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A comparison of the raman spectra of some complex anions in the crystalline phase and in solution
SpectrochidcaActs. 1067,Vol. 22A, pp. 2871to 2874.
Pergamon Press Ltd.Printed is Northern Ireland
A comparison of the Raman spectra of some complex anions in the crysMine phase and in solution P. J. HENDRA Department of Chemistry, University of Southampton (Received 26 April 1967) Abstr&-Raman spectra of solutions and solids of the type K,,M’X, snd K,M’X, are reported (M’ = Au, Pt and Pd; X = Cl or Br). On dissolving a solid in water or acid the stretching vibration frequencies are relatively unaffeoted but the deformations may fall by as much as 26 cm-l. In using solid-phase measurements of vibration frequency for structural diagnosis allowancemust be made for these large shifts but they do not appear to be w seriousas has been recently forecast. RECENTLY, HLRAISIII and SHIMANOUCIII [l] have given an account of the effect of lattice modes of vibration on the internal modes of a series of complex anions. They demonstrated mathematicelly that in such compounds as potassium tetrachloroplatinum (II) and potassium hexachloroplatinum (IV) a considerable cation-anion interaction is to be expected which would have the effect of drastically raising the frequency of some of the modes of vibration of the anion above their norms1 “free” values. They point out that although this interaction should have little effect on the stretching modes, the deformations could well shift from their “free ion” values As it has become convenient and, in fact, customary to by as much as 60 cm-l. use data accumulated from i&a-red (and now Raman) studies of solids for structural diagnoses in co-ordination chemistry, and to assume that phase change shifts are negligible it is clearly essential to know just how serious are these interactions in order that one can gauge the reliance to be put on solid-phase measurements. It is frequently the case that co-ordinstion compounds are difficult to dissolve in organic solvents thus precluding solution phase infra-red measurements. The authors were in the unfortunate position of having solid-phase i&a-red data and solution Raman results on molecules with a centre of symmetry, thus, they had no direct experimental method of checking their suggestions. Recently, it has become convenient to record Raman spectra of deeply coloured solids [2, 31 and solid-phase data on some of the molecules examined by Riraishi and Shimanouchi is available in the literature [2]. A careful comparison of the two sets of results is therefore required to clarify the situation. A series of square planar and octahedral complexes were either obtained commercially from Johnson Mathey and Co. or, in the cases of K,PdCl, and K,PtBr, prepared by standard means [4] from the tetrehalo species. Raman spectra of K,PtCl,, K,PtBr,, KAuCl, and KAuBr, were obtained in the solid (as [l] J. HIRAISHI and T. SHIBXANOUCHI,Spectrochim.AC&Z22, 1483 (1966). [2] P. J. HIENDRA, Nature 212, 179 (1966). [3] P.J. HENDRA~~~E.R.LIPPINCOTT,N&M~ 212,1448 (1966); P.J. HENDRA~II~Z.JOVIC, J. Chem. Sot. A, 1127 (1967). [4] A. GUTBIER and A. KRELL, Ber. 38,2385 (1906). 2871
2872
P.
J.
HENDRA
small pressed pellets) and solution states. Solid phase spectra of K,PtCI,, K,PtBr, and K,PdCl, and of an aqueous solution of chloroplatinic acid were also measured. Where possible a record was made of the effect of polarization on line intensity. In all solution spectra a single polarized (polarization ratio
Potassium salt as B pellet
1
Aqueous solution
Previous solution Results
shift Liq. + Solid
ptcy-
329 8 302 w 194 mw 76s
332 s(p) 314 m 17ow
PtBr,=-
206 * 190m 126m
206 s(p) 194 m
AuCl,-
349 8
359 s(p)
347’
-1
324 183mw m
326 broad 173 ms
324 171
370
213s 196 m 108w
215 S(P) 197 m 104w
212’ 196 102
-2 ;:
$3”;; 2)
344t
+6
169 w
320 162
y-f2
AuBr,-
PtcI,‘-
349 8 320 ms 1718
3358 304 164
-3 -12 +24 -3 -4
PtBr,”
216 snl 8 191 1lOwm
207t 190 97
:: +13
PdCl,‘-
324 m s 294 1721.11
317 292 174
$: -2
* Rssultg due to STAMJ~REICHand FOBNEBIS [6]. Results due to \~OODWARD et al. [6]. $ Results from examinstion of H,PtCI, in H,O (~10% (p) Polarized line.
t
cont.).
