π-Frontier molecular orbitals in S = 2 ferryl species and elucidation of their contributions to reactivity

S = 2 Fe^IV=O species are key intermediates in the catalysis of most nonheme iron enzymes. This article presents detailed spectroscopic and high-level computational studies on a structurallydefined S = 2 Fe^IV =O species that define its frontier molecular orbitals, which allow its high reactivity. Importantly, there are both π- and σ-channels for reaction, and both are highly reactive because they develop dominant oxyl character at the transition state. These π- and σ-channels have different orientation dependences defining how the same substrate can undergo different reactions (H-atom abstraction vs. electrophilic aromatic attack) with Fe ^IV =O sites in different enzymes, and how different substrates can undergo different reactions (hydroxylation vs. halogenation) with an Fe ^IV =O species in the same enzyme.