Vibrational spectra of ligands and complexes—IV Infrared spectra of o-phenylenebisdimethylarsine complexes of zinc, cadmium and mercury

Spectrochimica Acta, Vol. 24A, pp. 959 to 964. PergamonPress 1968. Printed in Northern Ireland

Vibrational spectra of ligands and complexes---IV Infrared spectra of o-phenylenebisdimethylarsine complexes of zinc, cadmium and mercury G. B. DEACON Chemistry Department, Monash University, Clayton, Victoria, Australia

J. H . S. GREEN Chemical Standards Division, National Physical Laboratory, Teddington (Received 13 November 1967}

Abstract--The infrared spectra (3500-500cm -1) of the o-phenylenebismethylarsine (D) complexes, MD~(C1Oa)2 (M = Zn, Cd, or Hg) and (3650-200 cm -1) ofMDX~ (M = Zn, Cd, or H g ; X = C1 or Br) and Z n D I 2 have been recorded. Assignments of the observed frequencies, including those for the metal-halogen stretching frequencies, are proposed, and are discussed in relation to the structures of the complexes. INTRODUCTION

IN spite of the considerable importance of o-phenylenebisdimethylarsine as a ligand and the wide range of complexes of it that have been described, few investigations of the vibrational spectra of the complexes have been reported. Studies of parts of the spectra of the ligand and some complexes have shown characteristic shifts of the CH 3 rocking and CH out-of-plane vibrations b y ~ 2 0 cm -1 on chelation [1], and there has been a recent study of the far infrared spectra (450-200 cm -1) of some transition metal complexes of the ligand [2]. A detailed assignment of the vibrational frequencies of the free ligand has now been made [3], hence a similar interpretation is possible for the complexes. In this study, the infrared spectra of the complexes 1~D2(C104)2 {IV[ = Zn, Cd, or Hg; D = o-phenylenebisdimethylarsine), MDX~ (l~ ---Zn, Cd, or Hg; X ~ C1 or Br,), and ZnDI 2 have been recorded, assignments are proposed, and the structures of the complexes are discussed. RESULTS AND DISCUSSION

For purposes of discussion, it is convenient to divide the spectra into three regions (3100-2800 em -1, 1650-500 cm -1, and 500-200 cm-1). Data and assignments for o-phenylenebisdimethylarsine are taken from Ref. [3].

1. The region 3100-2800 cm -1 Five bands are generally observed in this region, typically at 3060, 3040, 2970, 2905, and 2852 em -1. The first two are the ring CH stretching vibrations. Similarly, only two such bands are observed for the free ligand [3]. The pair at 2970 and 2905 [1] R. J. H. CLARK, J. LEWIS and R. S. NYHOLM~,J. Chem. Soc. 2460 (1962). [2] J. LEwis, R. S. NYHOL~f and G. A. RODLEY, J. Chem. Soc. 1483 (1965). [3] J. H. S. GREEN, W. KYI~ASTON and G. A. RODLEY, Spectroehim. Acta 24A~ 853 (1968). 959

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cm -1, of which the latter is invariably the stronger and sharper, are attributed to the CH a antisymmetrie and symmetric vibrations, respectively. The much weaker band at 2852 cm -1 almost certainly is the overtone of the CH 8 antisymmetric deformation mode.

2. The region 1650-500 cm -1 The observed frequencies of representative compounds in the region 1650-500 cm -1 are listed in Table 1; the values for the other compounds are very similar. Table

ZnDBr 2

D 1610 1563 1441 1421 1261 1250 1235 1160 1119

1. I n f r a r e d

w w m s m m sh w m

1615 1546 1448 1418 1266 1253 1244 1164 1124

vw vw m s, b r m m w w w

frequencies (1650-500 cm -1) and assignments MDX 2 and MD2(C104) 2 complexes CdDCI s 1615 1550 1448 1414 1265 1258 1244 1163 1126

vw vw m s, b r m w w w w

HgDC12 1612 1550 1450 1416 1277 1256 1245 1163 --

vw vw m m, br m m w w

885 846 826 744 719 692 645

s s v~ s w w w

1100 1035 1029 912 874 858 818 754 703

m m m vs vs s w s w

576 s

606 m

568 s

588 m

1094 1033 1029 905 861 848 806 753 697

605 597 588 582

m m m s s m w s w

m m m m

1106 1038 1029 917 881 861 822 741 --

m m

Assignments

H g D u (ClO~)2 1615 1570 1450 1418 1276 1263 1244 1163 1131

1093 1090 m 1033 m 1024 w

for some

vw vw m s, b r m'~ m) w w m vs, br

-1038 m

w

--

s s m w s

926 889 878 833 767 --

s s ~ In) w s

va(al) v~a(ba) va(al) C H a asym. def. -]- ~24(ba) Cl~[a s y m . def. + Ys(al) ~2a(b2)

ve(al) V~6(ba)

~s(Cl04-) r ~ ( a 1) ~'8(al) P2~(b2) CH a

rock

Combination ~,ls(b 1)

V14(as)

621 s}

Y2s(b~) v9 ( a l ) ~4(C10 _ )

595

r(CHa--As )

608 m 587 m

Intensities: s strong, m m e d i u m , w weak, v very, br broad, sh shoulder.

