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@C₂₈ (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@C₂₈. The HOMO–LUMO gap of all these structures ranges from 1.35 eV to 2.31 eV, in which [V@C₂₈]⁺ 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 ΔE_elstat and the orbital interaction term ΔE_orb, in which the orbital interaction term ΔE_orb contributes more than the Coulomb term ΔE_elstat. The addition of one or two CF₃ groups to [V@C₂₈]⁺ could increase the HOMO–LUMO gap and further increase the stability of [V@C₂₈]⁺.