Abstract
The nature of the Si-Si double bond in a number of substituted disilenes (R2Si=SiR2, R=H, CH3, t-butyl mesityl) was investigated using the Fenske-Hall molecular orbital method. The results of the electronic structure calculations indicate that alkyl substituents serve to stabilize the Si-Si double bond electronically as well as sterically, while aryl substituents appear to destabilize the Si-Si bond. Calculations including d-orbital functions in the Si basis set show that the availability of d-orbitals on Si plays an important role in altering the compositions and lowering the energies of the virtual molecular orbitals. This result is in agreement with the experimentally observed spectroscopic transitions in the visible region and may also account for the relatively low rotational barriers that have been observed in the experimentally determined disilenes. Our calculations show that the bonding in disilenes is very sensitive to the substituents on the silicon atoms. Bulky substituents on the isolated disilenes serve not only to protect the Si-Si double bond sterically but also to stabilize the bond electronically. Alkyl groups are more stabilizing than aryl groups, providing greater electron donation into the double bond. Aryl substituents appear to be electon withdrawing, and therefore destablizing. The enhanced reactivity of disilenes compared with that of alkenes may be attributed to the existence of low-lying unoccupied orbitals that have substantial Si d-orbital character. These virtual orbitals may also provide a low-energy pathway for rotation about the Si-Si axis. © 1994, Walter de Gruyter. All rights reserved.