Kinetics of aquation of Ni(II) oxydiacetate complex induced by Fe(III) ions

Abstract

The kinetics of the aquation reaction of [Ni(ODA)(H2O)3] (where ODA = oxydiacetate) promoted by [Fe(H2O)6]3+ ions were investigated. Spectrophotometry studies were carried out at temperatures within the range 293.15–313.15 K. The concentration of [Ni(ODA)(H2O)3] was 8.67 × 10–4 mol L–1 and the concentration of the [Fe(H2O)6]3+ ions was kept within the range 0.75 × 10–4–7.50 × 10–4 mol L–1. The observable reaction rate constants increased with the increase of [Fe3+] according to the rate law: kobs = k + k1K{[Fe(H2O)6]3+}. Based on the kinetic data, the activation parameters were determined for the spontaneous step of the reaction and for the step dependent on the concentration of Fe3+ ions. A mechanism for the aquation reaction has been proposed. In the first step (the reversible part of the aquation reaction) a dinuclear intermediate product is produced and in the next step the products of the reaction, ([Ni(H2O)6]2+ and [Fe(ODA)(H2O)3]+), are formed. Nevertheless, it has turned out that the spontaneous path of the aquation reaction is also possible concurrently (independent of the presence of Fe3+ ions). However, this process is too slow to be accurately measurable.