An ab initio comparison of dichalcogen hydrides

Journal of Molecular Structure (Theochem), 124 (1985) 293-305 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

AN AB INITIO COMPARISON OF DICHALCOGEN

HYDRIDES

RISTO LAITINENT Institut ftir Anorganische Berlin 12 (B. R. D.)

un Analytische

Chemie,

Technische

UniversitiJt Berlin, D-1000

TAPANI PAKKANEN Department

of Chemistry,

University

of Joensuu,

SF-80100

Joensuu

10 (Finland)

(Received 25 February 1985)

ABSTRACT A theoretical study of HSH, HSeH, HTeH, HSSH, HSeSH, HSeSeH, HTeSH, HTeSeH, and HTeTeH was carried out by ab initio molecular orbital methods employing minimal Gaussian basis sets MINI-l and MINI-l* of Huzinaga and his group. Both basis sets yield accurate estimates on the equilibrium geometries of monochalcogen hydrides. In the case of dichalcogen hydrides, however, the inclusion of d-polarization functions for sulfur, selenium, and tellurium greatly improve the accuracy of the geometry prediction. The unpolarized MINI-l basis sets yield essentially correct orbital energies and therefore suffice for the comparative study on the electronic structures in similar molecules. The results with both basis sets imply close similarities in the electronic structures of SS, SeS, and SeSe bonds with more marked differences in bonds containing tellurium as a consequence of notably smaller orbital energy of the 5s- orbital of tellurium as compared to the corresponding orbitals in sulfur and selenium. The barriers to internal rotation about the chalcogen-chalcogen bond in all dichalcogen hydrides are similar. The cis- and trans-barrier heights are ca. 23 and 14 kJ mol-I, respectively. The relative stabilities of different hydrides are discussed. INTRODUCTION

The electronic structures and properties of SS, SeS, and SeSe bonds have recently been studied by ab initio molecular orbital methods employing minimal Gaussian basis sets and using chalcogen hydrides HSe,S, _,H (A = 2 or 3; II = 0 -A) as model compounds [l] . This comparison showed that the three bonds in question are very similar and that the energy change in various interconversion reactions between them is small thereby rationalizing the existence of a complex binary system of sulfur and selenium [2]. Minimal basis level treatment was found suitable for the qualitative comparison of similar molecules, and molecular parameters of various chalcogen hydrides were reasonably well reproduced as compared to the other *Permanent address: Department SF-02150 Espoo 15, Finland. 0166-1280/85/$03.30

of Chemistry,

Helsinki University

0 1985 Elsevier Science Publishers B.V.

of Technology,

294

theoretical and experimental data as available [ 11. For example, in HSSH the ordering of the molecular orbitals and the orbital energies were in agreement with those obtained from more sophisticated calculations. However, the chalcogen-chalcogen bonds were estimated to be 5-10% too long. Similarly, the dihedral angles assumed too large values. This seems to be a common deficiency in most calculations employing unpolarized basis sets for the third row and heavier elements as exemplified by Hinchliffe [3] who showed that the addition of 3d-polarization function to a sulfur basis set of double zeta quality contracted the SS bond in HSSH from 224.5 to 208.1 pm which only then is in agreement with the experimental value of 205.5 pm [4]. Interestingly, however, STO-3G calculations on HSSH reproduce the experimental geometry very accurately 15, 61. With the addition of the d-polarization function for sulfur the SS bond length is, in fact, underestimated. We utilized the detailed study of Huzinaga and his group [7-161 on the effects of the split basis sets and polarization functions and found that the addition of 3&-polarization function for sulfur in HSSH shortened the SS bond effectively even at minimal basis level [17]. Furthermore, the estimate for the dihedral angle reduced to a value closely in agreement with the experimental value [ 41. The existing ab initio literature on polyselanes and -telluranes is very sparse [l, 181 obviously owing to computational limitations. However, there is some work performed on dimethyl diselenide, which is the simplest experimentally characterized compound containing the SeSe bond [ 19 1. In the present work we have extended our earlier investigation [l] to the full range of dichalcogen hydrides and also expanded the minimal Gaussian basis sets of the chalcogen atoms to include polarization functions. The monochalcogen hydrides HSH, HSeH, and HTeH were included for comparison. CALCULATIONS

