Are Atomic-Oxygen-Like Intermediates the Key to Metal-Oxo Catalyzed O-O Bond Formation?

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(z²)¹ d(xy)¹ d(x²-y²)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.