Oxidative homolysis of a nitrosylchromium complex.

Metal(III)-polypyridine complexes M(NN)(3) (M = Ru or Fe; NN = bipyridine (bpy), phenanthroline (phen), or 4,7-dimethylphenanthroline (Me(2)-phen)) oxidize the nitrosylpentaaquachromium(III) ion, Cr(aq)NO, with an overall 4:1 stoichiometry, 4 Ru(bpy)(3) + Cr(aq)NO + 2 H(2)O --> 4 Ru(bpy)(3) + Cr(aq) + NO(3)(-) + 4 H(+). The kinetics follow a mixed second-order rate law, -d[M(NN)(3)]/dt = nk[M(NN)(3)][Cr(aq)NO], in which k represents the rate constant for the initial one-electron transfer step, and n = 2-4 depending on reaction conditions and relative rates of the first and subsequent steps. With Cr(aq)NO in excess, the values of nk are 283 M(-1) s(-1) (Ru(bpy)(3)), 7.4 (Ru(Me(2)-phen)(3)), and 5.8 (Fe(phen)(3)). In the proposed mechanism, the one-electron oxidation of Cr(aq)NO releases NO, which is further oxidized to nitrite, k = 1.04x10(6) M(-1) s(-1), 6.17x10(4), and 1.12x10(4) with the three respective oxidants. Further oxidation yields the observed nitrate. The kinetics of the first step show a strong correlation with thermodynamic driving force. Parallels were drawn with oxidative homolysis of a superoxochromium(III) ion, Cr(aq)OO, to gain insight into relative oxidizability of coordinated NO and O(2), and to address the question of the "oxidation state" of coordinated NO in Cr(aq)NO.