The assignments are given in the notation used previously for free o-phenylenebisdimethylarsine [3], the frequencies of which are included to facilitate the discussion. Absorption near 1600 cm -1 is very weak. With difficulty, a band at --~1610 cm -1 could be detected which may be one of the two expected frequencies ~3(al) and ~3(b2). Alternatively, these may be coincident at 1572-1550 cm-1; the spectrum of CdDBr~ shows a resolved doublet at 1570, 1558 cm -1 which may be attributed to these fundamentals. B y contrast, the strong band at ~-~1450 cm -1 is obviously the a 1 fundamental ~4- For the free ligand, much of the absorption at 1421 cm -~ has been shown to be due to the CH 3 antisymmetric deformation modes, which overlap a b2 ring fundamental. This band is considerably broadened for the complexes, and for EnDC12 two maxima (1424 and 1418 em -1) are resolvable. Likewise the CH a symmetric deformation modes overlap an a 1 fundamental. They are generally at somewhat higher frequencies in the MD2(C10~) 2 complexes than in the MDX 2 complexes, e.g. for ZnD2(C104) 2 and ZnD2C12 the values are 1280, 1266 and 1269, 1259 em -1, respectively. The five bands in the region 1200-1000 cm -~ are readily assigned

Vibrational spectra of ligands and complexes--IV

961

and require no comment. The strong band of the lVID~(C104)~ complexes at ~ 1 0 9 0 cm -1 is obviously ~3 of the perchlorate group. There is no evidence for splitting of this absorption or for activation of ~1(~ sym. CI--O) in any of the complexes, thus confirming the proposed [4] ionic constitution, (1~ID~)2+(C104-)2. The prominent bands in the range 930-850 cm -1 arise from the CH 3 rocking modes, which are displaced on chelation from their positions (885 and 846 cm -1) in the free ligand, as previously reported [1]. Three well-resolved features are observed for each complex, and the highest frequencies are attained in the MI)2(C104) 2 complexes. The weak band observed at --~825 cm -1 for all complexes seems too low to be a methyl rocking frequency, and probably corresponds to the very weak absorption of the free ligand at 826 cm -1, which is not shifted when the CH 3 rocking modes are displaced to longer wavelengths on deuteration [3]. For the ligand, the absorption has been explained as a combination involving a low-lying fundamental (125 em-1). The relevance of this explanation for the complexes cannot be verified from the present data. There can be no doubt that the strong band of o-phenylenebisdimethylarsine at 744 cm -1 is the bl, ?(CH) "umbrella" mode. As found for other complexes [1], this is appreciably shifted to higher wavenumbers on chelation; the increase is greatest for the MD~(C10~) 2 complexes. Three fundamentals in the region 730-650 cm -1 afford only weak bands for the free ligand. For the complexes, only one, or in some instances none, could be detected. The promlnent bands of the ligand at 576, 568 cm -1, assigned to predominantly CH3--As stretching vibrations, are appreciably raised in frequency on coordination. For the MD2(C104) 2 complexes, the higher component is obscured byabsorption due to ~4 of the anion. A similar shift of methyl-arsenic stretching frequencies is seen on conversion of dimethylphenylarsine into phenyltrimethylarsonium iodide [5]. 3. T h e region 500-200 cm -1 (ligand vibrations)

The spectra of the complexes MDX~ in this region have been obtained, and the frequencies are assembled in Table 2 together with those of the free ligand. The band ~ 4 3 5 cm -1, assigned to a b 1 fundamental which is essentially an out-of-plane ring deformation, shows no shift on coordination. Of the lower frequencies, those which are essentially independent of the metal halide are assigned to the coordinated ligand. I t is noteworthy that only a single strong band is observed at 358-350 cm -1, which clearly corresponds to that of o-phenylenebisdimethylarsine at 344 cm -1. The latter is assigned to two coincident fundamentals (~10(al)-}-~29(b2)). In other complexes [2] and in o-phenylenebisdimethylarsine dioxide [3], these modes are resolved as a pair of strong bands at 377-367 and 336-329 cm -1. Comparison with the forms of vibration for the corresponding fundamentals Cat 480 and 429 cm -1, respectively) of o-dichlorobenzene [6] indicates that these fundamentals (whose frequencies are clearly markedly sensitive to the mass and nature of the ring substitutent) involve out-of-plane and in-plane stretching of the ring carbon-arsenic bonds. The ligand absorption at 231 cm -1, which is probably due to deformation of [4] J. LEwis, R. S. NYHOLI~and D. J. PHILLIPS,J. Chem. Soc. 2177 (1962). [5] W. R. CUI~EN, G. B. DEACONand J. H. S. GREEN, Can. ,1. Chem. 44, 717 (1966). [6] J. R. SOH~RER,Planar Vibrations of Chlorinated Benzenes, Dow, Midland, Michigan (1963).

