Electronic structures of unsaturated silicon compounds and transition metal-silicon interactions

Abstract

The electronic structures of disilenes and tungstenocene complexes of disilenes, silenes, and alkenes are studied by gas phase valence photoelectron spectroscopy (PES). PES is reported for three stable disilenes: Mes2Si=SiMes2' (Mes)(ᵗBu)Si=Si(ᵗBu)(Mes), and [(Me₃Si)₂CH]Si=Si[CH(SiMe₃)₂] [Mes = 2,4,6- C₆H₃(CH₃)₃, Me = CH₃]. The PES of tetramesityldisilene is compared to that of tetraphenylethylene. Fenske-Hall calculations are also reported. This comparison shows that the primary difference between the interaction of a C=C bond and a Si=Si bond with aryl substituents is the direction of the "push" of the filled-filled interaction with the π orbitals of the aryl. While for an olefin, interaction with aryl groups stabilizes the C=C π ionization, for a disilene this interaction destabilizes the Si=Si π ionization. This is the direct influence of the difference in energy between the C=C π bond and Si=Si π bond. The PES of the two alkyl substituted disilenes indicates that some type of decomposition is occurring in the gas phase, possibly forming silylene. More study is needed to confirm the gas phase process that is occurring. The PES of the complexes CP₂W(η ²-H₂C=CH2), CP₂W(η²-Me₂Si=CH₂), and CP₂W(η²-Me₂Si=SiMe₂) are also reported (Cp = η⁵-C₅H₅). The overall amount of electron donation by the alkene, silene, and disilene ligands appears to be approximately the same. For the alkene and silene complexes, the highest occupied molecular orbital (HOMO) is the a₁, metal based orbital associated with the d² metal center, and contains little to no contribution from the olefinic ligand. The HOMO of the disilene complex is the same molecular orbital, but contains a significant amount of silicon character due to increased interaction with the disilene π orbital. This additional interaction is due to greater overlap and better energy matching of the ligand P and metal fragment a₁ orbitals. The relationship between these differences in bonding and differences between the reactivity of the disilene complex and the reactivity of the alkene and silene complexes is also discussed.