Some fast reactions in mixed solvents

The stability constants and the rate constants for the formation of the one-to-one complexes of nickel and several of the lanthanides with singly charged murexide in H[lowered 2]O, 25% DMSO-H[lowered 2]O, 50% DMSO-H[lowered 2]O and 50% ethanol - H[lowered 2]O mixed solvents have been determined at different temperatures. The formation constants were calculated from spectrophotometric data and the kinetic studies were made on a temperature jump apparatus. The thermodynamic and kinetic parameters obtained from these measurements for the nickel system are interpreted in terms of dielectric constant of the mixed solvents and intermolecular interactions in the solvents. The slowness of nickel murexide complexation reactions compared to other nickel systems reported in the literature is explained by the fast stepwise dissociation of metal-ligand bonds rather than by the chelate ring closure mechanism. After analysis of these data and some literature values for the lanthanide ligand substitution reactions, it is concluded that the process of eliminating the water molecule from the solvated metal ions is coupled to an outer sphere ion pair formation reaction and may be subject to a ligand effect.