It will be seen from the table that the measurements reported here agree well with those given previously by STAMMREICH [5], WOODWARD [6], and co-workers. In this deterThe only major discrepancy seems to occur in the case of K,PtCl,. mination, a large shouldered band was found peaking at Av = 330 cm-l. Inclusion [6] H. STAMMXEICH and R. FORNERIB,Spectrochim. Acta 16, 363 (1960). [B] L. A. WOODWAELD and A. CREIUEITON, Spectrochim. Acta 17,594 (1961); and M. J. WBE, Spectrochim. Acta 20, 711 (1964).
L.
A.
WOODWARD
A oomparison of the Raman spectra of some complex anions
IPtCC,I’-
I
I
I
I
[PtCC,12-
_I
;’
____---- /’
,JLJf, ,“:
/’ :
:
:
,,’
l.,’
400
:
x5
:
i,-Z
:
‘\
300‘
200
AU,
cm-’
KM
Fig. 1. The Raman spectra of the PtCI,Z- and PtCl,* ions in solution and in the solid phase is the potassium salts. (Solid line, 11polarizer; dotted line 1 polarizer on solutions. Vertical lines, previous results. Broken lines, solid phase spectra.)
2873
2874
P. J. HENDRA
of a crossed polarizer in the system altered this to a band at the position of the shoulder (Av = 314 cm-l). No emission was observed near 304 cm-l. The band at 170 cm-l in the solution spectrum of K,PtCl, is weak and broad and must have been difficult to detect photographically. In all the solutions examined only one line was polarized and that always the one furthest from the exciting line. Clearly these lines arise from the A,, modes yl. * In the last column of the table the shift on changing phase is recorded. Two points can be made. (a) Of all of the vibrations effected by a change of phase, those at the lowest frequency seem to be most sensitive. These are the XMX bending mode (Z&)Y* and the similar X,M deformation rs(FZs) in MX, and MX, respectively. And (b) the shifts seem to be random in nature. Hiraishi and Shimanouchi proposed that in the square planar system MX, the in-plane deformation Y,(E~ class) may shift by more than 25% on changing phase. The closely related vibration Ye should be similarly effected. The experimental results reported here suggest strongly that shifting can and does occur but its magnitude is very variable. Thus, in KAuBr,, dissolving in water has little or no effect on the vibration frequencies of the anion. It was possible in two cases to detect a lattice mode in the Raman spectra. This E, class vibration has frequencies of 76 cm-l in the K,PtCI, species and 59 cm-r in KAuCl,. The wide difference in frequency suggests that the interactions between cation and anion may differ considerably in these two cases (of course, the species are not isostructural); PtC1,2shows large phase effects and AuCI,- shows very little. Unfortunately it was not possible to observe the lowest frequency band in the solution of K,PtBr, to ascertain whether large phase-change shifts are characteristic of the platinum containing species. Turning to the octahedral complexes, it is clear that again the vibration vp is effected most by dissolving the ion. The F,, stretching mode vq varies in the series M,PtCI, from 345 cm-1 where M = K, to 331 where M = Cs, yet the Raman active stretching modes seem to be relatively insensitive to the presence of a cation. As forecast previously, the deformation modes of vibration are not as sensitive to interaction as they are in the square planar complexes. In conclusion, it must be admitted that the effects of lattice interaction on internal modes of vibration of a complex ion as forecast by Hiraishi and Shimanouchi are important but their magnitude is not as great as was anticipated. It is suggested that allowance must be made, when considering solid phase spectra, for a shift of up to 25 cm-l from the free isolated ion value for the deformation modes of vibration. This is probably an overestimate of the effect in the majority of cases. Aclcnowledgemen~The author wishes to thank E.A.T.O. used for the purchase of materials used in this research.
for their award of a grant which W&S
* Mode numbering in this paper is that due to NAEAMOTO
[7].
[7] K. NAKAMOTO, InfraredSpectra of Inorganic and Coordination John Wiley
(1963).