The calculations were carried out by standard ab initio molecular orbital methods involving Gaussian lobe functions [20, 211. The basis sets for the chalcogen atoms were those of Huzinaga and his group [ 12,15,16] f and the hydrogen basis set was a standard three Gaussian expansion [ZO]. The combination of these unpolarized basis sets in various molecules are denoted MINI-l [8-161. When d-polarization functions are included for sulfur, selenium, and tellurium the basis sets are denoted MINI-l*. The orbital exponent of 0.46 for the 3d-polarization function of sulfur is according to Sakai et al. [ 131. The orbital exponents for the corresponding polarization functions of selenium and tellurium were calculated by maximizing the $The basis set of Tavouktsoglou and Huzinaga for sulfur [7] used in our earlier calculations [I ] has been replaced by the newer MINI-l expansion [12] which has been found to give better atomic and orbital energies.

295

overlap integral between the polarization function and the valence orbital as described by Tatewaki and Huzinaga [lo]. The values of these exponents are 0.37 and 0.29 for selenium (4d) and tellurium (54, respectively. The scaling factor for the 1s orbital of hydrogen was optimized in HSH and yielded the value of 1.295 as described previously [l]. The optimum scaling factors of the selected valence orbitals [7] of S, Se, and Te converged to a value near unity and thus the scaling was omitted as also suggested by Sakai et al. [13]. Full geometry optimization was performed on HSH, HSeH, and HTeH. The equilibrium geometries of dichalcogen hydrides were obtained by simultaneous optimization of the chalcogen-chalcogen bond length and the dihedral angle. The hydrogen--chalcogen bond lengths and the bond angles were held constant at their experimental values as observed in HSSH, HSeH, and HTeH (see footnote a in Table 1). In order to estimate the rotational barriers in the different dichalcogen hydrides the chalcogen-chalcogen bond length was optimized both for the cis- and truns-conformations of each molecule. The calculations were carried out by the CRAY-1M computer at the Technical University of Berlin and by the CRAY-1S computer at the University of London. RESULTS

AND DISCUSSION

Molecular geometry The energy optimized equilibrium geometries of the hydrides studied in this work are shown in Table 1 together with their total energies. Experimental information is included where applicable. However, there are known molecular structures only for HSH, HSeH, HTeH, and HSSH. For other hydrides the experimental chalcogen-chalcogen bond lengths and the dihedral angles have to be approximated with a suitable model compound exhibiting similar type of bonding. It is seen from Table 1 that both basis sets predict the equilibrium ‘geometries of the monochalcogen hydrides HXH well with MINI-l* geometries in slightly better agreement with experimental values. In dichalcogen hydrides the MINI-l results yield, predictably, too large estimates for both the chalcogen-chalcogen bond lengths and the dihedral angles. The discrepancy between the calculated and observed values is smallest in HTeTeH and gets larger towards the lighter hydrides. A particularly serious case is HOOH which shows an equilibrium angle of 180” in most unpolarized minimal basis level calculations (for a review of ab initio results, see ref. 1). With MINI-l* basis sets the deficiencies in geometry predictions are significantly reduced. All chdcogen-chalcogen bond lengths are estimated to be within 5% of their experimental values as judged by the limited experimental evidence available. The residual overestimation in the bond lengths

296 TABLE 1 Equilibrium geometries and total energies of mono- and dichalcogen hydridesa Molecule

Parameter

HSH

r (HS) (pm) Q (degrees) E (a.u.)

