A Stable Silylene with a σ², π- Butadiene Ligand

The synthesis of a new type of silylene 1 is reported. It adopts a bicyclo[2.1.1]­hexene structure in which a hafnocene group is incorporated. The silylene is stabilized by homoconjugation with the remote CC double bond. This is indicated by its highly shielded 29Si NMR chemical shift (δ29Si = −155) and is firmly established by its experimental molecular structure from XRD analysis. The results of a detailed bonding analysis based on DFT calculations suggest for model compounds of silylene 1 and for its heavier germanium, tin, and lead homologues uniformly electronic structures of carbene analogues that are stabilized by homoconjugation. This stabilization mode is equivalent to a σ2, π-coordination of the butadiene ligand to the element atom as it is typical for zirconocene or hafnocene butadiene complexes.