K18Tl20Au3, A Novel Derivative of K8Tl11 with the Unprecedented Polyanion Tl9Au29-, the Parent Tl117- and an Isolated Au- Ion

Abstract

Attempts to derivatize the novel Tl{sub 11}{sup 7-} ions in K{sub 8}Tl{sub 11} 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{bar 6}m2, Z = 1, a = 10.880 (1), c = 14.552 (2) {angstrom}, R/R{sub w} = 3.8/4.0% for 52 variables and 703 independent reflections). The compound contains separate close-packed layers of Tl{sub 9}Au{sub 2} and Tl{sub 11} clusters, each with D{sub 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{sub 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{sub 2}{close_quotes} to become strongly antibonding and empty. The Tl{sub 9} portion of the cluster is otherwise slightly perturbed. EHMO calculations indicated closed shell configurations for Tl{sub 9}Au{sub 2}{sup 9-} and, as before, Tl{sub 11}{sup 7-}, leading to formulation of the compoundmore » as (K{sup +}){sub 18} (Tl{sub 9}Au{sub 2}{sup 9-})(Tl{sub 11}{sub 7-})(Au{sup -})e{sup -}. The phase is appropriately a metallic conductor (Q{sub 290} {approximately} 43 {mu}{Omega}{center_dot}cm) and Pauli-paramagnetic, although some localization below 100 K is suggested by its Curie-Weiss behavior.« less