In the remainder of this article we explore the electron donation in the ground state of PuO 2 by a combination of theoretical calculations at the multi-configuration wavefunction theory (MC-WFT) and Kohn-Sham density functional theory (DFT) levels and experimental spectroscopy. It is shown that the 5f n f depends on the approximations used for the calculations. A Pu n f around 5 is consistent with the donation in the system, which includes considerable donation into the diffuse regions of the Pu 5f shell, along with additional donation into the 6d and 7s shells. Despite the considerable excess 5f electron count, we advocate for the description of PuO 2 as featuring a 5f plutonium ion, because the magnetism and the system's low-energy valence electronic states as well as the X-ray spectroscopic data reported herein are consistent with this description. The distinction between n open and n f highlighted in this work has implications for actinide and lanthanide chemistry more generally.