New Co-III-nitrosyl complexes bearing N-tetramethylated cyclam (TMC) ligands, [(12-TMC)Co-III(NO)](2+) (1) and [(13-TMC)Co-III(NO)](2+) (2), were synthesized via [(TMC)Co-II(CH3CN)](2+) + NO(g) reactions. Spectroscopic and structural characterization showed that these compounds bind the nitrosyl moiety in a bent end-on fashion. Complexes 1 and 2 reacted with KO2/2.2.2-cryptand to produce [(12-TMC)Co-II(NO2)](+) (3) and [(13-TMC)Co-II(NO2)](+) (4), respectively; these possess O,O'-chelated nitrito ligands. Mechanistic studies using O-18-labeled superoxide (O-18(2)center dot-) showed that one O atom in the nitrito ligand is derived from superoxide and the O-2 produced comes from the other superoxide O atom. Evidence supporting the formation of a Co-peroxynitrite intermediate is also presented.