HSeH

MINI-l

MINI-l*

Expt. 133.56c 92.11

138.0 96.2 -396.67587

136.9 93.4 -396.68933

r (HSe) (pm) OL(degrees) E (a.u.)

147.2b 94.5 -2391.21439

146.6 92.7 -2391.42416

146.0d 90.57

HTeH

r (HTe) (pm) (Y(degrees) E (a.u.)

166.5 94.1 -6591.30751

165.8 92.5 -6591.40208

165.8e 90.25

HSSH

r (SS) (pm) 7 (degrees) E (au.)

229.4 109.5 -792.20592

215.7f 92.9 -792.29552

HSeSH

r (SeS) (pm) 7 (degrees) E (au.)

240.5 105.3 -2786.75653

226.9 92.1 -2787.00571

221.4h 94.8

HSeSeH

r (SeSe) (pm) 7 (degrees) E (a.u.)

252.2b 102.0 -4781.30498

238.2 92.7 -4781.71683

232.5’ 84.7

HTeSH

r (TeS) (pm) 7 (degrees) E (a.u.)

259.0 103.2 -6986.86297

246.3 94.5 -6986.99407

239.11 89-99

HTeSeH

r (TeSe) (pm) 7 (degrees) E (a.u.)

272.3 99.0 -8981.41083

258.0 92.5 -8981.70652

HTeTeH

r (TeTe) 7 (degrees) E (a.u.)

293.3 100.4 -13181.51718

279.4 92.8 -13181.69570

205.5’ 90.6

269.7k 85.7

aThe notation: r bond length, OLbond angle, 7 dihedral angle. In the energy optimization of dichalcogen hydrides the HX (X = S, Se, Te) bond lengths and the uX bond angles were held fixed at their experimental values as follows: r(HS) = 132.7 pm and QS = 91.3” as in HSSH [4], r(HSe) = 146.0 pm and “se = 90.6” as in HSeH [22], and r(HTe) = 165.8 and ~~~ = 90.3” as in HTeH [23]. bLaitinen and Pakkanen [l]. ‘Cook et al. [ 241. dHill and Edwards [22]. eMoncur et al. [ 231. ‘Laitinen and Pakkanen [ 17 1. gWinnewisser et al. [4]. FHauge: NCSSeSCN [ 251. ‘D’Antonio et al.: CH,SeSeCH, [ 261. IGjerrestad and Ma$y: [(O,S)STeS(SO,)]Z[ 271. kSpirlet et al.: CH,(C,H,)TeTe(C,H,)CH, [28].

297

again diminishes towards the heavier hydrides. The dihedral angles calculated with MINI-l* basis sets turn out to be almost constant throughout the dichalcogen hydride series. They agree well with the experimental value observed for HSSH [ 41 and also with the original proposal of Pauling that in such molecules the dihedral angle should be near 90” as a consequence of the minimum repulsion between the p-lone-pairs of the adjacent chalcogen atoms [ 291.

Binding energies While the chemistry of polysulfanes is very extensive [30] there is only passing evidence on the existence of polyselanes and -telluranes [31, 321. It has been suggested that this is because the polyselanes and -telluranes have a strong tendency to decompose into the free element and the simplest dihydrogen compound [ 331. The total binding energies of all hydrides studied in this work are given in Table 2. Though both basis sets indicate that the hydrides are stable with respect to the atoms they are composed of, MINI-l* calculations, as expected, yield better binding energies. In fact, MINI-1 basis sets fail to show an increase in the total binding energy from monochalcogen hydrides to dichalcogen hydrides. Despite this, the trends provided by both basis sets within the two series (HXH and HXYH; X,Y = S, Se, Te) are very similar and the same qualitative conclusions can be drawn from both sets of data. The MINI-l* results indicate a small net increase in the total binding energy from monochalcogen hydrides to corresponding dichalcogen hydrides.

