Na₁₂K₃₈Tl₄₈Au₂: A Metallic Zintl Phase with Naked Icosahedral Fragments Tl₇⁷^- and Tl₉⁹^- Plus Au^-

Abstract

The title compound is synthesized by direct reaction of the elements at 500 °C followed by slow cooling. Na12K38Tl48Au2 crystallizes hexagonal [space group P6̄2m (No. 189), Z = 1, a = 19.343(2) Å, c = 11.498(5) Å] and is constituted (in terms of oxidation states) as (Na+)12(K+)38(Tl77-)3(Tl99-)3(Au-)2. Both clusters have C2v symmetry and can be viewed as fragments of a centered icosahedral Tl13. The Tl77- cluster is an oblate pentagonal bipyramid with an apex−apex bond distance of 3.39 Å, and Tl99- can be best derived from the centered icosahedron by removal of four adjoining vertexes (or as two fused pentagonal bipyramids). The isolated Au- is bound in a trigonal antiprism of potassium. The compound is structurally electron-precise (Zintl phase), but it shows characteristics of a very poor metal (ρ298 ∼ 760 μΩ〈cm) with a Pauli-like susceptibility of ∼1.4 × 10-3 emu·mol-1 over 50−300 K after correction for Larmor precession of cluster orbitals. EHMO results for the bonding in Tl77- and factors that stabilize this compound are presented and discussed.