Hypermetallation is ubiquitous: MX6 molecules (MCPb, XLiK)

The octahedral MX6 hypermetallated clusters (M  C, Si, Ge, Sn, Pb and X  Li, Na, K) all are stable minima. The bonding in the entire set is similar, with considerable M− X+ ionic character and cage XX interactions. Density functional computations (Becke3LYP/TZP; Sn, Pb = pseudopotential calculations) show Li compounds to be exceptionally stable; the differences in the atomization energies between the other clusters (in the order MLi6 > MNa6 > MK6 and CX6 > SiX6 ≈ GeX6 > SnX6 ≈ PbX6) are smaller. This stability order follows the variation of the Allred-Rochow electronegativity down group 14; the MX6 cluster size and relativistic effects also are important.