Structural and Mechanistic Basis of the Fast Metathesis Initiation by a Six-Coordinated Ruthenium Catalyst

The N-heterocyclic carbene coordinated ruthenium benzylidene complex [(H₂IMes)­(PCy₃)­(Cl)₂RuCHPh], a highly active olefin metathesis catalyst, is known to convert into a very fast initiator in the presence of pyridine or 3-bromopyridine. Computational studies presented in this work reveal the mechanistic basis of this phenomenon. Depending on the size of the olefin, the reaction follows either a dissociative or associative mechanism.