TABLE

2

Total binding energies of monochalcogen Molecule

MINI-l

MINI-l*

HSH HSeH HTeH

0.147 0.127 0.075

0.157 0.167 0.138

HSSH HSeSH HSeSeH HTeSH HTeSeH HTeTeH

0.143 0.135 0.125 0.096 0.085 0.047

0.225 0.210 0.196 0.192 0.179 0.162

aAtomic energies: MINI-l: [15], Te -6590.238309 -6590.27011.

and dichakogen

hydrides (in au.)”

H -0.496979 [20], S -395.52461 [16]; MINI-l*: S -395.53840,

[12], Se -2390.09320 Se -2390.23626, Te

298

In case of HSSH, HSeSeH, and HTeTeH this increase can be attributed to the following three reactions HSSH + HSH + S HSeSeH + HSeH + Se HTeTeH + HTeH + Te

AE = 178 kJ mol-’ AE = 77 kJ mol-’ AE = 62 kJ mol-’

It seems that HSSH can be considered somewhat more stable than other dichalcogen hydrides which is of course in agreement with the experimental observations. None of the dichalcogen hydrides are, however, likely to be very stable as the “decomposition products” are further stabilized by the formation of the chalcogen molecules from the free atoms (Ss rings from sulfur and polymeric Se, or Te, chains from selenium and tellurium). The experimental information on the decomposition of HSSH is well in accord with these conclusions [ 341 HSSH(I) + HSH(Z) + l/8 SB (s,ort) HSSH(I) + HSH(g) + l/8 Ss (s,ort)

AH = 18.5 kJ mol-’ AH = 2.5 kJ mol-’

The decreasing stability towards the heavier hydrides within the two series can also be judged from the total binding energies given in Table 2. Rotational barriers There is only very little experimental information on the rotational barriers of HSSH and none at all for the heavier dichalcogen hydrides. Winnewisser et al. [4] assumed that the rotational barriers in HSSH are higher than in HOOH with cis- and tram- barriers roughly equal. Fraser et al. [35] have concluded, on the basis of the NMR studies, that in the absence of steric effects the barriers to internal rotation about the SS bond is ca. 28 kJ mol-’ with trans-barrier probably higher than &-barrier. Steudel [36] has estimated from the thermochemical vapour phase data for homocyclic heptasulfur molecule that in cyclic S, molecules (n > 7) the barrier height is ca. 24 kJ mol-l with trans-barrier lower than &-barrier. There is a multitude of ab initio calculations for the rotational barriers of HSSH [ 1, 5, 361 but only two reports on heavier dichalcogen hydrides [1, 181. In Table 3 the barriers to internal rotation in the full range of dichalcogen hydrides are compared as calculated with both the MINI-1 and MINI-l* basis sets. The values calculated in this work agree well with other calculations of similar sophistication as available [l, 5, 18, 361. The prominent feature in the data of Table 3 is the similarity of the barriers in different hydrides. Particularly the MINI-l* calculations yield practically equal barrier heights in all hydrides. Ewig et al. [ 181 have found that the XX overlap populations (X = S, Se, Te) are nearly equal and behave similarly as a function of the dihedral angle thereby reflecting the similarity of the barrier heights, if not quantitatively justifying it. The cis-barrier seems to be slightly affected by

299

TABLE 3 Rotational barriers of dichalcogen hydrides (kJ mol-‘) MINI-l

MINI-l*

22.5 4.1

26.4’ 14.3

trans

20.5b 5.3

24.5 14.1

HSeSeH

cis trans

19.2b 6.8

23.1 13.9

HTeSH

cis tram

18.6 6.2

22.1 14.3

HTeSeH

cis trans

17.5 6.8

21.0 14.1

16.4 6.9

19.7 14.1

Molecule

Barrier

HSSH

CiS

trans

HSeSH

HTeTeH

CiS

CL?

trans aReference 17. bReference 1.

