Scientific Review: Single Crystal Analyses of the Structure and Bonding in Transition-Metal σ-Complexes

Transition-metal _σ-complexes [2] are coordination compounds in which two electrons in an X-H _σ-bond form a dative bond with a transition metal as illustrated in Figure 1. This bond can be further stabilized by π-backbonding from the metal to the X-H σ* antibonding orbital. Transition-metal σ-complexes are typically reactive intermediates that precede oxidative addition of substrates having an X-H bond. σ-complexes are, therefore, intermediates in catalytic hydrogenation (X = H), activation and functionalization of hydrocarbons (X = C), hydro-silylation (X = Si), and hydroboration (X = B) reactions.