• Relaxation of the structure of simple metal ion complexes in aqueous solutions at up to supercritical conditions

Our previous x-ray absorption fine structure studies of aqueous solutions revealed relaxation of the structure of complexes of ${\mathrm{Zn}}^{\text{2+}},$ ${\mathrm{Fe}}^{\text{2+}},$ ${\mathrm{La}}^{\text{3+}},$ and ${\mathrm{Yb}}^{\text{3+}}$ 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 $\mathrm{M}({\mathrm{H}}_2\mathrm{O}{)}_m^{\mathrm{n+}}$ $\text{(m=6–9}$ at room temperature, $\text{n=2,\hspace{0.5em}3)}$ 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.