The kinetics and mechanism of the decomposition reaction of the bis(oxalato)manganese(III) complex in an aqueous solution

Article abstract of DOI:10.1246/bcsj.63.151

The kinetics and mechanism of the decomposition of the bis(oxalato)manganase(III) complex ion ([Mn(ox)₂]^-) were studied in acid solution in the absence and in the presence of oxygen at temperatures from 10 to 35°C. The decomposition reaction of [Mn(ox)₂]^- in the absence of oxygen was described by the first-order rate law of -d[[Mn(ox)₂]^-]/dt=k(obsd)[[Mn(ox)₂]^-], where the observed rate constant, k(obsd), increased proportionally with the increasing hydrogen-ion concentrations, being expressed as k(obsd)=k[H⁺] in the [H⁺] range of 0.006-0.1 M. The enthalpy and entropy changes of activation (ΔH≠ and ΔS≠) were 73.4±2.0 kJ mol^-1 and -8.6±0.2 J K^-1 mol^-1 respectively. The rate of the decomposition of [Mn(ox)₂]^- decreased greatly upon the addition of a radical scavenger for CO₂^-. such as [Co(NH₃)₆]³⁺, [CoCl(NH₃)₅]²⁺, or molecular oxygen. In the presence of oxygen, the rate deviated greatly from the first-order rate law. On the other hand, the addition of [Co(NH₃)₆]³⁺ or [CoCl(NH₃)₅]²⁺ in the absence of oxygen did not change the first-order rate law, but did decrease the rate of reaction up to 40% of that in the absence of the radical scavenger. The mechanisms for the decomposition reaction of [Mn(ox)₂]^- are discussed in the light of the results obtained.