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, σ2, and their ligand exchange reaction rate constant, k1, in aqueous solution. A good correlation between these two factors was obtained for various transition metal ions. The relation between σ2 and log k1 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 σ2 and log k1, which is in accordance with the experimental results. It is also found that k1 correlates not only with σ2 but also a third order cumulant in EXAFS, C3; the greater the k1 value, the greater the increase in C3 with temperature. The behavior of C3 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.