Actinide 5f Occupations: The Case of PuO₂

In actinide chemistry, the formal number of open-shell 5f electrons, n_open, is a well-defined quantity with an integer value. The effective 5f occupation, n_f, additionally takes donation and back-donation into account, generally has a non-integer value, and has varying numerical definitions. The present study explores the important distinction between n_f and n_open in actinide chemistry with the example of PuO₂, by using electronic structure methods with a relativistic Hamiltonian in combination with experimental Pu M₅-edge high-energy-resolution X-ray absorption and emission spectroscopic data. The total donation to the metal in PuO₂ is between 3.1 and 2.4 electrons, depending on the type of calculation, most of which is to the Pu 6d and 5f shells. The donation into 5f is sensitive to the approximations in the electronic structure model but likely amounts to 1.6/0.8 electrons when the diffuse regions of the 5f shell are included/excluded. Valence band resonant inelastic X-ray scattering experiments demonstrate that Pu 5f electron density is present in the valence band; thus, there is a clear experimental signature of covalent bonding in PuO₂. Pu M₅-edge and M₃-edge high-energy-resolution X-ray absorption near-edge structures for Pu³⁺ and Pu⁴⁺ in an aqueous solution are compared to PuO₂, showing that Pu in PuO₂ has an n_f closer to Pu⁴⁺(aq).