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Vibrational spectra of ligands and complexes~IV

963

the CI-Ia--As--CH 3 angles, becomes rather variable in intensity in the complexes, and is not readily resolvable for the mercury derivatives. 4. The region 500-200 cm -1 (metal-halogen stretching vibrations)

Absorption frequencies (Table 2) which move to lower wavenumbers with increase in mass of the halogen are assigned to metal-halogen stretching modes. For CdDBr~, the ~(Cd--Br) vibrations are evidently below 200 cm -1. The frequency ratios, ~(~--Br)/~(~I--C1) --~ 0.83 (M ---- Zn) and 0.80 (M ---- Hg) and ~(Zn---I)/ ~(Zn--CI) ~-- 0.69 (using average ~(]~[--X) frequencies where necessary), are within the expected range [7]. I n particular the ratio v(Zn--Br)/V(Zn--C1) is close to the value (0.79) for (phen or bipy) ZnX~ complexes (phen-----1,10-phenanthroline; bipy = 2,2'-bipyridyl; X ---- C1 or Br) [8]. 5. Structures of M D X 2 complexes

The ~(M--X) stretching modes for the zinc and the mercury complexes are at frequencies similar to those for tetrahedral or distorted tetrahedral complexes of these metals [7-12]. Values for ZnDX~ and HgDCl~ are very close to those for the corresponding (phen or bipy) MX~ complexes [8]. Although the two v(R---X) frequencies expected for an As~MX~ skeleton of C2~ symmetry are not always resolved, this is not unusual for tetrahedral L~MX2 (L is a neutral unidentate or L~ a neutral bidentate ligand) or MX~Y2~- (X =fi Y ~ C1, Br or I) complexes [8-10]. Thus tetrahedral or distorted tetrahedral stereochemistry is indicated for the ZnDX2 and HgDX~ complexes. For the latter, any distortion is likely to involve opening of the C1HgC1 bond angle towards the stable linear C1--Hg--C1 configuration [13]. By contrast, v(Cd--C1) of CdDCl~ is found at significantly lower frequencies than observed for tetrahedral cadmium complexes (~270-245 cm -1 [8, 10, 11]), but is near values (228-213 cm -1) for L2CdCI 2 complexes (L 2 = phen, bipy, (M%NCH~)2, or (EtSCH~)2, for which polymeric octahedral structures have been proposed [8]. Thus CdDC12 is probably an oetahedral polymer in the solid state; a possible structure follows. Similar stereochemistry is likely in CdDBr2, but any definite conclusion must await location of the cadmium-bromine stretching frequencies.

[7] R. J. H. CT.AR~,Spectrochim. Acta 21, 955 (1965). [8] G. E. COATESand D. RXDLEY,J. Chem. Soc. 166 (1964). [9] R. J. H. C ~ ] ~ and C. S. W'ILLIAMS,Inorg. Chem. 4, 350 (1965); C. W. FaANx and L. B. ROOERS,Inorg. Chem. 5, 615 (1966); G. B. DEACON, J. H. S. GREEN and F. B. TxYT~OR,Australian J. Chem. 20, 2069 (1967). [10] G. B. DEACONand J. H. S. GREEN,Chem. Commun. 629 (1966). [11] M. L. D~-LWA~LE, Bull. Soc. Chim. France 1294 (1955) D. M. ADAMS,J. CHA~T, ;[. M. DAVIDSO~rand J. GE~m~r, J. Chem. Soc. 2189 (1963). [12l G. B. DEACO~r,J. H. S. GRE~, and W. :KY~S~O~r, Australian J. Chem. 19, 1603 (1966). [13] A. T. McPm~L and G. A. S~M,Chem. Commun. 21 (1966); D. GRDENIC,Quart. Rev. 19, 303 (1965).

964

G . B . DEACOlV and J. H. S. GR~.v.~ EXPERI~CIEI~TAL

Compounds Dichloro-o-phenylenebisdimethylarsinezine(II) was prepared by a method similar to that of SUTTO~ [14], and was obtained analytically pure. The other complexes were provided by Professor Sir Ronald Nyholm, and the preparations have been reported [4].

Spectra The infrared spectra of the MD~(C104)~ complexes were obtained using a GrubbParsons GS2A (3500-500 em -1) spectrophotometer. Those of 1VIDX~ compounds were recorded with Unicam SP 100/130 (3650-375 cm -1) and Grubb-Parsons DM2 (420-200 cm -1) instruments. The listed frequencies were obtained for samples dispersed in Nujol or hexaehlorobutadiene; in some instances rather different results were obtained using the KC1 disc technique, which m a y therefore be unreliable for these compounds. Acknowledgements--We are grateful to Professor Sir RONALD NYHOLM, F.R.S., for providing samples of the complexes. One of us (G. B. D.) thanks Imperial Chemical Industries Limited for financial support. The work of G. B. D. and J. H. S. G. was carried out at University College London and the National Chemical Laboratory, respectively. [14] G. J. SUTTO~, Australian J. Chem. 14, 545 (1961).