An electrostatic approach to the structure of hydrated lanthanoid ions. [M(OH2)9]3+versus[M(OH2)8]3+

Abstract

The bond energies of ennea- and octa-aqua-coordinated polyhedra of lanthanoid ions have been calculated on the electrostatic model. Sums of coordination bond energies and Born hydration enthalpies of the aqua complexes predict an exclusive stabilization of the enneahydrate regardless of whether the central metal ion is of the earlier or later elements of the series. However, the difference between the hydration free energies evaluated for octa- and ennea-hydrate ions shows that there is a preference to form the enneahydrate for the earlier elements, but the octahydrate is formed for the later elements. A small difference in hydration free energy results in the transitional coordination number for the elements near to gadolinium. As an application of the present model, the equilibrium constant for the 5dâ†� 4f excited states of [Ce(OH₂)₈]³⁺ and [Ce(OH₂)₉]³⁺ in aqueous media have been calculated.