Spectrochimica Acta, Vol. 24A, pp. 1031 to 1035. Pergamon Press 1968. Printed in Northern Ireland

Spectroscopic studies el organophosphorous compounds--IV In~rared spectra and structure of tetramethyl diphosphine

disulphide-metal halide complexes

M. ARSHAD A. BEe and S. H. KHAWAJA Central Laboratories, Pakistan Council of Scientific and Industrial Research

Karaehi-32, Pakistan

(Received 13 May 1967; revised 2 January 1968)

Abstract--The infra-red spectra of the tetramethyl diphosphine disulphide complexes with zinc chloride, cadmium chloride, mercuric chloride, bromide and iodide, stannic chloride, bismuth trichloride and cupric chloride and bromide have been studied in the 4000-525 cm -1 region. While the similarity of the spectra of the cadmium complex with the diphosphine disulfide, suggests a trans structure with bonding to the metal through sulphur, the complexity in the methyl rocking frequency for the other complexes should be due to a cis structure arising out of chelation. The shift in the P - -CH 3 stretching to higher frequency implies a gain in the P- -C bond order as in phosphonium compounds. The splitting of the P ~ S second band at 568 cm -1 into one at 535 cm -1 and another at 571 cm -1 supports the chelate structure.

INTRODUCTION

IN an earlier communica t i on [1] we have r epor t ed the isolat ion of 1 : 1 me ta l hal ide complexes of t e t r a m e t h y l d iphosphine disulphide (TMDPDS) . The infra-red spec t ra of these compounds faci l i ta te the ass ignment of the P~-~S frequencies since the char- acter is t ic f requencies shif t on co-ordinat ion. I n the recent s tudies of the spec t ra of phosphine sulphides, two bands occurr ing in the 685-862 and 550-730 cm -1 regions have been no ted to be character is t ic of the PF~S bond [2]. Fo r the T M D P D S the P - - C H 3 f r equency also absorbs in this region and hence is l ikely to create confusion in the ass ignments . An a t t e m p t has been m a d e here to correlate the spec t ra and to suggest a possible s t ruc tu re on this basis.

EXPERIMENTAL

A 1:2 complex of c a d m i u m chloride which was no t r epor t ed earlier has been ob t a ined b y mix ing s to ichiometr ic p ropor t ions of a chloroform solut ion of T1KDPDS wi th methanol ic c a d m i u m chloride. Addi t ion of excess chloroform prec ip i t a t ed the complex which was washed wi th a m e t h a n o l - e t h e r mix ture . Analysis, F o u n d : C, 8.7; I-I, 1.98; CI, 26.3; ca lcula ted for C4H12P2S2Cd2C14: C, 8.68; H, 2.1; C1, 25-7; M.P. 265°(d).

The spec t ra were recorded on a P e r k i n - E l m e r model 237 spec t ropho tome te r and also on a Lei tz Gra t ing S pec t ropho t om e t e r b y Mr. W. Theiss of the E. Leitz , Wes t G e r m a n y , in the 4000 to 525 cm -1 range.

[1] M. A. A. BEG and S. H. KHAWAJA, Chem. Ind. 1181 (1966). [2] L. C. THOMAS and R. A. CHITTENDEN, Speetroehim. Acta. 20, 1679 (1964).

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Spectroscopic studies of organophosphorous compounds--IV 1033

RESULTS

Table 1 lists the absorption maxima of the infra-red spectra of the various com- pounds along with assignment of the bands. The range between 1500 and 2800 cm -1 has very weak absorptions possibly due to overtones and has not been included in the Table.

DISCUSSION

Split bands corresponding to the methyl rocking and P- -S stretching absorptions dominate the spectra of all the complexes reported earlier. The cadmium chloride complex seems to be different from these in that its spectrum is similar to the one of TMDPI)S, and this leads us to modify our earlier suggestion that only the copper complexes are chelates [1]. In the former group of compounds the methyl rocking frequencies are split into at least twice the number of bands observed for TMDPDS. The P ~ S stretching is noted to occur at two frequencies viz. at 733 cm -I and 568 cm -1 in TMDPDS. The band I at 733 cm -1 has been confused by COWLEY and ST~I~I~X [3] with the P~-CH a antisymmetric stretching. I t is found that on co- ordination this band moves to lower frequencies as in other phosphine sulphide complexes [4]. The bonding in the present complexes therefore occurs through the sulphur atom. The 747 cm -1 band however, is definitely a P - -CH 3 stretching absorption since it shifts to higher frequencies on co-ordination as observed in our earlier studies [5].

TMDPDS may be either a monodentate or a bidentate ligand i.e. the metal atom may be linked through one or both the sulphur atoms. The sulphur atoms are placed irans to one another in the crystal lattice of the ligand [6]. In case of unidentate function therefore, the symmetry of the ligand wonld not be altered. This, for example would be the case with the 1 : 2 complex of cadmium chloride, and accordingly the complex may be assigned structure II.

