Structure of Hydrated Cerium(III) Ion in the Excited State

Abstract

Hydrated cerium ions in aqueous media, are predominantly in a tricapped trigonal prism ([Ce(OH₂)₉]³⁺). Upon 5 d←-4 f excitation, however, the excited hydrated ion dissoc iates one of the coordinated water molecules to form *[Ce(OH₂)₈]³⁺. The destablization of coordination bonds in the excited complex is attributable to an increase in the antibonding character of 5 d orbitals compared with 4f orbitals. The angular-overlap model calculations were carried out on various geometries of octaaquacerium ions by taking into account the spin-orbit coupling. The theory predicts that the lowest 5 d ←-4 f excited Kramers doublet of [Ce(OH₂)₈]³⁺ is energetically much lower than the lowest component state of [Ce(OH₂)₉]³⁺, provided that the Ce-O distance is common for [Ce(OH₂)₈]³⁺ and [Ce(OH₂)₉]³⁺. A ligand-field stabilization is obtained at the expense of the bond dissociation. In the lowest substate of ²D(5d¹), the promoted electron occupies the d_z2 orbital which extends in the direction of the C₃ symmetry axis. The excitation results in a shift of the vertex water molecules to the equatorial and thus one of the equatorial water molecules is eliminated. The structure of the excited octacoordinate species in solution must be square antiprism, dodecahedron and/or cube. Due to their small energy differences, however, the structure must be fluxional.