Kinetics of oxidation of nitrogen(III) by an oxo-bridged dinuclear manganese(III,IV) complex in weakly acidic media

The heterovalent oxo-bridged dinuclear manganese(III,IV) complex [Mn2O2(phen)(4)](3+) 1 aquates to [Mn2O2(phen)(3)(H2O)(2)](3+) 2 (phen = 1,10-phenanthroline) in phen-Hphen(+) buffer (pH 4-5). The equilibrium constant for the reaction was kinetically evaluated as K-1H = 35 +/- 10. Nitrite ion formed adducts 1N and 2N with 1 and 2 respectively. Their respective association constants, K-1N = 14.1 +/- 2 dm(3) mol(-1) and K-2N = 50 +/- 10 dm(3) mol(-1), are small and indicate the outer-sphere nature of the adducts. Adduct 2N is reduced via one-electron steps, ultimately to Mn-II with a rate constant k(2N) = (3.33 +/- 0.6) x 10(-2) s(-1) at 30.0 degrees C and I = 0.10 mol dm(-3) for the first step. No kinetic activity of 1N was detectable under the experimental condition employed. Added Mn2+ has been found to scavenge phen from the reaction media and thus promotes the formation of 2N and hence the reaction rate.