The role of three-center/four-electron bonds in superelectrophilic dirhodium carbene and nitrene catalytic intermediates

Three-center/four-electron (3c/4e) bonds are important bonding motifs that dictate the electronic structure, and thereby the reactivity, of metal–metal bonded carbene and nitrene intermediate complexes that are crucial to the dirhodium-catalyzed functionalization of hydrocarbons. In this Perspective article, general features of the 3c/4e bond are presented and discussed in comparison to two-center/two-electron (2c/2e) bonds. Specifically, 3c/4e bonding interactions lead to longer distances between the atoms involved and measurably weaker bonds. Additionally, excited states derived from the 3c/4e bonding manifold are lower in energy than those derived from a 2c/2e manifold, signifying a greater degree of reactivity in the former case. Three coterminous 3c/4e Ru–Ru–N bonds are present in metal–metal/metal–ligand multiply bonded diruthenium terminal nitrido compounds. This bonding situation results in an unusual superelectrophilic character of the nitride nitrogen atom, exemplified by its insertion into aryl C–H bonds via an electrophilic aromatic substitution mechanism. The key catalytic intermediates in dirhodium-catalyzed C–H functionalization reactions, dirhodium carbene and dirhodium nitrene complexes, may also be described as superelectrophilic by virtue of 3c/4e Rh–Rh–C(or N) σ and π bonds. These 3c/4e bonding interactions set apart dirhodium carbene and nitrene intermediates from other, less electrophilic, carbene or nitrene species.