Compounda, pp. 113, 118.
Pergamon Press Ltd.Printed is Northern Ireland
A comparison of the Raman spectra of some complex anions in the crysMine phase and in solution P. J. HENDRA Department of Chemistry, University of Southampton (Received 26 April 1967) Abstr&-Raman spectra of solutions and solids of the type K,,M’X, snd K,M’X, are reported (M’ = Au, Pt and Pd; X = Cl or Br). On dissolving a solid in water or acid the stretching vibration frequencies are relatively unaffeoted but the deformations may fall by as much as 26 cm-l. In using solid-phase measurements of vibration frequency for structural diagnosis allowancemust be made for these large shifts but they do not appear to be w seriousas has been recently forecast. RECENTLY, HLRAISIII and SHIMANOUCIII [l] have given an account of the effect of lattice modes of vibration on the internal modes of a series of complex anions. They demonstrated mathematicelly that in such compounds as potassium tetrachloroplatinum (II) and potassium hexachloroplatinum (IV) a considerable cation-anion interaction is to be expected which would have the effect of drastically raising the frequency of some of the modes of vibration of the anion above their norms1 “free” values. They point out that although this interaction should have little effect on the stretching modes, the deformations could well shift from their “free ion” values As it has become convenient and, in fact, customary to by as much as 60 cm-l. use data accumulated from i&a-red (and now Raman) studies of solids for structural diagnoses in co-ordination chemistry, and to assume that phase change shifts are negligible it is clearly essential to know just how serious are these interactions in order that one can gauge the reliance to be put on solid-phase measurements. It is frequently the case that co-ordinstion compounds are difficult to dissolve in organic solvents thus precluding solution phase infra-red measurements. The authors were in the unfortunate position of having solid-phase i&a-red data and solution Raman results on molecules with a centre of symmetry, thus, they had no direct experimental method of checking their suggestions. Recently, it has become convenient to record Raman spectra of deeply coloured solids [2, 31 and solid-phase data on some of the molecules examined by Riraishi and Shimanouchi is available in the literature [2]. A careful comparison of the two sets of results is therefore required to clarify the situation. A series of square planar and octahedral complexes were either obtained commercially from Johnson Mathey and Co. or, in the cases of K,PdCl, and K,PtBr, prepared by standard means [4] from the tetrehalo species. Raman spectra of K,PtCl,, K,PtBr,, KAuCl, and KAuBr, were obtained in the solid (as [l] J. HIRAISHI and T. SHIBXANOUCHI,Spectrochim.AC&Z22, 1483 (1966). [2] P. J. HIENDRA, Nature 212, 179 (1966). [3] P.J. HENDRA~~~E.R.LIPPINCOTT,N&M~ 212,1448 (1966); P.J. HENDRA~II~Z.JOVIC, J. Chem. Sot. A, 1127 (1967). [4] A. GUTBIER and A. KRELL, Ber. 38,2385 (1906). 2871
2872
P.
J.
HENDRA
small pressed pellets) and solution states. Solid phase spectra of K,PtCI,, K,PtBr, and K,PdCl, and of an aqueous solution of chloroplatinic acid were also measured. Where possible a record was made of the effect of polarization on line intensity. In all solution spectra a single polarized (polarization ratio
Potassium salt as B pellet
1
Aqueous solution
Previous solution Results
shift Liq. + Solid
ptcy-
329 8 302 w 194 mw 76s
332 s(p) 314 m 17ow
PtBr,=-
206 * 190m 126m
206 s(p) 194 m
AuCl,-
349 8
359 s(p)
347’
-1
324 183mw m
326 broad 173 ms
324 171
370
213s 196 m 108w
215 S(P) 197 m 104w
212’ 196 102
-2 ;:
$3”;; 2)
344t
+6
169 w
320 162
y-f2
AuBr,-
PtcI,‘-
349 8 320 ms 1718
3358 304 164
-3 -12 +24 -3 -4
PtBr,”
216 snl 8 191 1lOwm
207t 190 97
:: +13
PdCl,‘-
324 m s 294 1721.11
317 292 174
$: -2
* Rssultg due to STAMJ~REICHand FOBNEBIS [6]. Results due to \~OODWARD et al. [6]. $ Results from examinstion of H,PtCI, in H,O (~10% (p) Polarized line.
t
cont.).
