Ca@Pb₁₂: A Viable All-Main-Group-MetalEndofullerene That Imitates the Transition Metals

Herein, we report the Ca@Pb₁₂ cluster, an all-main-group-metal endofullerene possessing the highly symmetric icosahedral (<i>I</i>_<i>h</i>) Platonic solid structure. The endohedral environment endows the Ca atom with concrete transition-metal-like bonding behaviors, as evidenced by the 18e (in 3d¹⁰4s²4p⁶ configuration) valence shell structure and five strong Pb₁₂ → Ca(3d) dative bonds, whose orbital interaction energy (Δ<i>E</i>_orb, −134.2 kcal/mol) represents 54.9% of total Δ<i>E</i>_orb (−244.4 kcal/mol) between Ca²⁺ and [Pb₁₂]^2–. Moreover, since the favorable orbital interactions can compensate for the enlarged steric repulsion during encapsulation, Ca@Pb₁₂ is thermodynamically stable, dynamically rigid (up to 1300 K), and electronically robust, as verified by large HOMO–LUMO gaps (4.58 eV), high vertical detachment energy (6.39 eV), and low electron affinity (−0.71 eV). It is the first viable all-main-group-metal endofullerene, where the guest alkaline earth metal exhibits the typical bonding behaviors of transition metals.