Geometries, electronic structures, and bonding properties of endohedral Group-14 Zintl clusters TM@E10 (TM = Fe, Co, Ni; E = Ge, Sn, Pb)

The geometries, electronic structures, and bonding properties of the title endohedral Zintl clusters have been studied by using ab initio calculations. [Fe@Ge-10](4-) and [Co@Ge-10](3-) have D-5h-symmetric pentagonal prismatic structure and [Fe@Sn-10](4-) adopts the C-2v-symmetric structure as their ground-state structures, whereas all the other clusters possess D-4d bicapped square antiprismatic structures, in consistent with the experimental values when available. Natural bonding orbital and electron localization function disclosed that the negative charges are localized on the central atoms rather than the cages while the TM-E ionic bonding interactions increase in the order of Ge < Sn < Pb. The energy decomposition analysis revealed that the total bonding energy increment E-int between central TM and E-10 cage is above 150 kcal/mol. The ionic bonding interaction termed as electrostatic interaction increment E-elstat increases in the order of Ge < Sn < Pb and becomes higher than the covalent bonding interactions termed as total orbital interactions increment E-orb. Among the total orbital interactions, the pi back donations from the TM-d orbitals to the empty cage orbitals consisting of E-p orbitals, the magnitude of which is importantly affected by the cage symmetry, are dominant contributions.