The controversy regarding PuO 2 , viz., 5f 4 vs. 5f , and related debates about the metal ONs in other Ln and An systems reflect, in part, the fact that for better or worse the chemical concepts of ON or the charge of an atom in a molecule (formal or otherwise) do not have 'hard' reductionist definitions in quantum mechanics (QM)-there are no self-adjoint QM operators representing these chemical concepts. These conceptual issues, paired with the approximations that are necessary in all electronic structure calculations, means that quantum chemical calculations alone are often not sufficient to establish f-element ONs. X-ray wavelength spectroscopic methods such as X-ray absorption near edge structure (XANES), core-to-core or valance-band resonant inelastic X-ray scattering (CC-RIXS, VB-RIXS), and related techniques, have emerged as key experimental tools in this field, especially when paired with quantum theoretical calculations. A 2021 XANES and theoretical study of Pu and PuO 2 concluded that there is comparatively little 5f-shell covalency in PuO 2 , similar to what was noted in the earlier magnetism study, which appears to be at odds with the 5f assignment from Reference 7. In addition to addressing the n open vs. n f question, we show herein how this apparent discrepancy came about.