Unsupported non-bridged uranium–carbon double bonds have long been sought after in actinide chemistry as fundamental synthetic targets in the study of actinide-ligand multiple bonding. Here we report that, utilizing I_h(7)-C₈₀ fullerenes as nanocontainers, a diuranium carbide cluster, U=C=U, has been encapsulated and stabilized in the form of UCU@I_h(7)-C₈₀. This endohedral fullerene was prepared utilizing the Krätschmer–Huffman arc discharge method, and was then co-crystallized with nickel(II) octaethylporphyrin (Ni^II-OEP) to produce UCU@I_h(7)-C₈₀·[Ni^II-OEP] as single crystals. X-ray diffraction analysis reveals a cage-stabilized, carbide-bridged, bent UCU cluster with unexpectedly short uranium–carbon distances (2.03 Å) indicative of covalent U=C double-bond character. The quantum-chemical results suggest that both U atoms in the UCU unit have formal oxidation state of +5. The structural features of UCU@I_h(7)-C₈₀ and the covalent nature of the U(f¹)=C double bonds were further affirmed through various spectroscopic and theoretical analyses.

The nature of the actinide-actinide bonds is of fundamental importance to understand the electronic structure of the 5felements. It has attracted considerable theoretical attention, but little is known experimentally as the synthesis of these chemical bonds remains extremely challenging. Herein, we report a strong covalent Th-Th bond formed between two rarely accessible Th³⁺ions, stabilized inside a fullerene cage nanocontainer as Th₂@I_h(7)-C₈₀. This compound is synthesized using the arc-discharge method and fully characterized using several techniques. The single-crystal X-Ray diffraction analysis determines that the two Th atoms are separated by 3.816 Å. Both experimental and quantum-chemical results show that the two Th atoms have formal charges of +3 and confirm the presence of a strong covalent Th-Th bond insideI_h(7)-C₈₀. Moreover, density functional theory and ab initio multireference calculations suggest that the overlap between the 7s/6dhybrid thorium orbitals is so large that the bond still exists at Th-Th separations larger than 6 Å. This work demonstrates the authenticity of covalent actinide metal-metal bonds in a stable compound and deepens our fundamental understanding offelement metal bonds.