The fundamental vibrational spectra of the formates of the main group elements

Spectrochimica Acta, 1964, Vol. 20, lap. 847 to 851. PergamonPress Ltd. Printed in Northern Ireland

The fundamental vibrational spectra of the ~ormates of the main group elements J . D. DONALDSON, J . F . KNIFTON a n d S. D . R o s s D e p a r t m e n t of Chemistry, Chelsea College of Science and Technology, S.Vv'.3 (Received 13 November 1963)

A b s t r a c t - - T h e infra-red spectra of twenty formates have been measured as mulls in Nujol and hexachlorobutadiene. The influence of the metal ion on the carboxylate stretching frequencies has been investigated, and possible correlations between spectra and structure considered. INTRODucTIoN STUDIES o f t h e f o r m a t e i o n h a v e b e e n m a d e u s i n g t h e R a m a n effect [ 1 - 8 ] ; t h e i n f r a - r e d s p e c t r a o f s o m e a l k a l i a n d a l k a l i n e e a r t h f o r m a t e s h a v e also b e e n r e c o r d e d [ 9 - 1 2 ] . T h e r e a r e six f u n d a m e n t a l s , all o f w h i c h a r e b o t h i n f r a - r e d a n d R a m a n active, and the generally accepted assignments are given below. Table 1. The fundamental vibrational spectrtun of the formate ion Description C - - H stretch • Symmetric OCO stretch Symmetric OCO deformation Asymmetric OCO stretch Asymmetric OCO deformation Out-of-plane deformation

Frequency (cm -1)

Assignment

2801 1370 774 1592 1381 1066

v1 u2 ua u4 u5 us

Type AL A1 A1 B1 B1 Bu

(Note: The frequencies are our observed frequencies for rubidium formate.) The vibrations of the carboxylate ion have been treated as a separate system by a number of workers, and relationships have been suggested between the frequency of v~ a n d v a r i o u s p r o p e r t i e s o f t h e c a t i o n . KAOARISE [13] f i n d s in p h e n y l s t e a r a t e s a l i n e a r r e l a t i o n b e t w e e n v2 a n d t h e e l e c t r o n e g a t i v i t y o f t h e m e t a l , f o r m e t a l s o f [1] J. LECOMTE, Compt Rend. 208, 1401 (1939). [2] J . LECOMT]~, Cahiers Phys. 17, 1 {1943). [3] R. FONTEYNE, Natuurw. Tijdschr. (Ghent) 25, 173 (1943). [4] R. FONTEYNE, Natuurw. Tijdsch. (Ghent) 81, 441 (1943). [5] J. W. EDSALL, J. Chem. Phys. 4, 1 (1936). [6] S. V~.NXAT]~SWARAN,Proc. Indian. Acad. Sci. 2, 615 (1935). [7] J. GUPTA, Indian J. Phys. 10, 117 (1936). [8] J. GUPTA, Indian J. Phys. 10, 313 (1936). [9] C. DUVAL, J. LECOMTE and F. DOUVILLE,Ann. Phys. 17, 5 (1942). [10] K. ITO and H. J. BERNSTEIN. Can. J. Chem. 34, 170 (1956). [11] R. NEWMAn, J. Chem. Phys. 20, 1663 (1952). [12] K. B. HARVEY, B. A. MORROW and H. SHURVELL, Can. J. Chem. 41, 1181 (1963). [13] R. E. KAOARISE, J. Phys. Chem. 5 9 , 271 (1955). 847

848

J.D.

DONALDSON, J. F. KNIFTON and

S. D . R o s s

groups I A and II A. THEIMER and THE~ER [14] have concluded that stretching frequencies should depend on the ionic radius of the cation, but do not suggest a quantitative relation; while ELLIS and PYSZORA [15] after a study of v2 in stearates have found no obvious relation between electro-negativity, ionic radius and frequency, but observe that if the atomic weight of the metal is plotted against the wavelength of the v2 mode, a smooth curve results. No attempt is made to assess the significance of this. RESULTS AND DISCUSSIOI~ With a few exceptions, all the formates studied show all six bands. In many cases ~5, ~2 and ~3 are split, and occasionally ~4 and v6 appear as doublets. A number of overtones and combinations are also found. Because of the low symmetry (C2~) of the formate ion, no gross differences between the spectra of formates of different structural types are expected. T a b l e 2. F u n d a m e n t a l b a n d s of f o r m a t e s ~1 2857

