Theory of chemical bonds in metalloenzymes XII: Electronic and spin structures of metallo-oxo and isoelectronic species and spin crossover phenomena in oxygenation reactions

The broken-symmetry (BS) and multideterminant approaches to atomic oxygen (O), molecular oxygen (O₂) and iron-oxo (Fe(IV){double bond, long}O) core in P450 have elucidated electronic structures of the ground triplet and excited singlet states, which indicate isoelectronic characteristics of the species. The dissociation processes of the O-O and Fe-O double bonds are also examined to clarify the radical character, namely O-atom property responsible for radical mechanism of hydroxylations of alkanes and epoxidation of alkenes. This isolobal analogy has indeed enabled us to propose possible reaction mechanisms of oxygenation reactions by the Fe(IV){double bond, long}O species on the basis of available theoretical and experimental results for O and O₂. Similarly, an isolobal analogy of the σ^* bond among Fe(IV){double bond, long}O, dioxirane, peracids, etc. indicates the common electrophilic property for the oxygenation reactions. The small energy gaps between the high- and low-spin states of the transition structures and intermediates generated in the oxygenation reactions are found to be origins for spin crossover phenomena along the reaction pathways of these reactions.