KINETICS AND MECHANISM OF COMPLEX-FORMATION BETWEEN CO(II) AND (3-NITROSALICYLATO)(TETREN)COBALT(III) ION IN AQUEOUS-MEDIUM

The kinetics of reversible complexation of Co(II) with (3-nitrosalicyl ato)(tetren)cobalt(III) ion has been investigated by the stopped flow techniques at 15-35-degrees-C, pH = 5.80-6.90 and I = 0.30 mol dm-3 (C lO4-). The formation of the binuclear species, (tetren)CoO2CC6H3(3NO2) OCo3+, occurs via the reaction of Co(OH2)6(2+) with the phenoxide form of the cobalt(III) substrate. At 25-degrees-C, k(f) = (8.29 +/- 0.07) x 10(4) dm3 mol-1 s-1, DELTAHnot-equal = 30.6 +/- 1.9 kJ mol-1, DELTA Snot-equal = -48 +/- 6 JK-1 mol-1, and k(r) = (35.9 +/- 0.4) s-1, DELT AHnot-equal = 52.7 +/- 1.9 kJ mol-1, DELTASnot-equal = -39 +/- 6 JK-3 mol-1, where k(f) and k(T) denote the rate constants for the formation and dissociation of the binuclear species, (tetren)CoO2CC8H3(3NC2)OCo 3-, respectively. Data are consistent with I(d) mechanism. The first b ond formation between Co(OH2)6(2+) and the coordinated salicylate of t he cobalt(III) substrate in the encounter complex is rate limiting. Th e binuclear species exists in dynamic equilibrium between its monodent ate and chelate form.