Metal nitrosyl complexes are key intermediates involved in many biological and physiological processes of nitric oxide (NO) activation by metalloproteins. In this study, we report the reactivities of mononuclear cobalt(III)-nitrosyl complexes bearing N-tetramethylated cyclam (TMC) ligands, [(14-TMC)Co-III(NO)](2+) and [(12-TMC)Co-III(NO)](2+), in NO-transfer and dioxygenation reactions. The Co(III)-nitrosyl complex bearing 14-TMC ligand, [(14-TMC)Co-III(NO)](2+), transfers the bound nitrosyl ligand to [(12-TMC)Co-II(NO)](2+) via a dissociative pathway,-[[(14-TMC)Co-III(NO)](2+) (14-TMC)Co center dot center dot center dot NO}(2+)}, thus affording [(12-TMC)Co-III(NO)](2+) and [(14-TMC)Co-II](2+) as products. The dissociation of NO from the [(14-TMC)COIII(NO)](2+) complex prior to NO-transfer is supported experimentally and theoretically. In contrast, the reverse reaction, which is the NO-transfer from [(12-TMC)Co-III(NO)](2+) to [(14-TMC)Co-II](2+), does not occur. In addition to the NO-transfer reaction, dioxygenation of [(14-TMC)Co-III(NO)](2+) by O-2 produces [(14-TMC)Co-II(NO3)](+), which possesses an O,O-chelated nitrato ligand and where, based on an experiment using O-18-labeled O-2, two of the three O-atoms in the [(14-TMC)Co-II(NO3)](+) product derive from O-2. The dioxygenation reaction is proposed to occur via a dissociative pathway, as proposed in the NO-transfer reaction, and via the formation of a Co (II) peroxynitrite intermediate, based on the observation of phenol ring nitration. In contrast, [(12-TMC)Co-III(NO)](2+) does not react with O-2. Thus, the present results demonstrate unambiguously that the NO-transfer/dioxygenation reactivity of the cobalt(III)-nitrosyl complexes bearing TMC ligands is significantly influenced by the ring size of the TMC ligands and/or the spin state of the cobalt ion.