Inner-sphere oxidation of ternary nitrilotriacetatocobalt(II) complexes involving oxalate and phthalate by periodate

The oxidation of Co^II(nta)(ox)(H₂O)₂]³⁻ and Co^II(nta)(ph)(H₂O)₂]³⁻ (nta = nitrilotriacetate, ox = oxalic acid and ph = phthalic acid) by periodate have been studied kinetically in aqueous solution over 20–40 °C and a variety of pH ranges. The rate of oxidation of Co^II(nta)(ox)(H₂O)₂]³⁻ by periodate, obeys the following equation: dCo^III]/dt = Co^II(nta)(ox)(H₂O)₂³⁻]H₅IO₆] {k ₄ K ₅ + (k ₅ K ₆ K ₂/H⁺]} while the reaction of Co^II(nta)(ph)(H₂O)₂]³⁻ with periodate in aqueous acidic medium obeys the following rate law: dCo^III]/dt = k ₆ K ₈Co^II]_T I^VII]_T/{1 + H⁺]/K ₇ + K ₈I^VII] _T }. Initial cobalt(III) products were formed and slowly converted to final products, fitting an inner-sphere mechanism. Thermodynamic activation parameters have been calculated. A common mechanism for the oxidation of ternary nitrilotriacetatocobalt(II) complexes by periodate is proposed and supported by an excellent isokinetic relationship between ΔH* and ΔS* values for these reactions.