Kinetics and mechanism of the oxidation of a ternary complex involving nitrilotriacetatocobaltate(II) and malonic acid by N-bromosuccinimide

The kinetics of oxidation of Co^IINM(H₂O)]^3– (N = nitrilotriacetate, M = malonate) by N-bromosuccinimide (NBS) in aqueous solution have been found to obey the equation: dCo^III]/dt = k ₁ K ₂NBS]Co^II]_T/{1 + K₂NBS] + (H⁺/K₁)} where k ₁ is the rate constant for the electron transfer process, K ₁ the equilibrium constant for dissociation of Co^IINM(H₂O)]^3– to Co^IINM(OH)]^4– + H⁺, and K ₂ the pre-equilibrium formation constant. Values of k ₁ = 1.07 × 10^–3 s^–1, K ₁ = 4.74 × 10^–8 mol dm^–3 and K ₂ = 472 dm³ mol^–1 have been obtained at 30 °C and I = 0.2 mol dm^–3. The thermodynamic activation parameters have been calculated. The experimental rate law is consistent with a mechanism in which the deprotonated Co^IINM(OH)]^4– is considered to be the most reactive species compared to its conjugate acid. It is assumed that electron transfer takes place via an inner-sphere mechanism.