Structural study of the cyclic chlorosiloxanes and chlorosilathianes†

Abstract

Crystal structures of the cyclic (OSiCl₂)₃ and (OSiCl₂)₄ molecules showed that the bonding around silicon remains close to tetrahedral in each case, with varying oxygen angles providing for cyclisation. The trimer ring is planar, but the tetramer ring is buckled, with valence angles at O of 170.7 and 148.5°. HF/6-31G* and MP2/6-31G* calculations on the (OSiCl₂)_n, (SSiCl₂)_n, (OSiH₂)_n and (SSiH₂)_n polymer series were performed. Silicon shows a strong tendency towards retention of regular tetrahedral bonding throughout. For the oxo- and thio-chloride series, calculated energies of formation expressed relative to the monomer are in line with preparative experience. The non-appearance of a four-membered oxo ring, as would be required in a dimer (OSiCl₂)₂, is attributable to too great a charge separation within a small structure, but reduction of these unfavourable factors permits the formation of a four-membered ring in the thio series to give (SSiCl₂)₂. Features of ring development in the oxo series can be correlated with structures shown by silicates.