2257
Organometallics 1986,5, 2257-2259
Gas-Phase Molecular Structure of Tetramethyldistibine, Attila G. Csbsz6r,t Lise Hedberg,+ Kenneth Hedberg,"+ Edward G. Ludwig, Jr.,t and Arthur J. Ashe, 111s Departments of Chemistry, Oregon State University, Corvallis, Oregon 9733 1, and University of Michigan, Ann Arbor, Michigan 48109 Received February 25, 1986
The structure of tetramethyldistibine has been investigated in the gas at 74 OC by electron diffraction. Although gauche and anti forms of the molecule probably exist, it was not possible to determine the rotomeric composition because of the small relative scattering from the torsion-sensitive distances. The results for the more important distances (ra), bond angles (L), and amplitudes of vibration (1) with estimated 2a uncertainties are r(Sb-Sb) = 281.8 (4)pm, r(Sb-C) = 216.6 (4)pm, r(C-H) = 109.8 (10) pm, 4SbSbC) = 95.5 (7)O, L(CSbC) = 102.7 (65)O, L(SbCH) = 114.3 (23)O,l(Sb-Sb) = 6.9 (4)pm, I(Sb-C) = 5.4 (5)pm, and I(C-H) = 5.9 (11)pm. The Sb-Sb bond length is definitely shorter than it is in the crystal. The longer bonds in the crystal probably result from electron delocalization along chains of Sb atoms which results in a decrease of the bond order of the Sb-Sb bonds. Table I. Structural Results for Tetramethyldistibineolb Tetramethyldistibine (hereafter TMDS) has long been known' to undergo a striking color change on melting: the model A model B crystals are deep red; t h e liquid is pale yellow. The rat La 1 ra, La 1 molecules in the crystal exist in the anti conformation with r(Sb-Sb) 281.8 (4) 6.9 (4) 281.8 (4) 6.9 (4) t h e antimony atoms arranged in linear chains;2 at 367.8 r(Sb-C) 216.6 (4) 5.4 (5) 216.3 (5) 5.6 (5) (1) p m t h e intermolecular Sb.-Sb distances are about 70 r(C-H) 109.8 (10) 5.9 (11) 109.6 (10) 5.9 (11) p m shorter than a van der Waals contact. The vibrational 4SbSbC) 95.5 (7) 95.3 (15) 102.7 (65) 102.5(102) spectra of the solid and liquid have also been i n ~ e s t i g a t e d ~ , ~ 4CSbC) L(SbCH) 114.3 (23) 114.6 (23) from which it appears that there may exist more than one r(SbSbCH) 29.7 (219) conformer in t h e latter. r (SbC) 371.6 (21) (15.31 370.7(44) (15.3) It seemed possible that the color change on melting r(C.C) 338.2 (154) (15.01 337.4 (240) (15.0) could be associated with a change in molecular structure r(Sb.H) 280.2 (29) 12.4 (46) 280.3 (29) 11.8 (49) large enough to measure. Since t h e color of TMDS vapor r(Sb-.H) 472.3(52) [18.0] r(Sb9-H) 341-477' 10-2OC 356.2(124) [21.0] is similar to that of t h e liquid, an electron diffraction r (Sb-H) 406.0 (272) [21.0] investigation of the gas was indicated. Our main interest r(C-C) 419.8 (111) (26.0) 418.6 (139) (26.0) was the length of the Sb-Sb bond for comparison with that r(C-C) 539.1 (30) (14.8) 537.7 (61) (14.8) in t h e crystal. We were also interested in a possible der(C.4) 374.2 (90) (26.0) 372.3 (179) (26.0) termination of the temperature dependence of t h e conr(C.4) 531.0 (84) 114.8) 529.3 (158) (14.8) formational composition of the gas and ultimately of the Xanti:Xgauche [0*46:0*461 [0.95:0.05] thermodynamics of the conformational equilibrium. As x(C2H,Br2ld [0.081 Re 0.079 0.084 it turned out, the composition could be measured only very imprecisely, and we therefore limited our work t o the data "Distances ( r ) and amplitudes (1) in picometers; angles (L) in gathered at a single temperature. degrees. Quantities in parentheses are estimated 2a. Amplitudes in braces were calculated from an