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

The kinetics of oxidation of [(CoNM)-N-II(H2O)](3-) (N = nitrilotriacetate, M = malonate) by N-bromosuccinimide (NBS) in aqueous solution have been found to obey the equation: d[Co-III]/dt = k(1)K(2)[NBS][Co-II](T)/{1 + K-2[NBS] + (H+/K-1)} where k(1) is the rate constant for the electron transfer process, K-1 the equilibrium constant for dissociation of [(CoNM)-N-II(H2O)](3-) to [(CoNM)-N-II(OH)](4-) + H+, and K-2 the pre-equilibrium formation constant. Values of k(1) = 1.07 x 10(-3) s(-1), K-1 = 4.74 x 10(-8) mol dm(-3) and K-2 = 472 dm(3) mol(-1) have been obtained at 30 degreesC 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 [(CoNM)-N-II(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.