the identity of the chalcogen atoms and gets somewhat lower towards the heavier hydrides. This can be attributed to the decreasing interaction of the I-IX bonds [ 181. It is also interesting to compare those barriers with those in analogous molecules. Renugopalakrishnan and Walter [19] have reported that the barriers of CH$SSCHJ and CH3SeSeCH3 are very near to each other in height which is in agreement with the present findings. For comparison it is also worthwhile to note that the rotational barriers about the central SS bond in tetrasulfane HSSSSH are of roughly equal height at ca. 30 kJ mol-’ as deduced from MINI-l* calculations [ 171. Another feature connected with the barrier heights is the lengthening of the chalcogen--chalcogen bonds from the single bond lengths when the dihedral angles are near to 0 or 180”. This is readily observed in the homocyclic heptasulfur molecule which has one unique dihedral angle of 0” [ 371. The central bond defining this dihedral angle is abnormally long (218 pm as contrasted to an unstrained single bond length of ca. 205 pm in 5s [38] or 205.5 pm in HSSH [4] ). Conversely the bonds adjacent to this bond are somewhat shorter than normal (200 pm) [ 371. The MINI-l* calculations on HSSSSH [17] showed analogous variations in different SS bond lengths as the central dihedral angle was varied. A plausible explanation involves, in addition to the lone-pair repulsion between the two adjacent central atoms defining the dihedral angle, the hyperconjugation of these same lone-pairs with the antibonding orbitals of the bonds connecting the planar S4 fragment to the rest of the molecule [ 391.

We compared the behaviour of the chalcogen-chalcogen bond lengths in different hydrides as a function of the dihedral angle. These results are summarized in Table 4 for both basis sets. All bonds seem to be extended in a similar fashion from their equilibrium values as the dihedral angle is either 0 or 180”. This further emphasizes the similar barriers of rotation about the different chalcogen--chalcogen bonds. It probably also indicates that the conformations in analogous chalcogen molecules should be similar. For example, the geometries of S8 and Se, molecules are known to be analogous. Also Se,, when prepared, will probably carry the same conformation and bond length relationships within the molecule as has been found in S, [ 371. This conclusion has been applied in the spectroscopic identification [40] of a seven-membered selenium sulfide ring molecule which has been produced in the reaction between (C5H5)2TiS5 and Se&l, [41] as an isomer of 1,2-Se& having the SeSe bond adjacent to the bond bearing the unique dihedral angle of ca. 0”. Electronic structures The orbital energies obtained from MINI-l* calculations highest occupied orbitals (viz. valence orbitals) and for the orbital in different dichalcogen hydrides are shown in Fig. 1. tributions to the characters of these orbitals are given in Table

TABLE

for the seven lowest virtual The main con5.

4

The chalcogen-halcogen Molecule

bond length as a function of the dihedral angle (pm)

Basis set

Dihedral angle (degrees) 0

Topt

180

HSSI?

MINI-l MINI-l*

231.5 219.2

229.4 215.7

229.8 217.3

HSeSHb

MINI-1 MINI-l*

243.8 230.4

240.5 226.9

241.2 228.8

HSeSeHb

MINI-l MINI-l*

255.0 242.0

262.2 238.2

253.3 240.4

HTeSH

MINI-l MINI-l*

261.8 249.7

259.0 246.3

260.5 248.4

HTeSeH

MINI-l MINI-l*

275.0 261.8

272.3 258.0

273.8 260.6

HTeTeH

MINI-l MINI-l*

296.1 283.4

293.3 279.4

294.9 282.2

aMINI-l* results: ref. 17. b MINI-l results: ref. 1.

301

IF-

m-

250Z&_

17b_ 180

350 3La_

rla-

33a-

!307b70-

T+,_ 220-

16b,&3-

32Q3,a-

.I390-

*Lb-

ZLoSb60-

HSSH

HSSeH

HSeSeH

HSTeH

L

HTeTeH

Fig. 1. The MINI-l* orbital energies of the dichalcogen hydrides.

