Formation of High-Oxidation-State Metal–Carbon Double Bonds

This tutorial explores the major pathways of forming metal–carbon double bonds in high-oxidation-state alkylidene complexes that began with the alkylidene chemistry of tantalum complexes in the 1970s and continued with the organometallic chemistry of Mo, W, and Re and the development of homogeneous catalysts for the metathesis of olefins. It also explores recent findings in surface organometallic chemistry and discusses the link between molecularly defined and heterogeneous catalysts. Recent results suggest that heterogeneous olefin metathesis catalysts that are activated toward metathesis upon exposure to olefins produce a d 0 alkylidene through formation of a metallacyclopentane ring at d 2 metal sites followed by “a ring contraction” to a metallacyclobutane, a reaction that was first observed in tantalum chemistry.