Oxidation state and covalency in f-element metallocenes (M = Ce, Th, Pu): a combined CASSCF and topological study

Andrew Kerridge

doi:10.1039/c3dt52279b

Abstract

CASSCF calculated wavefunctions are presented for three f-element metallocenes, MCOT2 (M = Ce, Th, Pu; COT = η(8)-C8H8). The configurational admixture of these systems is investigated and, where the ThCOT2 ground state is well-defined as a monodeterminantal Th(IV) state, the cerocene ground state is found to be strong multiconfigurational and to bear strong similarities to that of plutonocene. Associated electronic densities are studied using QTAIM topological analysis and compared to CASSCF-derived densities of the aromatic systems benzene and the COT dianion. This analysis provides evidence of enhanced covalent character in plutonocene, supporting structural data calculated previously. Evidence of charge localisation in found in cerocene, this being most pronounced in its excited state of A(g) symmetry. QTAIM analysis reveals that the ligand electronic structure is very similar in all metallocenes, and density differences show little variation in the ligand between the cerocene ground and excited state. Orbital contributions to integrated QTAIM properties are considered, and excellent agreement with experimentally determined f-orbital occupation is obtained. All methods of analysis support a Ce(IV) or mixed valence assignment of the cerocene ground state, whereas the A(g) excited state is best described as a Ce(III) state.

Highlights

Metal–ligand covalency is difficult to quantify in complexes containing open shell ions,[1,2] with traditional views based on the mixing of degenerate orbitals failing in the presence of strong electron correlation
The results presented here demonstrate, in a quantifiable manner, that plutonocene exhibits larger metal– ligand covalency than thorocene, whilst such covalent character is weakest in cerocene
Both plutonocene and cerocene exhibit populations close to 1, with the cerocene excited state population

Summary

Introduction

Metal–ligand covalency is difficult to quantify in complexes containing open shell ions,[1,2] with traditional views based on the mixing of (near) degenerate orbitals failing in the presence of strong electron correlation. This (static) correlation manifests itself in the form of multiconfigurational character in the electronic wavefunction, and the independent particle approximation breaks down. Current methods for the separation of minor actinides from lanthanides in spent nuclear fuel are based on the fact that the more radially extended 5f shell of the former results in enhanced covalent interactions with soft donor ligands[3] when compared to those of the latter, which are characterised by a contracted, core-like, 4f shell.

Methods

Results

Conclusion