The octahedral MX 6 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 MLi 6 > MNa 6 > MK 6 and CX 6 > SiX 6 ,.~ GeX 6 > SnX 6 ~-PbX 6) are smaller. This stability order follows the variation of the Allred-Rochow electronegativity down group 14; the MX 6 cluster size and relativistic effects also are important. Supersonic expansion of lead-sodium vapor from a hot oven source led to mass spectrometric observation of 'an exceptionally abundant and stable PbNa 6 cluster' [1]. In reporting this discovery, Yeretzian, Rt~thlisberger and Schumacher called for 'accurate calculations' to explain the nature of this species. However, molecules of this type are not new. CLi 6, first described computationally [2-5], was observed recently as a product formed from the vaporization of C2Li 2 [6] and its thermochemical properties were determined in the gas phase [7].
The highest occupied molecular orbital (AIg) of CLi 6 has anti-bonding Li-C character, but is Li-Li bonding [2][3][4][5]8]. Hence, such clusters do not violate the octet rule and there is considerable ionic character in the bonding [2][3][4][5]8]. A very large number of * Corresponding author.