Na K Cs T11

2833 2833 2801 2801 2786

NH 4 Be

2825 2907

Mg

2907

Ca

2874

Sr

2874

Ba

2865

P b II

2833

Sn II

2874

A11H

2907

Ga HI

2849

I n IH

2874

1383 1368

Sn I v Bi I I I

2841 2841

1368 1333

Sb I I I

2841

1307 1325

Rb

[ 1 4 ] 1%. T H E I M E R a n d [15] B. ELLIS and

|~2

Li

1385 1374 1366 1370 1351 1374 1364 1351 1362 1357 1348 1333 1339 1311 1387 1366 1383

P3

~4

P5

~6

791

1587

1072

775 775 774 775 758

1592 1590 1592 1595 1590

775 800 735 761

1592 1613

1410 1399 1381 1385 1383 1379 1406 1387 1406 1399

1603

1404 1391

1081

803 800 781 786 781 763 719

1581

1408 1395

1075 1064

1572 1553

1404 1399

1087

769 767 759 763 758 781

1585 1577

1403 1389

1075 1066

1577

1075 1066 1064

792 774 784 772 730 800 794 728 719 788 772 769 740

1613 1597

1389 1372 1385 1370 1420 1406 --

1587 1582

1404 1391

1072

1608 1550

1404 1399

-1075

1541

--

1026

1563

Monatsh. 8 1 , 3 1 3 ( 1 9 5 0 ) . Nature 181, 181 ( 1 9 5 8 ) .

O. THEIMEI¢,

H. PYszo~,

1387 1374 1364 1350 1350 1351 1355 1333 1364 1383

1071 1064 1066 1099 -1072 --

1121 1094 1050

The fundamental vibrational spectra of the formates of the main group elements

849

There will be two main types of difference; firstly, of the degree of splitting of the bands due to crystal structure effects, and secondly, shifts in the OCO stretching frequencies ~2 and v4 which, if related to the M - - O bond strength would imply that some form of co-valent bonding exists. The connexion between crystal-field splitting and structure suggested b y HARVEY et al. [12] appears to be confirmed b y our results. I t seems probable that all the alkali metal formates except Li have similar crystal structures, and that, among the formates of divalent metals, Sr, Ba and Pb H are similar while Be, Mg, Ca and Sn H are different. The most important correlation between structure and spectra deducible from these results is the effect of co-valent bonding on %,(0C0) and ~s~n (OCO), which are v4 and v2 respectively. Previous work has suggested three types of structure which involve co-valent bonding: H H H

I

L

c

/\ O

O

{ (A)

c

,/\

,/\

O

O

O

\/

M

I

c

M

(B)

O

I

I

M

M

(C)

Type (A) can be distinguished from types (B) and (C) in that, for type (A) compounds, va~(0CO) increases as the M--O bond strength increases. It is also claimed that the frequency separation between %s and Vsrm increases b u t we find no conclusive evidence of this. In type (C) compounds it appears that both these frequencies should move in the same direction. In the only case so far investigated NAKAMOTO et al. [16] have observed that, in the spectra of hydrated copper (II) and chromium (II) acetates, both frequencies are higher than the corresponding frequencies in sodium acetate. This is certainly the expected direction of shift of %s. I t is difficult to institute direct comparisons between the frequencies in ionic and type (B) compounds, since there is, in such compounds, coupling of the OCO stretching modes with the bending and M - - O stretching modes. In the following discussion we have taken the frequencies of rubidium formate as a standard, since this is an ionic compound whose spectrum is free from crystal-field splitting effects. In this compound, v2 ---- 1350 cm -1 and ~4 = 1592 cm -1. We find three groups of formates in which either or both of these frequencies differ substantially from those for rubidium formate. In the formates of beryllium, magnesium, aluminium, gallium and tin (IV), ~2 and ~4 are both higher than the corresponding frequencies in rubidium formate; in those of lithium and indium, ~2 is markedly higher while ~4 is slightly lower; and in the formates of antimony (III), bismuth (III), tin (II), lead (II) and thallium (I), v~ and ua are both lower. We should not except any systematic effect of the [16] K. NAKAMOTO,Y. MO~IMOTOand A. E. MART~.LL,J. Am. Chem. Soc. 8;], 4528 (1961).

