Electron Deficient Bridges Involving Silylenes:  A Theoretical Study

Ab initio and density functional studies show that silylenes can form complexes with BH<sub>3</sub> and the resultant complexes possess 3c−2e bridges. The complexation energy for the formation of the these H-bridged structures is in the range of 18−46 kcal/mol. The characteristics of the electron deficient bridges depend on the substituents attached to the silylenes. With an increase in the π-donating capacity of the substituents, the exothermicity of complex formation gets reduced but the kinetic stability of the H-bridged structures increase. The natural bond orbital analysis shows that all the H-bridged structures are associated with σ<sub>B</sub><sub>-</sub><sub>H</sub>→pπ<sub>Si</sub> second-order delocalization, which is responsible for the origin of the 3c−2e bonds. The complexation energies of the silylene−BH<sub>3</sub> complexes have been shown to have a correlation to the singlet−triplet energy gaps of silylenes.