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In order to shed light on metal-dependent mechanisms for O–O bond cleavage, and its microscopic reverse, we compare herein the electronic and geometric structures of O2-derived binuclear Co(III)– and Mn(III)–peroxo compounds. Binuclear metal peroxo complexes are proposed to form as intermediates during Mn-promoted photosynthetic H2O oxidation, as well as a Co-containing artificial leaf inspired by nature’s photosynthetic H2O oxidation catalyst. Crystallographic characterization of an extremely activated peroxo is made possible by working with substitution-inert, low-spin Co(III). Density functional theory (DFT) calculations show that the frontier orbitals of the Co(III)–peroxo compound differ noticeably from the analogous Mn(III)–peroxo compound. The highest occupied molecular orbital (HOMO) associated with the Co(III)–peroxo is more localized on the peroxo in an antibonding π*(O–O) orbital, whereas the HOMO of the structurally analogous Mn(III)–peroxo is delocalized over both the metal d-orbitals and peroxo π*(O–O) orbital. With low-spin d6 Co(III), filled t2g orbitals prevent π-back-donation from the doubly occupied antibonding π*(O–O) orbital onto the metal ion. This is not the case with high-spin d4 Mn(III), since these orbitals are half-filled. This weakens the peroxo O–O bond of the former relative to the latter.