Electron transfer mechanism for the oxidation of ternary N-(2-acetamido)iminodiacetatocobaltate(II) complexes involving succinate and maleate as secondary ligands by periodate

Abstract

The kinetics of oxidation of the ternary complexes [Co^II(ADA)(Su)(H₂O)]²⁻ and [Co^II(ADA)(Ma)(H₂O)]²⁻ (ADA = N-(2-acetamido)iminodiacetate, Su = succinate and Ma = maleate) by periodate have been investigated spectrophotometrically at 580 nm under pseudo-first-order conditions in aqueous medium over 30–50 °C range, pH 3.72–4.99, and I = 0.2 mol dm⁻³. The kinetics of the oxidation of [Co^II(ADA)(Su)(H₂O)]²⁻ obeyed the rate law d[Co^III]/dt = [Co^II(ADA)(Su)(H₂O)]²⁻[H₅IO₆] {k ₄ K ₅ + (k ₅ K ₆ K ₂/[H⁺)}, and the kinetics oxidation of [Co^II(ADA)(Ma)(H₂O)]²⁻ obeyed the rate law d[Co^III]/dt = k ₁ K ₂[Co^II] _T [I^VII] _T /{1 + ([H⁺]/K ₇) + K ₂[I^VII] _T }. The pseudo-first-order rate constant, k _obs, increased with increasing pH, indicating that the hydroxo form of maleate complex, [Co^II(ADA)(Ma)(OH)]³⁻, is the reactive species. The initial Co(III) products were slowly converted to the final products, fitting an inner-sphere mechanism. Thermodynamic activation parameters were calculated using the transition state theory equation. The initial cobalt(II) complexes were characterized by physicochemical and spectroscopic methods.