Role of metals in enzymatic reactions. Part 4.—Kinetics of complex formation in model systems involving cobalt(II) and pyridine-2-azo-p-dimethylaniline

Abstract

The temperature-jump relaxation method has been used to measure the rate constants and activation parameters for the formation and dissociation of the 1 : 1 complex between cobalt(II) and pyridine-2-azo-p-dimethylaniline (PADA) and of the ternary complexes between PADA and the cobalt(II)-iminodiacetate, -nitrilotriacetate and -polytriphosphate complexes. Charge on the first ligand has little influence on the reaction of cobalt(II) with the second ligand. The stability constant for the metal-PADA complex is almost invariant across the series, but rate constants and activation enthalpies vary markedly. Implications of these results with respect to the role of cobalt(II) in enzymatic reactions are discussed.