SYNTHESIS, STRUCTURE, AND BONDING OF THE NOVEL CLUSTER COMPOUND KTL WITH ISOLATED TL6(6-) IONS

The unprecedented naked cluster Tl6(6-) occurs in the incongruently melting line compound KTl (Cmca, Z = 24, a = 15.239(4) angstrom, b = 15.069(4) angstrom, c = 8.137(2) angstrom; R/R(w) = 3.7%/3.9% for 462 independent reflections and 33 variables). The C2h anion consists of a thallium octahedron tetragonally compressed to produce shorter axial-waist distances (3.06-3.08 angstrom), an enlarged rectangular waist (3.34-3.47 angstrom), and a 1.0-angstrom difference in trans distances. Potassium atoms bridge between clusters and show somewhat specific orientations on cluster vertices, edges, and faces. The compound is diamagnetic. The hypoelectronic cluster structure with 2n 6p electrons in skeletal orbitals derives from the observed compression, in analogy to recent results for two indium clusters. Extended Huckel calculations show the expected splitting of the p-orbital-based t(lu) on compression of octahedral Tl6 to elevate the a(2u) component, although the available orbital parameter sets are not completely satisfactory. Relationships with some corresponding liquid alloy systems are noted.