of the USSR, Siberian Branch. Translated from Zhurnal Strukturnoi Khimii, Vol. 18, No. 3, pp. 573-577, May-June, 1977. Original article submitted July 27, 1976. Thi~ material is protected by copyright registered in the name of Plenum Publishing Corporation, 227 West 17th Street, New York, N.Y. 10011. No part 1 of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by an), means, electronic, mechanical, photocopying,| f microfilming, recording or otherwise, without written permission of the publisher. A cop), of this article is available from the publisher for $ 7.50. |
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