Na6TISb4:: Synthesis, structure, and bonding.: An electron-rich salt with a chain conformation and a TlI-Tl bond length determined by the cation

The title compound forms on fusion and annealing of a stoichiometric mixture of the elements. The structure was determined by single-crystal X-ray diffraction methods in the monoclinic space group C2/c, with a = 15.154(3) Angstrom, b = 10.401(2) Angstrom, c= 17.413(4) Angstrom, and beta = 113.57(3)degrees, Z = 8. Four-membered Tl-Sb1-Sb4-Sb3 rings interlinked by pairs of Sb2 bridges generate swinglike repeat units [Tl2Sb8] that are further interlinked through external Tl-Tl bonds to form infinite one-dimensional chains. Cations play a major role in the structure. In contrast to the Zintl-phase K6Tl2Sb3 with similar swinglike [Tl4Sb6] repeating units and Tl-Tl interlinkages, Na6TlSb4 has a more compact conformation of the chains and a notably smaller cell volume than expected. The new phase is metallic with two excess cations according to empirical electron counting, EHTB band calculations for the anion, and the compound's measured resistivities and magnetic susceptibilities. The notably shorter Tl-Tl bond in the present salt (2.954 Angstrom) can be directly attributed to the smaller cation and reduced intercation repulsions across that bond.