Actinide Tris(tetramethylcyclopentadienide) Complexes: Bonding and Electronic Structures Across the Isostructural [An(C5Me4H)3] (An = Th, U, Np, Pu, Am) Series

Abstract

Abstract

Understanding how metal–ligand bonding evolves across the actinide series is central to the rational development of separations, nuclear fuel cycle, and waste management strategies, yet systematic experimental studies remain scarce owing to the difficulty of handling transuranium elements. Here we report the synthesis and isolation of [NpIII(Cptet)3] and [PuIII(Cptet)3] (Cptet = {C5Me4H}), which complete a rare isostructural series of trivalent triscyclopentadienide complexes [MIII(Cptet)3] spanning five early actinides (M = Th, U, Np, Pu, Am). Metal–carbon distances from thorium to americium show remarkably little variation, despite the decrease in metal ionic radius across this series, and unlike previously reported lanthanide analogues with similar metal ionic radii, which rules out steric congestion as the cause. Density functional theory and multi-reference ab initio calculations reveal that the 5𝑓 orbitals stabilise and contract markedly across the series, crossing below the ligand π-dominant manifold by americium. Topological analysis of the electron density exposes a polar covalent metal–ligand interaction that peaks around uranium/neptunium. These factors are tensioned across the series, such that covalency-induced bond length shortening is key for early actinides, while for later examples, the ionic bonding precludes bond length contraction due to electron-electron repulsion between the ligands and the non-bonding metal 5𝑓 electrons. UV-Vis-NIR spectroscopy, supported by CASSCF/RASSCF calculations, tracks the spectroscopic consequences of this electronic reorganization. Together, these results provide a self-consistent experimental and computational benchmark for electronic structure evolution across the first half of the actinide series within a single, structurally invariant ligand platform.