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 k298.2f 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 kRC[double greater-than, compressed]k–2 in the kinetic scheme of the reaction. However, the effect on kf is small, such that kf 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 k298.2 was found to be 6.4 × 106 dm3 mol–1 s–1.