Geometries, Electronic Structures, Bonding Properties, and Stability Strategy of Endohedral Metallofullerenes TM@C₂₈ (TM = Sc^-, Y^-, La^-, Ti, Zr, Hf, V⁺, Nb⁺, Ta⁺)

We performed quantum chemical calculations on the geometries, electronic structures, bonding properties, and stability strategy of endohedral metallofullerenes TM@C-28 (TM = Sc-, Y-, La-, Ti, Zr, Hf, V+, Nb+, Ta+). Our calculations revealed that there are three different lowest-energy structures with C-2v, C-3v, and T-d symmetries for TM@C-28. The HOMO-LUMO gap of all these structures ranges from 1.35 eV to 2.31 eV, in which [V@C-28](+) has the lowest HOMO-LUMO gap of 1.35 eV. The molecular orbitals are mainly composed of fullerene cage orbitals and slightly encapsulated metal orbitals. The bonding analysis on the metal-cage interactions reveals they are dominated by the Coulomb term Delta E-elstat and the orbital interaction term Delta E-orb, in which the orbital interaction term Delta E-orb contributes more than the Coulomb term Delta E-elstat. The addition of one or two CF3 groups to [V@C-28](+) could increase the HOMO-LUMO gap and further increase the stability of [V@C-28](+).