Trends in actinide electronic structure revealed from asymmetric, isostructural transuranic metallocenes

The study of actinide electronic structure and bonding within rigorously controlled environments is fundamental to advancing nuclear applications. Here, we report a new set of isostructural actinide organometallics; An(COT^big)₂, (An = Th, U, Np, and Pu), where COT^big is the bulky 1,4-bis(triphenylsilyl)-substituted cyclooctatetraenyl dianion (1,4-(Ph₃Si)₂C₈H₆)^2-. The actinide(IV) metallocene sandwiches have a clam-shell structure, offering a new molecular symmetry to explore f-orbital contributions in bonding. Combined experimental and computational studies reveal that An(COT^big)₂ complexes strongly differ from the previously published coplanar An(COT)₂ sandwiches due to the bent geometry and electron-withdrawing nature of the substituents. While COT^big displays comparatively weaker electron donation, the low-energy f-f transitions in An(COT^big)₂ have increased molar absorptivity consistent with the removal of the parity selection rule and better energetic matching between ligand and actinide 5f orbitals as the series is traversed. For Pu(COT^big)₂, covalent mixing of donor 5f metal orbitals and the ligand-π orbitals is especially strong.