A nickel(II) thiolate complex incorporating three N-donor types (amino, amido, and imidazole) has been synthesized and characterized, The (N3S)Ni complex, [N-{2-[(2-mercapto-2-methylpropyl)amino]ethyl)1-methylimidazole-2-carbox amido]nickel(II) (1), is stable in the presence Of O-2 but readily forms the sulfinato (RSO2-) derivative 2 upon the addition of H2O2. Electrochemical investigations of 1 reveal an irreversible sulfur-based oxidation at +0.17 V vs Fc(+)/Fc (200 mV/s) that shifts to +0.81 V upon oxidation to 2. Density functional theory investigations of 1 reveal a highest occupied molecular orbital that is predominantly sulfur-based, consistent with the observed sulfur-based oxidation and O-2 stability.