AB INITIO INVESTIGATIONS ON SB4 ANALOGOUS ZINTL CLUSTERS

The validity of the Zintl-Klemm-Busmann (ZKB) principle is examined for the cluster series ${\mathrm{Sb}}_{3}$SnA, ${\mathrm{Sb}}_{3}$In${\mathit{A}}_{2}$ (A=alkali-metal atom) and ${\mathrm{Sb}}_{3}$TeHalo (Halo=halogen atom), which have been recently discovered by Knudsen effusion mass spectrometry. In the Zintl anion systems ${\mathrm{Sb}}_{3}$SnA and ${\mathrm{Sb}}_{3}$In${\mathit{A}}_{2}$, the alkali-metal atoms function, according to the ZKB principle, as electron donors. Since sizable electron transfer occurs from the alkali-metal-atom system to the ${\mathrm{Sb}}_{3}$Sn or ${\mathrm{Sb}}_{3}$In cluster nuclei, these units approach a 20-valence-electron configuration and therefore tetrahedral symmetry. From our ab initio treatment of the systems ${\mathrm{Sb}}_{3}$SnA and ${\mathrm{Sb}}_{3}$In${\mathit{A}}_{2}$ with A=Na, K, or Cs it turns out that for both types of Zintl clusters a geometry is energetically favored which allows for maximal electron transfer and thereby for the closest possible approximation to tetrahedral symmetry of the cluster nuclei. This finding confirms that the ZKB principle is applicable to free clusters. For the Zintl cationic system ${\mathrm{Sb}}_{3}$TeHalo (Halo=Cl,Br,I), again a clear correspondence between electron transfer and cluster nucleus geometry is observed, but in contrast to the Zintl anion systems discussed in this work, the tetrahedral structure of the cluster nucleus does not turn out to be the stablest one. Quantitative comparisons between theoretical and measured ionization energies yield excellent agreement. \textcopyright{} 1996 The American Physical Society.