Described are further studies directed towards elucidating the mechanism of the nitric oxide reduction of the copper(II) model system, Cu(dmp)2(2+) (I, dmp=2,9-dimethyl-1,10-phenanthroline). The reaction of I with NO in methanol results in the formation of Cu(dmp)2+ (II) and methyl nitrite (CH3ONO), with a second order rate constant kNO=38.1 M-1 s-1 (298K). The activation parameters for this reaction in buffered aqueous medium were measured to be DeltaH(double dagger)=41.6 kJ/mol and DeltaS(double dagger)=-82.7 kJ/mol deg. The addition of azide ion (N3-) as a competing nucleophile results in a marked acceleration in the rate of the copper(II) reduction. Analysis of the kinetics for the NO reduction of the bulkier Cu(dpp)(2)2+ (IV, dpp=2,9-diphenyl-1,10-phenanthroline) and the stronger oxidant, Cu(NO2-dmp)2(2+) (V, NO2-dmp=5-nitro-2,9-dimethyl-1,10-phenanthroline), gave the second order rate constants kNO=21.2 and 29.3 M-1 s-1, respectively. These results argue against an outer sphere electron transfer pathway and support a mechanism where the first step involves the formation of a copper-nitrosyl (Cu(II)-NO or Cu(I)-NO+) adduct. This would be followed by the nucleophilic attack on the bound NO and the labilization of RONO to form the nitrite products and the cuprous complex.