Strong covalent Th-Th bond inside C80
An unprecedented thorium-thorium bond has been characterized inside the icosahedral Ih(7)-C80 carbon cage. The single-crystal X-ray diffraction of Th2@Ih(7)-C80 unambiguously determined that the two encapsulated Th atoms are separated by 3.816 Å, accurately reproduced by DFT calculations at SR-PBE0/TZP level. The overall agreement between the crystallographic, XAS, and quantum-computational results conclusively shows that the two encapsulated Th ions have formal charges of +3 and confirm the existence of a strong covalent Th-Th bond, which has been estimated to have an effective bond order (EBO) of 0.99 and a bond energy higher than 40 kcal/mol. The computational studies reveal that the overlap between 7s/6d hybrid thorium orbitals is so large that the bond still prevails at Th-Th separations larger than 6 Å, suggesting that this Th-Th bond is, up to now, the strongest one among the known actinide-actinide or lanthanide-lanthanide interactions.
