f-Orbital covalency in the actinocenes (An = Th–Cm): multiconfigurational studies and topological analysis

Abstract

The CASSCF methodology is used to calculate the ground state electron densities of a series of seven actinocenes, AnCOT₂ (An = Th–Cm, COT = η⁸-C₈H₈). The multiconfigurational character of these complexes is found to be substantial and topological analysis of the electron density via the QTAIM approach is therefore chosen in order to investigate the electronic structure in more detail. Topological analysis reveals increased values of the electron density at the An–C bond critical point for An = Pa–Pu, suggesting enhanced covalent character in metal–ligand bonding for these complexes. In order to investigate the origins of this covalency, integrated one- and two-electron properties are evaluated. A trend for increased electronic charge, spin density and electron localisation on the An centre as one traverses the actinide series is found. The difference between atomic number and the electron localisation index is considered and found to correlate well with the expected oxidation state in these complexes, with a tendency towards trivalent character for the later actinides. Total and orbitally resolved delocalisation indices are evaluated, and increased electron delocalisation is found for the complexes containing Pa–Pu centres. It is shown that, while 5f contributions to covalency in these complexes are smaller in magnitude than 6d contributions, the variation in covalency is almost entirely accounted for by the variation in the 5f contribution.