In order to have a bidentate function the ligand has to acquire a cis structure. Such a structural change will reduce the symmetry of the molecule. Normal vibra- tions of a molecule depend upon the bond length, bond angle and distance between the non-bonded atoms and hence a change in the chelated metal atom would induce changes in the vibrations. Rocking frequencies are the most susceptible to such changes and hence coupling would be introduced. Similar relationship has been demonstrated for the ammonia rocking frequency [7]. With a change from the sym- metrical trans to a less symmetrical cis structure of the ligand one should expect a complexity in the spectrum [8]. This is reflected in the shifts and splittings in the methyl rocking frequencies. In the TMDPDS and its cadmium chloride complex there are four bands corresponding to this mode: two strong ones at 943 and 883 cm -1, one medium at 862 and one weak at 824 cm -1. In the 1 : 1 complexes the band

[3] A. H. COWLEY and H. STEIN~INK, Inorg. Chem. 4, 1827 (1965). [4] R. A. ZING~O, Inorg. Chem. 2, 194 (1963). [5] M. A. A. BEG and M. S. SIDDIQUI, Can. J. Chem. 3, 608 (1965). [6] J . GOUBEAU, H. REIN~HARDT and D. BIA~CHI, Z. Physik. Chem. 12, 387 (1957). [7] F. SVATOS, D. M. SWEEZ~-Y, S. MIZVSHX~, C. C~RSAN and J. V. QUAGLIA~O, J. Am. Chem.

Soc. 79, 3313 (1957). [8] F. A. COTTO~r, The Infrared Spectra of Transitional Metal Complexes, Moder~r~ Coordination

Chemistry (Edited by J. L~.wm and R. G. WILKINS). Interscience (1960).

1034 M. ARs~tAn A. B~.G a n d S. H. KKAWAJA

at 943 cm -1 usually splits into a doublet at 950 ± 4 cm -1 while the one at 862 cm -1 is little affected by complex formation. Two new bands appear one at 930 ± 5 and the other at 850 cm -1. The 883 and 824 cm -~ bands are sensitive to the co-ordinated metal atoms and shift approximately in the following order:

ZnCla > CuBr 2 > CuC12 > SnC14 > HgCI~ > HgBr2 > HgI 2 > BiCI a > CdC12.

The cis structure is amply supported by the P = S stretching frequencies. The P-----S absorption band at 733 cm -1 shifts to lower frequencies on co-ordination. In CuC12, CuBr 2, BiC1 a and SnC14, this is split into a doublet. The second band at 568 em -1 suffers the maximum shift and in all the cases this band is split into two of nearly equal intensity (CdC12 being an exception), one at 580 + 10 and the other at 540 ± 10 em -1. Only a single band occurs at 545 cm -1 in the CdCl~ complex. The I I band at 540 ± 10 cm -1 is therefore assigned to the P = S co-ordinated bond P----S -~M.

The shift of the bands to lower frequencies and their splitting reminds one of the chelates of the fl-diketones [9]. From the study of the spectra of such chelates the structure of the present series may be formulated as follows:

Cd ++

S S H3C-. ][ /CH3 H3C- ~ / C H 3

" P - - P " " ' P - - r " H3C/~ 'NCH~ H~CJ~ "%CH 3

Cd ++ (I) (11)

M+4- M~+ M*÷

S S S S H~c.. I~ ii .cH~ Hr. . f¢ i . . . c . ~ H~C... I. tl ..CH~ "p ~ p " . " ' p - -p" ~ " p - - p "

H3C f" NvCH 3 - H3C~ "~CH 3 H3C ~ "N~CH 3

(ill) (Iv) (v)

These structures have a positive charge on the phosphorus atoms which is borne out by the fact tha t the reactions of the metallic compounds give te t ramethyl disphos- phine [1] which implies tha t the centre of at tack in the case of reduction with metals is the P----S bond. This is similar to the reduction of the phosphonium compounds by metals. Further , it is seen that the P - - C bond order is intermediate between tha t of the phosphonium and the phosphine sulphide as is apparent from the P - - C antisymmetric stretch which moves from 747 to 760 =[= 3 cm -1. This frequency is found at 780 cm -1 for te t ramethyl phosphonium iodide and at 755 cm -1 for the addition compound of t r imethyl phosphine and silver iodide. The overall structure has contributions from both P = S - - M and P - - S - - M linkages. This should result in

[9] L. J . BELLAYYY and R. F. BI~A~CH, J. Chem. Soc. 4491 (1954). [10] R. A. ZINOARO and 1%. M. ]~DOV, S, J. Phys. Chem. 65, 1132 (1961).

Spectroscopic studies of organophosphorous compounds--IV 1035

a lower bond order for the P ~ S link (absorbing at 568 cm -1) with a consequent shift to lower frequencies. I f the chelate structure is correct, we should expect two P~-~S bands one nearly but not necessarily constant and the other shifted to lower frequency as is observed in the case of fl-diketones [9]. The shift to higher frequency though small is nevertheless anomalous. An actual increase in bond order has pre- viously been noted in the case of phosphine sulphide halogen complexes. In the present metal complexes the symmetry of the p or d orbitals taking part in co-ordina- tion produces a better overlap with the 3d orbitals of sulphur. The back donation process may occur through the dv~ -* d~s and P~s -* d~rp. A higher bond order for the P - - P bond and contribution from I I I is thus indicated. A shift to 588 cm -1 in the complexes other than those of CuCI 2 and SnC14 suggests further gain in the P - - P bond order. There is a possibility of interaction among the ionic moeities in the crystal lattice which would explain the small splitting of the order of 10 cm -1 at P = S band I in both copper chloride and bromide. The spectra of the remaining complexes are almost identical and hence their structure should be similar.

CONCLUSION

The P ~ S absorption occurs at two frequencies viz. 733 and 568 cm -1. The I band is not very sensitive to co-ordination but the I I band is split into two: one to a higher and the other to a lower frequency. Since this pattern is observed in eight compounds, it may be taken as a characteristic feature of the chelates formed by TMDPDS.

Acknowledgements--We should like to express our thanks to Dr. S. H. ZAIDI for kindly arranging with Mr. W. Tn~is who recorded the spectra.