Disentangling Electronic Interactions in f7 Systems: A Comparative Study on GdIII, TbIV, CmIII, and BkIV

In this work, we demonstrate through computational spectroscopic analysis of four experimentally accessible f7 ions within complexes (GdIII, TbIV, CmIII, and BkIV) the extent to which electron-electron repulsion, spin-orbit coupling, and the identity of the ligand field influence the energy and intermediate coupling of an f7 ion's low-lying excited states. As expected, the energy of excited states is primarily governed by electron-electron repulsion in the lanthanides (GdIII and TbIV), whereas spin-orbit coupling and ligand field effects play an increasingly important role in the actinides (CmIII and BkIV). Additionally, the stabilization caused by a ligand field is significantly greater for ions in higher oxidation states, as evidenced by the decreased energy of the emissive state and the nephelauxetic reduction. Finally, mixing Russell-Saunders terms in an intermediate coupling scheme is shown to be more significant in actinides and at higher oxidation states, with TbIV exhibiting signs of behaving spectroscopically like an actinide. These results reveal a clear correlation between composition, excited-state energy, and covalency, which supports a direct correlation between f-orbital covalency and the f-f multiplet structure. Furthermore, these findings contribute to spectroscopic studies on high-valent f-element complexes.