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. alpha > 100 degrees represents silaallenes with Si=Si double bonds and C-2 symmetry. In the 80 degrees < alpha < 100 degrees region the Si=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 pi-system. A cyclic silylene with single Si-Si bonds and C-2 symmetry can be found in the alpha < 80 degrees region. With systematic variation of the substituents using pi-donor or -acceptor groups both silylenes can be stabilized. Some possible stabilized structures of each type are suggested.