Dihydrogen and Other σ Bond Complexes

Abstract

Coordination of a dihydrogen molecule in W(CO)3(PR3)2(H2) is the prototype for a new class of metal complexes commonly referred to as “σ-complexes.” Molecules containing only strong “inert” σ-bonds such as H–H had previously been believed to be incapable of stable binding, although metals readily break these bonds in catalysis, and σ-complexes were proposed as transients in oxidative addition (OA) processes. Metal hydrides formed by OA of H2 are catalytic intermediates, and it is now clear that metal–H2 and other σ-complexes can also be important in catalytic mechanisms. Silanes, boranes, germanes, stannanes, and alkanes form sigma complexes, which are distinct from intramolecular agostic interactions. Elongation of the coordinated X–H bond along the entire reaction coordinate for OA is observable on variation of metal/ligands, allowing “snapshots” of the bond-breaking process. Interaction of any X–Y sigma bond with a metal center is in principle possible and has been observed, for example, for C–C, Si–C, and Si–Si bonds. Catalytic and enzymatic processes employing sigma complexes are discussed, particularly those involving heterolytic activation of sigma bonds.