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

<p>S = 2 FeIV?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 structurally-defined S = 2 FeIV?O species that define its frontier molecular orbitals, which allow its high reactivity. Importantly, there are both ?- and ?-channels for reaction, and both are highly reactive because they develop dominant oxyl character at the transition state. These ?- and ?-channels have different orientation dependences defining how the same substrate can undergo different reactions (H-atom abstraction vs. electrophilic aromatic attack) with FeIV?O sites in different enzymes, and how different substrates can undergo different reactions (hydroxylation vs. halogenation) with an FeIV?O species in the same enzyme.</p>