Exohedral Complexes of Lanthanide Atoms with the C₆₀ Fullerene Cage: Hexagonal versus Pentagonal Ring Coordination

The formation, properties, and applications of lanthanide (Ln) endohedral fullerenes remain the most extensively studied aspect of the interactions between the metal species and carbon nanoclusters. In comparison, the related information on the exohedral Ln-fullerene systems is limited. We analyzed theoretically the geometries, bonding, and selected electronic properties for the entire lanthanide series, for both η⁵ and η⁶ patterns of their exohedral coordination to C₆₀. We found that the bonding strength, the geometries, and the positive charge acquired by Ln atoms follow similar qualitative and semi-quantitative trends in the exohedral η⁵ and η⁶ coordinations. The spin behavior of the complexes constitutes a particularly interesting aspect, with both ferromagnetic and antiferromagnetic coupling observed. The η⁵ coordination exhibits a stronger trend to increase the molecular spin. Thus, further studies are needed for spintronic applications and in superconductive lanthanide-fullerene complexes, in which the Ln coordination to pentagonal rings of C₆₀ plays an important role.