Abstract:
Our previous x-ray absorption fine structure studies of aqueous solutions revealed relaxation of the
structure of complexes of Zn21, Fe21, La31, and Yb31 ions with increasing temperature. These
complexes in general exhibit reduction of cation–ligand bond lengths with increasing temperature
due to hydrogen bond breaking and loss of water of solvation. Because this results in an overall
lowering of the equilibrium state of the complex, we refer to the variation of the structure as
relaxation. In the case of M(H2O)m
n1 (m56 – 9 at room temperature, n52, 3) aquo ion complexes
~M: Zn, Fe, La, Yb!, there is a similar reduction in the number of coordinating water molecules with
temperature. The relaxation of the structure is shown to result in a lowering of the binding energy
per cation–ligand pair of a complex with increasing temperature. A comparison of the rate of
structure relaxation with temperature, which is framed in the context of volume thermoelastic
constriction, shows this quantity to have a gradual and direct dependence on the difference in
Pauling’s electronegativity of the cation–ligand pairs making up the ion complex.
