Mechanisms of Reactions of ^.NO with Complexes with Metal-Carbon σ-Bonds and with Aliphatic Radicals

The competition kinetics between metal complexes and NO for aliphatic radicals is a convenient technique for determining the rate constants of the reactions of NO with aliphatic radicals. Thus, the rate constants of CH₃ and CH₂OH with NO were determined to be (3.4 ± 1.1) x 10⁹ and (5.9 ± 0.5) x 10⁹ M^-1 s^-1, respectively. The same rate constants can be determined, in several systems, from the decrease in the half-life of transient complexes with metal-carbon σ-bonds which decompose homolytically. In the presence of NO, the half-life of these transient complexes decreases, and their decomposition turns from second-order to first-order processes. From the dependence of the observed first-order rate constant on NO and on the metal complex concentrations, it is concluded that the mechanism of the decomposition of these transients in the presence of NO involves the reaction of NO with the carbon-centered radicals as well as with the transient with the metal-carbon σ-bonds to form the same products. The rate constants of the reactions of NO with (cyclam)(H₂O)Ni^IIICH₃²⁺ and (nta)Co^IIICH₂OH(H₂O)^- were determined to be (1.5 ±0.2) x 10⁵ and (3.6 ± 0.4) x 10⁸ M^-1 s^-1, respectively. The reactions of NO with complexes with metal-carbon σ-bonds are analogous to those of aliphatic radicals and dioxygen with the same complexes. This is not surprising as NO is a radical. The biological implications of these results are discussed.