A major challenge in lanthanide (Ln) and actinide (An) chemistry is the characterization of the valence f-shell properties, in particular the identification and quantification of Ln 4f and An 5f covalency. Therefore, f-shell covalency is the focus of many on-going experimental, theoretical, and joint efforts. This knowledge is of paramount importance for a wide range of applications, such as for the rational design of new felement materials. The quantification of covalency is also directly associated with the assignment of the metal oxidation numbers, which are sometimes hotly contested in felement chemistry. For example, the literature about the Ce oxidation number (ON) of cerocene (+III or +IV ?) and other formal Ce(IV) complexes stretches over many decades. Another example of an f-element system with contested metal ON is PuO 2 , which is the subject of the present study.