EXAFS DEBYE-WALLER FACTOR AND LIGAND-EXCHANGE REACTION OF HYDRATED METAL-COMPLEXES

The EXAFS (Extended X-Ray Absorption Fine Structure) spectra of hydrated transition metal complexes were measured to investigate the relation between the Debye-Waller factor in EXAFS, sigma(2), and their ligand exchange reaction rate constant, k(1), in aqueous solution. A good correlation between these two factors was obtained for various transition metal ions. The relation between sigma(2) and log k(1) is studied theoretically using the Arrhenius theory. The introduction of an anharmonic Morse function as the interaction potential was found to lead to a simple expression connecting sigma(2) and log k(1), which is in accordance with the experimental results. It is also found that k(1) correlates not only with sigma(2) but also a third order cumulant in EXAFS, C-3; the greater the k(1) value, the greater the increase in C-3 With temperature. The behavior of C-3 implies that the anharmonicity in the potential of the metal-oxygen interaction must be taken into consideration for the study of the ligand exchange reaction.