BOUND-LIGAND EFFECT IN THE COMPLEXATION KINETICS OF HYDROXOZIRCONIUM(IV) AND HYDROXOHAFNIUM(IV) COMPLEXES

The kinetics of the complexation reaction of zirconium(IV) and hafnium (IV) with Semixylenol Orange (H(4)L) have been studied spectrophotomet rically by means of a stopped-flow technique. The rate of formation of the complex is expressed as d[MHL(+)]/dt = H(5)L(+)][H+](-3)-k(3)[MHL (+)][H+]-K-4[MHL][H+](2) for both metal ions (M(4+)). With values of k (1) and k(2), assuming a dissociative interchange mechanism for the fi rst two reaction paths in this rate equation, the rate constants of th e loss of coordinated water molecules at MOH(3+) and M(OH)(2)(2+) were estimated. With these rate constants, the gamma parameters for both m etal ions, which describe the acceleration effect of the bound ligand A (A = OH- in this study) on the water exchange rate at MA, were also estimated. With the same parameter, some possibilities to design a new kinetic-catalytic method of analysis based on ligand-substitution rea ctions involving zirconium(IV) were proposed.