It is interesting to note that while MINI-l* basis sets provide superior estimates for the equilibrium geometries and binding energies in different chalcogen hydrides, the unpolarized MINI-l basis sets already yield sufficiently accurate description of the orbital energies. This is exemplified in Table 6 where the MINI-l and MINI-l* orbital energies of the valence orbitals of HTeTeH are compared. The agreement between the orbital energies of the occupied orbitals as obtained from MINI-l and MINI-l* calculations is equally good in all other molecules studied in this work. It can, therefore, be concluded that when the comparison of the electronic structures in analogous main group compounds is the objective, the less time-consuming MINI-l basis sets provide an attractive alternative. The valence orbitals shown in Fig. 1 and Table 5 can be divided into two groups. Lower in energy lies a group of two orbitals which are essentially composed of the non-bonding outer s-orbitals of the chalcogen atoms. The electron pairs in these orbitals are stereochemically inactive. The second group of orbitals constitutes five orbitals in the region -0.6 to -0.3 au. The lowest two are mainly hydrogen-chalcogen bonds, the middle one is the chalcogen-chalcogen bond and the two highest orbitals are mainly the lone-pair orbitals of the two chalcogen atoms. As these two orbitals are almost purely composed of the outermost p orbitals of each chalcogen atom, it is these orbitals which are responsible for the non-planar structure of all dichalcogen hydrides as suggested originally by Pauling [ 291. The overall similarity of the electronic structures of the bonding in HSSH, HSeSH, and HSeSeH can again be noted. The hydrides containing tellurium are somewhat different as the non-bonding 5sorbitals of tellurium has a notably smaller orbital energy than the corresponding orbitals of sulfur and selenium. The similar trend is exemplified by a photoelectron spectroscopic study of HSH, HSeH, and HTeH [ 421. The first two ionization

TABLE 5 The main contribution to the characters of the seven highest occupied orbitals and the lowest virtual orbitala HSSH 6a 3s(l) + 6b 3s(l) 7a u(SH) 7b u(SH) 8a o(SS) 9a Ip(1) + 3b fp(1) 9b o*(SS)

3s(2) 3s(2)

lp(2) lp(2)

HSeSH

HSeSeH

HTeSH

HTeSeH

HTeTeH

20a 3s(S) 21a 4s(Se) 22~1u(SH) 23a o(SeH) 24a u(SeS) 25a Ip(S) 26a @(Se)

15a 15b 16a 16b 17a 16a 17b

29a 30a 310 32a 33a 34a 35a

38a 4s(Se) 39a Ss(Te)

240 24b 25a 25b 26a 27a 26b

27a u*(SeS)

18b u*(SeSe)

4s(l) + 4s(2) 46(l) - 4s(2) u(SeH) u(SeH) u(SeSe) lp(l) + lp(2) Ip(1) -Ip(2)

36(S) Bs(Te) u(SH) u(TeH) u(TeS) MS) Ip(Te)

36 a o*(TeS)

40a 41a 42a 43a 44a

u(SeH) u(TeH) u(TeSe) Zp(Se) Ip(Te)

45a u*(TeSe)

5s(l) + 5s(2) 5s(l) - 5s(2) u(TeH) u(TeH) u(TeTe) Ip(1) + Ip(2) Ip(1) -Zp(2)

27b a*(TeTe)

aThe notation: ns(1) f ns(2) the linear combination of the non-bonding s orbitals of the two symmetry-related chalcogen atoms; ns(X) (X = S, Se, Te) the non-bonding s orbital of the chalcogen atom in brackets; lp the p lone-pair orbital of the chalcogen atom.

303 TABLE 6 The energies of the seven highest occupied Orbital 24a 24b 25a 25b 26a 27a 26b

MINI-l -0.840 -0.758 -0.490 -0.475 -0.385 -0.348 -0.338

molecular

orbitals of HTeTeH (a.u.)