approximate force field (for details see text); quantities in brackets were assumed. bThe first Experimental Section seven parameters were used to define the geometry. CRangeof Material. The sample of TMDS was prepared at the Univdistances and amplitudes in free rotation model. Mole fractions. ersity of Michigan as described elsewhere? Although the purity ' R = [CiwiAi2/Ciwi(siri(obsd))2]1'2 where Ai = sili(obsd) - siriof our particular sample was not checked, past experience suggests (calcd). at most only a few percent impurity, most likely trimethylstibine and f or 1,2-dibromoethane. Other structural parameters were the mole fractions of the possible Experiments. The electron diffraction patterns were made impurities mentioned earlier. Vibrational amplitude parameters in the Oregon State apparatus at two camera distances (long and were formed in the usual way following tests that indicated which intermediate,nominally 75 and 30 cm) with an accelerating voltage ones could be refined alone and which ones were likely candidates of about 44 kV, exposure times of 70-110 s (long camera) and for refinement in groups. For amplitudes involving H, we esti165-180 s (intermediate camera), and beam currents of 0.20-0.30 PA; the nozzle temperature was 74 OC. Four plates from the long (1) (a) Paneth, F. A. Trans. Faraday SOC.1934,30, 179. (b) Paneth, and five from the intermediate distance were selected for analysis. F.A.; Loleit, H. J. Chem. SOC.1935, 366. The procedures for obtaining the refined molecular structure have (2) (a) Ashe, A. J., 111;Ludwig, E. G., Jr.; Oleksyszyn, J.; Huffman, J. C. Organometallics 1984,3, 337. (b) Mundt, 0.;Riffel, H.; Becker, G.; been de~cribed.~,'The electron scattering amplitudes and phases Simon, A. Z . Naturforsch., B: Anorg. Chem., Org. Chem. 1984,39B, 317. were obtained from tables.s Figure 1 shows the final intensity (3)Breunia, H. J.: Breunip-Lvriti, V.: Fichtner, W. Z . Anorp. - - Allg. curves and Figure 2 the experimental radial distribution curve. Chem. 1982,287, 111. Structure Analysis. Structural parameters for the TMDS (4) Burger, H.; Eujen, R.; Becker, G.; Mundt, 0.;Westerhausen,M.; system were taken to be the bond lengths Sb-Sb, S H ,and C-H, Witthauer, C. J. Mol. Struct. 1983, 98, 265. (5) Meinema, H. A.; Martens, H. F.; Noltes, J. G. J. Organomet. Chem. the bond angles Sb-Sb-C, C-Sb-C, and Sb-C-H, the torsion 1973, 51, 223. angles C-Sb-Sb-C and Sb-Sb-C-H, and the anti-gauche com( 6 ) Hedberg, K.; Iwasaki, M. Acta Crystallogr. 1964, 17, 529. position represented by the mole fraction of the gauche conformer. (7) Gundersen, G.; Hedberg, K. J. Chem. Phys. 1969, 51, 2500. (8) Elastic amplitudes and phases from: Schafer, L.; Yates, A. C.; Bonham, R. L. J. Chem. Phys. 1971,55,3055. Inelastic amplitudesused 'Oregon State University. in the background removal from: Cromer, D. T.; Mann, J. B. J . Chem. University of Michigan. Phys. 1967,47, 1892. Cromer, D. T. Ibid. 1968, 50, 4857.
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0276-7333186f 2305-2257$01.50/0 0 1986 American Chemical Society
2258 Organometallics, Vol. 5, No. 11, 1986
Csciszcir et al.
Table 11. Correlation Matrix
r(Sb-Sb) r(Sb-C) r(C-H) 4SbSbC) 4CSbC) I(SbCH) l(Sb-Sb) 1(Sb-C) l(C-H) 1(Sb-H)
1 2 3 4 5 6 n
8 9 10
Ua
r1
0.09 0.14 0.35 0.26 2.30 0.80 0.10 0.15 0.36 1.62
100
r2 -3 100
for TMDS Model A
(XlOO)
r3
L4
4 5
4
17
18
19
9 -9 100
-16 -6 4 100
-21 -3 3 49 100
-49 -19 -26 -9 1 100
17 2 2 -5 -8 -33 100
4 -2 2 -6 -8 -14 38 100
10 -4 -6 -1
