K18TL20AU3 - A NOVEL DERIVATIVE OF K8TL11 WITH THE UNPRECEDENTED POLYANION TL9AU29-, THE PARENT TL-11(7-), AND AN ISOLATED AU- ION

Attempts to derivatize the novel Tl-11(7-) ions in K8Tl11 by Au substitution leads to the formation of the title compound from direct reaction of the elements in Ta tubing at 500 and then 350 degrees C. The crystal structure was established at room temperature by single crystal X-ray diffraction means (space group P (6) over bar m2, Z = 1, a = 10.880 (1), c = 14.552 (2) Angstrom, R/R(w) = 3.8/4.0% for 52 variables and 703 independent reflections). The compound contains separate close-packed layers of Tl9Au2 and Tl-11 clusters, each with D-3h point symmetry, together with isolated Au atoms and 18 K atoms that sheath and separate the units. The gold-containing cluster can be derived directly from the pentacapped trigonal prismatic Tl-11 through substitution on the two apical (axial) positions followed by a substantial axial compression to generate an Au-Au bond, d = 2.963 Angstrom, this action causing the antisymmetric equivalent MO a(2)'' to become strongly antibonding and empty. The Tl-9 portion of the cluster is otherwise slightly perturbed. EHMO calculations indicate closed shell configurations for Tl9Au(2)(9-) and, as before, Tl-11(7-), leading to formulation of the compound as (Ki)(18)(Tl9Au29-)(Tl-11(7-))(Au-)e(-). The phase is appropriately a metallic conductor (rho(290) similar to 43 mu Omega . cm) and Pauli-paramagnetic, although some localization below 100 K is suggested by its Curie-Weiss behavior.