MINI-l* -0.836 -0.739 -0.499 -0.478 -0.396 -0.337 -0.333

energies of HSH and HSeH are fairly similar while those of HTeH are smaller. It seems indeed, that in compounds containing sulfur or selenium the replacement of sulfur by selenium or vice versa has a minimal effect on the bonding in the molecule. This helps to understand the rearrangement reactions observed between various selenium sulfides [ 21. CONCLUSIONS

The development of efficient basis sets for the heavy elements and the improvements in the computing techniques have provided a possibility of comparing the full range of mono- and dichalcogen hydrides by ab initio molecular orbital methods involving minimal Gaussian basis sets both with and without inclusion of d-polarization functions for sulfur, selenium, and tellurium. The inclusion of d-polarization functions significantly improved the geometry prediction of the dichalcogen hydrides as judged from the limited experimental evidence available. The augmented basis sets also provided better estimates for the total and orbital energies and for the energy derived quantities like the total binding energies and the heights of the rotational barriers. However, despite their shortcomings the unpolarized basis sets were found to give adequate description on the orbital energies of all hydrides, and also to be suitable for the qualitative study of the trends in various molecular properties of the analogous molecules. The orbital energies in the valence regions of all dichalcogen hydrides are similar. The most notable difference is the relatively small orbital energy of the non-bonding 5s-orbital of tellurium as compared to the corresponding orbitals of sulfur and selenium. The total binding energies indicate that the stabilities of the dichalcogen hydrides with respect to the corresponding monochalcogen hydride and the free element decrease towards the heavier members of this series. The barriers to internal rotation about different chalcogen-chalcogen bonds appear to be of almost identical height to the cis- and trans-barriers ca. 23 and 14 kJ mol-‘, respectively. Also, each chalcogen-chalcogen bond

304

varied in length in a fashion similar to a function of the dihedral angle. The present work indicates that the properties of the various chalcogenchalcogen bonds are very similar in many respects, if only the low stability of the heavier dichalcogen hydrides makes their isolation difficult. However, derivatives are known for all dichalcogen hydrides which do illustrate the similarities in these different bonds. ACKNOWLEDGEMENTS

