Kinetics of the oxidation of substrate ligands by transition-metal cations. The oxidation of iodide ions by aquacations

Abstract

The kinetics of the oxidation of iodide ions by Ce^IV_aq have been investigated. The reaction is found to be first order in each of [Ce^IV], [I^–] and [H⁺], in contrast to the oxidation of Br^– by Ce^IV_aq, which is second order in both [Br^–] and [H⁺]. Nevertheless, the direct order of one in [H⁺] found for the Ce^IV+ I^– reaction can only be interpreted by assuming the involvement of intermediate Ce⁴⁺–I^– complexes. The second-order rate constant for the oxidation of I^– by Co^III_aq is shown to vary with ionic strength, so the enthalpy ΔH^* and entropy ΔS^* of activation have been determined in conditions where the rate is invariant with ionic strength. These values for ΔH^* and ΔS^* are compared with the values for other substrates. The oxidation of iodide ions is discussed in relation to a general mechanism for the oxidation of substrate ligands by aquatransition-metal cations.