Unexpected Mechanism for Formal [2 + 2] Cycloadditions of Metallacyclocumulenes

The formation of radialene complex 6M proceeds through a three-membered metallacyclopropene com-plex 7M, contrary to the prevailing notion of simple dimerization of metallacyclocumulene 1M. The 1M-7M equilibrium, which is predominantly governed by the size-dependent ligand binding of the metal atoms, plays a decisive role in the chemistry of Cp₂M-ligand complexes. This size dependency is further fine-tuned by the substituents on the substrates and helps in exploiting these classes of metallacycles to generate new chemistry.