Metal-dioxygen adducts, such as metal-superoxo and -peroxo species, are key intermediates often detected in the catalytic cycles of dioxygen activation by metalloenzymes and biomimetic compounds. The synthesis and spectroscopic characterization of an end-on nickel(II)-superoxo complex with a 14-membered macrocyclic ligand was reported previously. Here we report the isolation, spectroscopic characterization, and high-resolution crystal structure of a mononuclear side-on nickel(III)-peroxo complex with a 12-membered macrocyclic ligand, [Ni(12-TMC)(O 2)] (1) (12-TMC1,4,7,10-tetramethyl-1,4,7,10- tetraazacyclododecane). In contrast to the end-on nickel(II)-superoxo complex, the nickel(III)-peroxo complex is not reactive in electrophilic reactions, but is capable of conducting nucleophilic reactions. The nickel(III)-peroxo complex transfers the bound dioxygen to manganese(II) complexes, thus affording the corresponding nickel(II) and manganese(III)-peroxo complexes. Our results demonstrate the significance of supporting ligands in tuning the geometric and electronic structures and reactivities of metal-O 2 intermediates that have been shown to have biological as well as synthetic usefulness in biomimetic reactions.