From Silaallene to Cyclotrisilanylidene

Four possible structural isomers at the Si3H4 potential surface, a trisilene, a trisilaallene, and two structurally different silylenes, have been studied by quantum chemical methods. With the investigation of the Si−Si−Si bond angle the different structures could be distinguished and characterized. α > 100° represents silaallenes with Si&dbd;Si double bonds and C2 symmetry. In the 80° < α < 100° region the Si&dbd;Si bond order is about 1.5 and the molecule is planar or almost planar and can be characterized with an in-plane lone electron pair and a delocalized π-system. A cyclic silylene with single Si−Si bonds and C2 symmetry can be found in the α < 80° region. With systematic variation of the substituents using π-donor or -acceptor groups both silylenes can be stabilized. Some possible stabilized structures of each type are suggested.