Because of the interplay of LM donation and back-donation, actinide ONs can be ambiguous. A recent example is UCN@C 82 , an endohedral fullerene in which the uranium ON can be assigned as +III or +I depending on the interpretation of the ground state. As another example, the system [U(C 7 H 7 ) 2 ] -has long been of interest in the felement field. When conceptualizing the ligand as a 10-electron aromatic (C 7 H 7 ) 3-, the complex is formally U(V), but when taking the strong donation from the aryl ligands into consideration an assignment as U(III) would also be appropriate. Unless ON counting rules are applied naively, it is therefore a matter of interpretation and choice how the ON in an f-element system should be assigned when taking pronounced LM donation (and, potentially, back-donation) into account. At the core of the issue lies the distinction between n open and n f . As mentioned in the Introduction, f-element ONs are often taken to be the same as the nf-shell electron counts. Even then, however, the question remains whether the ON should conform to the formal electron count, n open , or to some measure of the electron count in the presence of the ligands as determined by integration of the electron density and some criterion by which to assign part of the density to the metal ion and its different valence shells (n f ).