NA2K21TL19, A NOVEL THALLIUM COMPOUND CONTAINING ISOLATED TL(5)(7-) AND TL(9)(9-) GROUPS - A NEW HYPOELECTRONIC CLUSTER

The title compound is synthesized in high yield by fusion of the elements in sealed Ta containers followed by slow cooling to and annealing at 250 degrees C. The structure was established by single crystal means (Cmcm, Z = 4, a = 11.345 (2) Angstrom, b = 13.807 (3) Angstrom, c = 41.832 (8) Angstrom, R/R(w) = 5.3/4.3%). The independent unit contains two Tl-5 and one Tl-9 units ordered in layers and well separated by the cations which bond to, and bridge between, faces, edges, and vertices of the clusters. The sodium appears essential in this role. The former cluster, a distorted trigonal bipyramid (C-m symmetry), is proportioned as is Pb-5(2-), etc. and should be analogously formulated as the isoelectronic Tl-5(7-) according to EHMO calculations. The new Tl-9 geometry (C-2v, d(Tl-Tl) = 3.13-3.44 Angstrom) can be obtained by substantial elongation of two side edges of a tricapped trigonal prism (D-3h) so that the intervening face-capping atom moves close to that face and becomes eight-bonded. MO calculations illustrate how this distortion of either Bi-9(5+)- or B9H92--like polyhedra affords a hypoelectronic state for Tl-9(9-) (2n skeletal electrons). More direct geometric and electronic relationships are found on removal of four adjoining vertices from a centered icosahedral Tl-13 The compound is diamagnetic and a very poor conductor (rho(293) similar to 800 mu Omega.cm).