Are <i>Atomic‐Oxygen‐Like</i> Intermediates the Key to Metal–Oxo Catalyzed O–O Bond Formation?

Abstract O–O bond formation catalyzed by a water‐oxidizing Co–O center was studied via state‐of‐the‐art multi‐configurational quantum chemical calculations. An unnoted electronic state was shown to guide water nucleophilic attack (WNA). Incoming water perturbs and over‐stretches the Co–O bond, generating a quartet state with five unpaired electrons, with a formal Co[d(yz) 2 d(xz) 2 d(z 2 ) 1 d(xy) 1 d(x 2 ‐y 2 ) 1 ] O[p(z) 2 p(y) 1 p(x) 1 ] representation and placing Co⋯O oxygen in a local atomic‐like character. Lone pair electrons of water flow into SOMOs of the atomic‐oxygen‐like center–a picture different than the classical WNA mechanism, where two electrons are transferred to LUMO. This atomic‐oxygen‐like electronic structure is a key to understanding O–O bond formation and originates from an internally excited state of the Co–O center in the absence of water. The conventional three‐electron quartet is restored when the O–O bond is generated. Local electronic structure of the atomic‐like oxygen can be foreseen by the symmetry of the water approach coordinate, enabling the prediction of catalytic activity. Our findings were successfully tested on eight experimentally available metal–oxo complexes with distinct catalytic profiles, comprising both active and inactive compounds. Generalization of the current results to other metal–oxo systems in nucleophilic attack and/or small molecule activation reactions might be possible.