Kinetics and mechanism of oxidation of thiosulphate by the complex ion [Mn3IV(μ-O)4(phen)4(H2O)2]4+ (phen=1,10-phenanthroline) in weakly acidic aqueous media

The trinuclear complex ion [Mn-3(IV)(mu -O)(4)(phen)(4)(H2O)(2)](4+) (1) is quantitatively reduced by an excess of S2O32- to Mn-II, but the binuclear complex [(MnMnIV)-Mn-III(mu -O)(2)(phen)(4)](3+), (2) is the only manganese product when [S2O32-] = 1.5 [(1)]. With an excess of S2O32- biphasic kinetics, (1) (k(1)(0)) under right arrow (2) (k(2)(0)) under right arrow Mn-II is observed, while the reaction with S2O32- = 1.5 [(1)], follows one-step second order kinetics with the second order rate constant k = k(1)(0)/[S2O32-]. The rate constant k(1)(0) is independent of c(phen) ( = [phen] + [Hphen(+)]) but directly proportional to [H+] and [S2O32-]. Rapid formation of an adduct between (1) and S2O32-, followed by rate-determining one-electron, one-proton reduction of the adduct, appears logical. A comproportionation reaction in one of the subsequent rapid steps leads to (2) without any Mn-II co-product. Kinetic dependences for the second step are same to those for an authentic complex of (2), and further support the assigned reaction sequence.