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 CeIVaq have been investigated. The reaction is found to be first order in each of [CeIV], [I–] and [H+], in contrast to the oxidation of Br– by CeIVaq, which is second order in both [Br–] and [H+]. Nevertheless, the direct order of one in [H+] found for the CeIV+ 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 CoIIIaq 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.