It will be seen from the table that the measurements reported here agree well with those given previously by STAMMREICH [5], WOODWARD [6], and co-workers. In this deterThe only major discrepancy seems to occur in the case of K,PtCl,. mination, a large shouldered band was found peaking at Av = 330 cm-l. Inclusion [6] H. STAMMXEICH and R. FORNERIB,Spectrochim. Acta 16, 363 (1960). [B] L. A. WOODWAELD and A. CREIUEITON, Spectrochim. Acta 17,594 (1961); and M. J. WBE, Spectrochim. Acta 20, 711 (1964).
L.
A.
WOODWARD
A oomparison of the Raman spectra of some complex anions
IPtCC,I’-
I
I
I
I
[PtCC,12-
_I
;’
____---- /’
,JLJf, ,“:
/’ :
:
:
,,’
l.,’
400
:
x5
:
i,-Z
:
‘\
300‘
200
AU,
cm-’
KM
Fig. 1. The Raman spectra of the PtCI,Z- and PtCl,* ions in solution and in the solid phase is the potassium salts. (Solid line, 11polarizer; dotted line 1 polarizer on solutions. Vertical lines, previous results. Broken lines, solid phase spectra.)
2873
2874
P. J. HENDRA
of a crossed polarizer in the system altered this to a band at the position of the shoulder (Av = 314 cm-l). No emission was observed near 304 cm-l. The band at 170 cm-l in the solution spectrum of K,PtCl, is weak and broad and must have been difficult to detect photographically. In all the solutions examined only one line was polarized and that always the one furthest from the exciting line. Clearly these lines arise from the A,, modes yl. * In the last column of the table the shift on changing phase is recorded. Two points can be made. (a) Of all of the vibrations effected by a change of phase, those at the lowest frequency seem to be most sensitive. These are the XMX bending mode (Z&)Y* and the similar X,M deformation rs(FZs) in MX, and MX, respectively. And (b) the shifts seem to be random in nature. Hiraishi and Shimanouchi proposed that in the square planar system MX, the in-plane deformation Y,(E~ class) may shift by more than 25% on changing phase. The closely related vibration Ye should be similarly effected. The experimental results reported here suggest strongly that shifting can and does occur but its magnitude is very variable. Thus, in KAuBr,, dissolving in water has little or no effect on the vibration frequencies of the anion. It was possible in two cases to detect a lattice mode in the Raman spectra. This E, class vibration has frequencies of 76 cm-l in the K,PtCI, species and 59 cm-r in KAuCl,. The wide difference in frequency suggests that the interactions between cation and anion may differ considerably in these two cases (of course, the species are not isostructural); PtC1,2shows large phase effects and AuCI,- shows very little. Unfortunately it was not possible to observe the lowest frequency band in the solution of K,PtBr, to ascertain whether large phase-change shifts are characteristic of the platinum containing species. Turning to the octahedral complexes, it is clear that again the vibration vp is effected most by dissolving the ion. The F,, stretching mode vq varies in the series M,PtCI, from 345 cm-1 where M = K, to 331 where M = Cs, yet the Raman active stretching modes seem to be relatively insensitive to the presence of a cation. As forecast previously, the deformation modes of vibration are not as sensitive to interaction as they are in the square planar complexes. In conclusion, it must be admitted that the effects of lattice interaction on internal modes of vibration of a complex ion as forecast by Hiraishi and Shimanouchi are important but their magnitude is not as great as was anticipated. It is suggested that allowance must be made, when considering solid phase spectra, for a shift of up to 25 cm-l from the free isolated ion value for the deformation modes of vibration. This is probably an overestimate of the effect in the majority of cases. Aclcnowledgemen~The author wishes to thank E.A.T.O. used for the purchase of materials used in this research.
for their award of a grant which W&S
* Mode numbering in this paper is that due to NAEAMOTO
[7].
[7] K. NAKAMOTO, InfraredSpectra of Inorganic and Coordination John Wiley
(1963).
Compounda, pp. 113, 118.