AB-INITIO INVESTIGATIONS ON SB-4 ANALOGOUS ZINTL CLUSTERS

The validity of the Zintl-Klemm-Busmann (ZKB) principle is examined fo r the cluster series Sb(3)SnA, Sb(3)InA(2) (A = alkali-metal atom) and Sb(3)TeHalo (Halo=halogen atom), which have been recently discovered by Knudsen effusion mass spectrometry. In the Zintl anion systems Sb(3 )SnA and Sb(3)InA(2), the alkali-metal atoms function, according to th e ZKB principle, as electron donors. Since sizable electron transfer o ccurs from the alkali-metal-atom system to the Sb3Sn or Sb3In cluster nuclei, these units approach a 20-valence-electron configuration and t herefore tetrahedral symmetry. From our ab initio treatment of the sys tems Sb(3)SnA and Sb(3)InA(2) with A=Na, K, or Cs it turns out that fo r both types of Zintl clusters a geometry is energetically favored whi ch allows for maximal electron transfer and thereby for the closest po ssible approximation to tetrahedral symmetry of the cluster nuclei. Th is finding confirms that the ZKB principle is applicable to free clust ers. For the Zintl cationic system Sb(3)TeHalo (Halo=Cl,Br,I), again a clear correspondence between electron transfer and cluster nucleus ge ometry is observed, but in contrast to the Zintl anion systems discuss ed 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 agreemen t.