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The uncatalyzed reactions of O 2 (S = 1) with organic substrates (S = 0) are thermodynamically favorable but kinetically slow because they are spin-forbidden and the one-electron reduction potential of O 2 is unfavorable. In nature, many of these important O 2 reactions are catalyzed by metalloenzymes. In the case of mononuclear non-heme iron enzymes, either Fe II or Fe III can play the catalytic role in these spin-forbidden reactions. Whereas the ferrous enzymes activate O 2 directly for reaction, the ferric enzymes activate the substrate for O 2 attack. The enzyme–substrate complex of the ferric intradiol dioxygenases exhibits a low-energy catecholate to Fe III charge transfer transition that provides a mechanism by which both the Fe center and the catecholic substrate are activated for the reaction with O 2 . In this Perspective, we evaluate how the coupling between this experimentally observed charge transfer and the change in geometry and ligand field of the oxidized metal center along the reaction coordinate can overcome the spin-forbidden nature of the O 2 reaction.