ADF reveals unusual uranium–carbon and thorium–carbon multiple bonds inside a fullerene

Multiple bonds between actinides and carbon are difficult to stabilize, particularly for thorium, where strongly polarized metal–ligand interactions complicate the formation of higher-order bonds. Fullerene cages offer a promising way to stabilize otherwise elusive bonding motifs and investigate their electronic structure.

In a recent JACS study, Meng and co-workers synthesized a heterobimetallic uranium–thorium carbide cluster encapsulated inside a C₇₈ fullerene. Single-crystal X-ray diffraction revealed exceptionally short U–C and Th–C distances, suggesting unusual multiple-bond character. To understand the nature of these interactions, the researchers combined experimental characterization with density functional theory (DFT) calculations using ADF.

The ADF calculations reproduced the experimentally observed structure and provided a detailed picture of the electronic interactions within the U–C–Th cluster. Using Natural Localized Molecular Orbital (NLMO) analysis and Wiberg bond orders, the authors identified one σ-bonding interaction and two three-center π-bonding interactions involving each metal. The analysis supports substantial triple-bond character for U–C and partial triple-bond character for Th–C, with the latter exhibiting considerably stronger polarization toward carbon.

Importantly, the calculations reveal how differences in orbital contributions and bond polarization distinguish uranium–carbon from thorium–carbon bonding. This illustrates how computational bonding analysis can go beyond experimental bond lengths to clarify unusual electronic structures in heavy-element chemistry.

Technically, AMS2023 was used for all-electron DFT calculations with PBE, PBE0, and B3LYP functionals, TZP Slater-type basis sets, scalar-relativistic ZORA, and D3(BJ) dispersion corrections. NLMO analysis was performed using NBO6 interfaced with AMS.