The monoanionic N₄O ligand N-methyl-N,N′-bis(2-pyridylmethyl)ethylenediamine-N′-acetate (mebpena⁻) undergoes oxidative C–N bond cleavage in the presence of Co(II) and O₂. The two resultant fragments are coordinated to the metal ion in the product [Co^III(2-pyridylformate)(mepena)]ClO₄ (mepena⁻ = N-methyl-N′-(2-pyridylmethyl)ethylenediamine-N′-acetato). Bond cleavage does not occur in the presence of chloride ions and [Co^III(mebpena)Cl]⁺, containing intact mebpena⁻, can be isolated. The oxidative instability of the mebpena⁻ in the presence of Co(II) and air stands in contrast to the oxidative stability of the family of very closely related penta- and hexa-dentate ligands in their cobalt complexes. Cyclic voltammetry on the matched pair [Co^IIICl(mebpena)]⁺ and [Co^IICl(bztpen)]⁺, bztpen = N-benzyl-N,N′,N′-tris(2-pyridylmethyl)ethylenediamine, shows that substitution of a pyridine donor for a carboxylato donor results in a relatively small cathodic shift of 150 mV in the E°(Co(II)/Co(III)) oxidation potential, presumably this is enough to determine the contrasting metal oxidation state in the complexes isolated under ambient conditions. DFT calculations support a proposal that [Co^II(mebpena)]⁺ reacts with O₂ to form a Co(III)-superoxide complex which can abstract an H atom from a ligand methylene C atom as the initial step towards the observed oxidative C–N bond cleavage.