Manganese(II)−Superoxide Complex in Aqueous Solution

Mn(II) aq −superoxide complex, MnO 2 +, was formed in pulse radiolysis by three distinct routes: Mn(I) + O 2, Mn(II) + O 2 - and Mn(III) + H 2 O 2 . The stability of this complex was found to be governed by the two equilibria: Mn 2+ + O 2 - ⇌ MnO 2 + (1,−1) and Mn 2+ + HO 2 ⇌ MnO 2 + + H + (6,−6). Both forward and reverse rate constants of the reactions involved in these equilibria were determined: k 1 = (1.5 ± 0.2) × 10 8 M -1 s -1, k - 1 = (6.5 ± 1.0) × 10 3 s -1; k 6 = (1.1 ± 0.2) × 10 6 M -1 s -1, k - 6 = (6.5 ± 1.0) × 10 6 M -1 s -1, yielding K 1, - 1 = (2.3 ± 0.5) × 10 4 M -1 and K 6, - 6 = 0.17 ± 0.06. The metal−oxy complex MnO 2 + decays by self-reaction with k (MnO 2 + + MnO 2 + ) = (6.0 ± 1.0) × 10 6 M -1 s -1 and in acid solutions also by reaction with HO 2, k (MnO 2 + + HO 2 ) = (1.0 ± 0.3) × 10 7 M -1 s -1 . In both cases stoichiometric amounts of H 2 O 2 are formed as the end product. Mn(I) was formed by reduction of Mn 2+ with H atoms. It has an absorption spectra with maxima at 290 and 340 nm with ε 290 = 1300 ± 200 M -1 cm -1 and ε 340 = 1000 ± 150 M -1 cm -1 . It reacts with oxygen with k (Mn(I) + O 2 ) = (6.0 ± 1.0) × 10 6 M -1 s -1 . Mn(III) reacts with hydrogen peroxide with k (Mn(III) + H 2 O 2 ) = (2.8 ± 0.3) × 10 3 M -1 s -1 .