Element effects in endohedral metal–metal-bonding fullerenes M2@C82 (M = Sc, Y, La, Lu)

Abstract

Endohedral metal–metal-bonding fullerenes have recently emerged, in which encapsulated metals form a metal–metal bond. However, the physical reasons why some metal elements prefer to form metal–metal bonds inside fullerene are still unclear. Herein, we reported first-principles calculations on electronic structures, bonding properties, dynamics, and thermodynamic stabilities of endohedral metallofullerenes M2@C82 (M = Sc, Y, La, Lu). Multiple bonding analysis approaches unambiguously reveal the existence of one two-center two-electron σ covalent metal–metal bond in M2@C82 (M = Sc, Y, Lu); however, the La–La bonding interaction in La2@C82 is weaker and could not be categorized as one metal–metal covalent bond. The energy decomposition analysis on bonding interactions between an encapsulated metal dimer and fullerene cages suggested that there exist two electron-sharing bonds between a metal dimer and fullerene cages. The reasons why La2 prefers to donate electrons to fullerene cages rather than form a standard σ covalent metal–metal bond are mainly attributed to two following facts: La2 has a lower ionization potential, while the hybridization of ns, (n − 1)d, and np atomic orbitals in La2 is higher. Ab initio molecular dynamic simulations reveal that the M–M bond length at room temperature follows the trend of Sc < Lu < Y. The statistical thermodynamics calculations at different temperatures reveal that the experimentally observed endohedral metal–metal-bonding fullerenes M2@C82 have high concentrations in the endohedral fullerene formation temperature range.