Kinetics and mechanism of fast metal–ligand substitution processes in aqueous and micellar solutions studied by means of a dye laser photochemical relaxation technique

Abstract

Rapid metal–ligand substitution reactions have been studied in aqueous and sodium lauryl sulphate (SLS) micellar solutions by means of a dye-laser induced photochemical relaxation method. Perturbation of the system involving Ni²⁺(aq) and the bidentate nitrogen ligand PADA produces two relaxation effects in both aqueous and micellar solutions. The faster relaxation process is concentration independent and is identified with a ring-closure reaction to reform the initial Ni(PADA)²⁺ complex. Rate constants and activation energies for this process are found to be similar in both media. The slower relaxation time is associated with the recombination reaction between Ni²⁺(aq) and PADA, for which k^298.2_f in water is found to be 1.4 × 10³ dm³ mol^–1 s^–1, which is in good agreement with previous measurements. In the surfactant solutions, the rate of complexation is greatly enhanced close to the c.m.c. followed by a decrease as the concentration of SLS is further increased. This is due to the increased local concentration of Ni²⁺ at the micelle surface as the c.m.c. is approached. It is shown that it may not be valid to make the assumption k_RCk_–2 in the kinetic scheme of the reaction. However, the effect on k_f is small, such that k_f is still dominated by the rate of loss of water from the metal ion.

The faster reaction between Zn²⁺(aq) and PADA was also studied, but only one concentration dependent relaxation time was observed. This was consistent with the complex re-formation reaction and k^298.2 was found to be 6.4 × 10⁶ dm³ mol^–1 s^–1.