Bond Stretch Isomerism in Bicyclo[1.1.0]tetrasilanes. Contrasts Between Strained Silicon and Carbon Ring Systems

Is bicyclotetrasilane expected to exhibit a markedly short or a markedly long central SiSi bond? Ab initio calculations at different levels of accuracy all gave two potential minima. The corresponding structures differ in the length of the central bond (by about 0.5 Å), the interflap angle (120 and 140°), and the HSiSi angle at the bridgehead atoms (140 and 90°). In all calculations, the structure with the long SiSi bond proved to be lower in energy. This order can be reversed, however, in derivatives with bulky substituents at the bridgehead atoms.