Oxidative Homolysis of a Nitrosylchromium Complex.
April 12th, 2008 | by admin |Oxidative Homolysis of a Nitrosylchromium Complex.
Metal(III)-polypyridine complexes [M(NN)(3)](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](2+), with an overall 4:1 stoichiometry, 4 [Ru(bpy)(3)](3+)+[Cr(aq)NO](2+)+2 H(2)O–>4 [Ru(bpy)(3)](2+)+[Cr(aq)](3+)+NO(3) (-)+4 H(+). The kinetics follow a mixed second-order rate law, -d[[M(NN)(3)](3+)]/dt=nk[[M(NN)(3)](3+)][[Cr(aq)NO](2+)], 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](2+) in excess, the values of nk are 283 M(-1) s(-1) ([Ru(bpy)(3)](3+)), 7.4 ([Ru(Me(2)-phen)(3)](3+)), and 5.8 ([Fe(phen)(3)](3+)). In the proposed mechanism, the one-electron oxidation of [Cr(aq)NO](2+) releases NO, which is further oxidized to nitrite, k=1.04×10(6) M(-1) s(-1), 6.17×10(4), and 1.12×10(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](2+), 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](2+).
Song W, Bakac A.
Iowa State University, Ames, IA 50011, USA, Fax: (+1) 515-294-5233.