850

J . D . DONALDSON,J. F. KNIFTONand S. D. Ross

m e t a l ion on the spectra e x c e p t among a group of compounds in which the same t y p e of eo-valent bonding exists. W e have observed t h a t , e x c e p t in the group a n t i m o n y (III), b i s m u t h (III), tin (II), lead (II) a n d thallium (I), va moves in the direction e x p e c t e d from consideration of eo-valent bond strength. CONCLUSION T h e most i m p o r t a n t spectral features which relate to the s t r u c t u r e of the formates studied are the crystal-field splitting and the shifts in the stretching frequencies of the c a r b o x y l a t e group. The absence of crystal-field splitting strongly suggests t h a t the formates of sodium, potassium, rubidium, caesium and a m m o n i u m are identical in s t r u c t u r e ; the only one of these to be established b y X - r a y crystallography is t h a t of sodium [17, 18]. I n lithium and thallium (I) r 2 and r 5 a p p e a r as doublets so these m u s t be of a different s t r u c t u r a l type. The frequencies observed for the formates of calcium, s t r o n t i u m and barium show these to be ionic. Their s t r u c t u r e is well established [19-21]. Although the X - r a y s t r u c t u r e of lead f o r m a t e [21] is identical with those of calcium, s t r o n t i u m and b a r i u m it would a p p e a r from the spectral observations t h a t there should be some distortion in this c o m p o u n d c o m p a r e d with the n o r m a l ionic structure. Since the frequencies r 2 and r 4 m o v e in the same direction in the formates of beryllium, magnesium, aluminium, gallium (III) and tin (IV), these compounds c a n n o t be of t y p e (A). The observed spectral d a t a are consistent with either (B) or (C), as there is no spectral criterion for distinguishing between them. As is to be e x p e c t e d from the greater ionic character of indium (III), there is no significant shift in r 4. The formates of a n t i m o n y (III), b i s m u t h (III), tin (II), lead (II) a n d thallium (I) show a t y p e of behaviour not otherwise observed, a n d for which no explanation has been previously offered. B o t h v2 and Vaare lower t h a n the corresponding frequencies in the ionic formates. The shifts are shown in Table 3 below: A n y co-valent bond properties of the members of this series are likely to be based on an o u t e r electronic configuration n s 2. The criterion for co-valent bonding is an increase in v4, b u t in this series Va actually decreases with the e x p e c t e d increase in Table 3. Shifts in r 2 and v4 in formates where both frequencies decrease

--A V4(em-1 ) A V2(cm-1) -

-

Sb(III)

Bi(III)

Sn(II)

Pb(II)

Ti(I)

51 34

42 17

29 25

15 9

2 6

(Note: where the bands are doublets, the mean frequency has been taken. ) [17] [18] [19] [20] [21]

W. H. ZAC~a~IASEN,J. Am. Chem. ~goc. 62, 1011 (1940). W. H. ZACI~'~IASEN, Phys. Rev. 53, 917 (1938). I. NITTA,X-rays, 5, 37 (1948). I. NITTA and Y. SAITO,X-rays, 5, 89 (1949). T. SIrGUWARA,3~. KAUDO, Y. SAI~TIand I. NITTA, X-rays, 6, 85 (1951).

The fundamental vibrational spectra of the formates of the main group elements

851

co-valent character. This decrease can be explained on the basis that any distortion of the regular octahedral field towards a less symmetrical configuration will cause the sterically active lone pair to point across the C--O bond. The effect of this will be to weaken the bond and thus lower the frequency. This should apply to both OCO stretching frequencies and Table 3 shows that the shifts A~a are quantitatively in the order expected due to increase in the lone pair effect. The shifts A~2 also follow this sequence with the exception that the positions of tin (II) and bismuth (III) and interchanged. Work now in progress on the vibrational spectra of acetates of elements displaying the lone pair effect appears to confirm this conclusion. EXPERIMENTAL

The spectra were run as mulls in Nujol and hexachlorobutadiene on a Grubb Parsons GS-2A grating spectrometer. The compounds were prepared b y recrystallisation of commercial reagents from formic acid solution, except for those of Be, A1, Ga, In, Sn(II), Sn(IV), Sb(III) and Bi(III) which were made b y refluxing the oxide with formic acid. Full preparative and analytical details will be found in [22]. [22] J. D. DONALDSONand J. F. K~IFTO~. TO be published.