Bonding pattern continuum from covalent to dative carbon-silicon bonds for substituted silenes: A theoretical study

A quantum chemical B3LYP/cc-pVDZ study on bisubstituted silenes indicates that substitution at the carbon atom is more effective than that at the silicon atom at inducing a bent structure. π-Electron-donating substituents at the carbon atom drive the polarized π electrons toward the silicon atom. This reversed polarization effect induces bending of the silylene fragment. The local bending can be interpreted in terms of the second-order Jahn-Teller effect with the "transition density" of πσ* localized substantially at the Si atom. From the series of silenes having increasing bending of the SiH₂ unit, we have found that the structure of the SiH₂ fragment gradually shifts to a singlet silylene-like geometry. Thus, the covalently bonded SiH₂ fragments can evolve into datively bonded fragments of the singlet valence state in the bent silenes, which is a finding that is in accord with the CGMT model.