We are grateful to Prof. S. Huzinaga for the basis set of tellurium and to Ms. Marina Lindblad for computational assistance. We also thank the Technical University of Berlin and the Ministry of Education of Finland for the CRAY-1 computer time. The financial help from the Academy of Finland and the research fellowship from Alexander von Humboldt Foundation (to R.L.) is gratefully acknowledged. REFERENCES 1 R. Laitinen and T. Pakkanen, J. Mol. Struct. (Theochem), 91 (1983) 337, and references therein. 2 R. Steudel and R. Laitinen, Top. Curr. Chem., 102 (1982) 177. 3 A. Hinchliffe, J. Mol. Struct., 55 (1979) 127. 4 G. Winnewisser, M. Winnewisser and W. Gordy, J. Chem. Phys., 49 (1968) 3466. 5 R. J. Boyd, J. S. Perkyns and R. Ramani, Can. J. Chem., 61(1983) 1082. 6 C. J. Marsden and B. J. Smith, J. Mol. Struct. (Theochem), 105 (1983) 385. 7 A. N. Tavouktsoglou and S. Huzinaga, J. Chem. Phys., 72 (1980) 1385. 8 H. Tatewaki and S. Huzinaga, J. Chem. Phys., 71 (1979) 4339. 9 H. Tatewaki and S. Huzinaga, J. Chem. Phys., 72 (1980) 399. 10 H. Tatewaki and S. Huzinaga, J. Comput. Chem., 1 (1980) 205. 11 H. Tatewaki, Y. Sakai and S. Huzinaga, J. Comput. Chem., 2 (1981) 96. 12 Y. Sakai, H. Tatewaki and S. Huzinaga, J. Comput. Chem., 2 (1981) 100. 13 Y. Sakai, H. Tatewaki and S. Huzinaga, J. Comput. Chem., 2 (1981) 108. 14 H. Tatawaki, Y. Sakai and S. Huzinaga, J. Comput. Chem., 2 (1981) 278. 15 Y. Sakai, H. Tatewaki and S. Huzinaga, J. Comput. Chem., 3 (1982) 6. 16 S. Huzinaga, personal communication. 17 R. S. Laitinen and T. A. Pakkanen, J. Mol. Struct. (Theochem), 108 (1984) 263. 18 C. S. Ewig, E. H. Mei and J. R. van Wazer, Mol. Phys., 40 (1980) 241. 19 V. Renugopalakrishnan and R. Walter, J. Am. Chem. Sot., 106 (1984) 3413. 20 J. L. Whitten, J. Chem. Phys., 39 (1963) 349. 21 J. D. Petke, J. L. Whitten and A. W. Douglas, J. Chem. Phys., 51 (1969) 256. 22 R. A. Hill and T. H. Edwards, J. Chem. Phys., 42 (1965) 1391. 23 N. K. Moncur, P. D. Willson and T. H. Edwards, J. Mol. Spectrosc., 52 (1974) 380. 24 R. L. Cook, F. C. De Lucia and P. Helminger, J. Mol. Struct., 28 (1975) 237. 25 S. Hauge, Acta Chem. Stand., Ser. A., 33 (1979) 313. 26 P. D’Antonio, A. H. Lowrey and J. Karle, J. Chem. Phys., 55 (1971) 1071. 27 K. Gjerrestad and K. Ma&y, Acta Chem. Stand., 24 (1970) 3402. 28 M. R. Spirlet, G. van der Bosche, 0. Dideberg and L. DuPont, Acta Crystallogr., Sect. B, 35 (1979) 1727. 29 L. Pauling, Proc. Natl. Acad. Sci. U.S.A., 35 (1949) 495. 30 Gmelin’s Handbuch der Anorganischen Chemie, “Sulfanes”, Sulfur, Suppl. B4a/b, 8th edn., Springer-Verlag, Berlin, Heidelberg, New York, 1983, 500 pp.

305 31 Gmelin’s Handbuch der Anorganischen Chemie, “Selen”, Suppl. Bl, 8th edn., SpringerVerlag, Berlin, 1981, p. 71. 32 Gmelin’s Handbuch der Anorganischen Chemie, “Tellur”, Suppl. Bl, 8th edn., SpringerVerlag, Berlin, 1976, p. 12. 33 J. R. van Wazer, Ann. N.Y. Acad. Sci., 159 (1969) Art. 1,5. 34 F. Feh& and G. Winkhaus, Z. Anorg. Allg. Chem., 292 (1957) 210. 35 R. R. Frazer, G. Boussard, J. K. Saunders, J. B. Lambert and C. E. Mixan, J. Am. Chem. Sot., 93 (1971) 3822. 36 R. Steudel, Z. Naturforsch., TeiI B, 38 (1983) 543, and references therein. 37 R. Steudel, J. Steidel, J. Pickardt, F. Schuster and R. Reinhardt, Z. Naturforsch., Teil B, 35 (1980) 1378. 38 R. Steudel, Top. Curr. Chem., 102 (1982) 149. 39 R. Steudel and F. Schuster, J. Mol. Struct., 44 (1978) 143. 40 R. Laitinen, R. Steudel and E.-M. Strauss, J. Chem. Sot., Dalton Trans., in press. 41 R. Steudel and E. -M. Strauss, Angew. Chem., 96 (1984) 356; Angew. Chem., Int. Ed. Engl., 23 (1984) 362. 42 S. Elbel, H. Bergmann and W. Ensslin, J. Chem. Sot., Faraday Trans. 2, 70 (1974) 555.