Antiaromatic Metallacyclopentatriene Complexes.

There is no report on the (anti)aromaticity of metallacyclopentatrienes, one kind of common and important five-membered metallacycles. This work presents the novel synthesis of osmium -biscarbene complexes and their oxidation to osmacyclopentatrienes. The osmacyclopentatriene unit is antiaromatic, as revealed by experimental and theoretical studies. This finding provides new insight into the discovery of antiaromatic species.