Unraveling the Sc3+ Hydration Geometry: The Strange Case of the Far-Coordinated Water Molecule

The hydration structure and dynamics of Sc3+ in aqueous solution have been investigated using a combined approach based on quantum mechanical (QM) calculations, molecular dynamics (MD) simulations, and extended X-ray absorption fine structure (EXAFS) spectroscopy. An effective Sc–water two-body potential has been generated from QM calculations and then used in the MD simulation of Sc3+ in water, and the reliability of the entire procedure has been assessed by comparing the theoretical structural results with the EXAFS experimental data. The outstanding outcome of this work is that the Sc3+ ion forms a well-defined capped square antiprism (SAP) complex in aqueous solution, where the eight water molecules closest to the ion are located at the vertexes of a SAP polyhedron, while the ninth water molecule occupying the capping position is unusually found at a very long distance from the ion. This far-coordinated water molecule possesses a degree of structure comparable with the other first shell molecules surrounding the ion at much shorter distances, and its presence gave us the unique opportunity to easily identify the geometry of the Sc3+ coordination polyhedron. Despite very strong ion–water interactions, the Sc3+ hydration shell is very labile, as the far-coordinated ligand allows first shell water molecules to easily exchange their positions both inside the solvation shell and with the rest of the solvent molecules.