The O-2 and NO reactivity of a Cr(II) complex bearing a 12-membered tetraazamacrocyclic N-tetramethylated cyclam (TMC) ligand, [Cr-II(12-TMC)-(Cl)](+) (1), and the NO reactivity of its peroxo derivative, [Cr-IV(12-TMC)(O-2)(Cl)](+) (2), are described. By contrast to the previously reported Cr(III)-superoxo complex, [Cr-III(14-TMC)(O-2)(Cl)](+), the Cr(IV)-peroxo complex 2 is formed in the reaction of 1 and O-2. Full spectroscopic and X-ray analysis revealed that 2 possesses side-on eta(2)-peroxo ligation. The quantitative reaction of 2 with NO affords a reduction in Cr oxidation state, producing a Cr(III)-nitrato complex, [Cr-III(12-TMC)(NO3)(Cl)](+) (3). The latter is suggested to form via a Cr(III)- peroxynitrite intermediate. [Cr-II(12-TMC)(NO)(Cl)](+) (4), a Cr(II)-nitrosyl complex derived from 1 and NO, could also be synthesized; however, it does not react with O-2.