Progress in the chemistry of metallabenzynes.

The synthesis and chemical and structural properties of metallabenzynes are reviewed. Reaction of [OsCl(2)(PPh(3))(3)] with HC(triple bonded)CSiMe(3) produces the osmabenzyne [Os((triple bonded)CC(SiMe(3))=C(CH(3))C(SiMe(3))=CH)Cl(2)(PPh(3))(2)], which undergoes electrophilic substitution reactions with HBF(4) and Br(2) to give new osmabenzynes. The reactivities and the X-ray diffraction data of osmabenzynes indicate that these metallacycles have aromatic properties. Unlike benzyne, which is thermally unstable, osmabenzynes are thermally much more stable and can be stored for months at room temperature without decomposition. The higher thermal stability of osmabenzynes compared to benzyne can be related to the relatively smaller ring strain and larger conjugation energy.