Electronic structure analysis of multistate reactivity in transition metal catalyzed reactions: the case of C–H bond activation by non-heme iron(iv)–oxo cores

This perspective discusses the principles of the multistate scenario often encountered in transition metal catalyzed reactions, and is organized as follows. First, several important theoretical concepts (physical versus formal oxidation states, orbital interactions, use of (spin) natural and corresponding orbitals, exchange enhanced reactivity and the connection between valence bond and molecular orbital based electronic structure analysis) are presented. These concepts are then used to analyze the electronic structure changes occurring in the reaction of C–H bond oxidation by FeIVoxo species. The analysis reveals that the energy separation and the overlap between the electron donating orbitals and electron accepting orbitals of the FeIVoxo complexes dictate the reaction stereochemistry, and that the manner in which the exchange interaction changes depends on the identity of these orbitals. The electronic reorganization of the FeIVoxo species during the reaction is thoroughly analyzed and it is shown that the FeIVoxo reactant develops oxyl radical character, which interacts effectively with the σCH orbital of the alkane. The factors that determine the energy barrier for the reaction are discussed in terms of molecular orbital and valence bond concepts.