It is argued that direct [2 + 2] cycloadditions and cycloreversions involving Mt.dbd;C and C.dbd;C species and quasi-metallacyclobutanes are the key steps in olefin metathesis. This theoretical concept is then extended in a novel fashion to describe the mechanisms of many other important reactions in homogeneous and heterogeneous catalysis, and especially the activation of C.dbd;C, Hâ H and Câ H bonds by metalâ alkyl complexes where the metal ion is electron deficient and the alkyl ligand assumes â carbenoidâ character.