A kinetic study of some rare earth ligand substitution reactions

The solution chemistry of the rare earth ions has been the subject of many discussions in the chemical literature. This is the result of the unusual variations observed in the stability of rare earth complexes with varying atomic number of the central ion involved. This type of behavior has also been observed in a number of independent thermodynamic studies of different rare earth complexes. A number of suggestions have been advanced in explanation of these variations. In an attempt to clarify the situation with respect to these complexes, the kinetics of the complex formation reactions between rare earth ions and the anions derived from acetylacetone and oxalic acid were studied by a relaxation technique—the pressure-jump method—which depends on the behavior of the equilibrium system after a sudden change in external conditions. Rate constants for the reaction of the anions with many of the rare earth ions were obtained and, in addition, activation parameters were determined for several of the rare earth/oxalic acid systems. These data are interpreted in terms of a model for the hydrated ions and variations are explained in terms of changes in solvent structure. The model suggested is shown to correllate well with a large body of experimental data.