ELECTRON-DIFFRACTION STUDY OF THE MOLECULAR-STRUCTURE OF GASEOUS TETRAMETHYL-1-STANNA-3,5-DISILA-4-OXACYCLOHEXANNE, BR2SN(CH(2)SIME(2))(2)O

The molecular structure of gaseous Br2Sn(CH(2)SiMe(2))(2)O was studied by electron diffraction. The six-membered ring has a chair conformati on whereas the entire molecule possesses C-s symmetry. The existence o f a boat conformer cannot be completely excluded. The results of theor etical calculations for a twisted-boat conformation are at variance wi th the experimental data. Steric strain caused by mutual repulsion of the two axial methyl groups is reduced to the tilt of the Me(2)Si frag ments in opposite directions. This results in an increase (up to 26 de grees C) in the angle formed by the bisector of the C-M-Si-C-M angle w ith the CcSiO plane. The main geometrical parameters are as follows: r (g) (Angstrom): Si-O 1.708(20); Si-C-M 1.862(20); Si-C-c 1.882(9); Sn- C 2.108(26); Sn-Br 29456(3); C-H 1.099(30); gamma(alpha) (degr.): C-Sn -C 105(2); Br-Sn-Br 107.9(1.2); Si-O-Si 129.6(3); C-M-Si-C-M 112; Si-C -H 113 (fixed value in accordance with experiment); C-c-Si-O 107(2); S n-C-Si 109(2); torsion angles: phi(Si-C) 52(2); phi(Si-O) 62(1); phi(C -c-Sn) 54(1). The average amplitudes were fixed at the values calculat ed from the force field. Structural parameters of molecules with simil ar structures were analyzed and compared.