Synthesis and structure of K10Tl7: the first binary trielide containing naked pentagonal bipyramidal Tl7 clusters

The title compound is synthesized by direct fusion of the elements at 400 degrees C followed by annealing at 330 degrees C, quenching to room temperature, and subsequent annealing at 120 and 100 degrees C for days to weeks. The compound crystallizes in the monoclinic space group P21/c (No. 14), with Z = 4, a = 10.132(1) A, b = 22.323(2) A, c = 13.376(1) A, and beta = 93.14(1)degrees, and consists of Tl7(7-) clusters embedded in a matrix of potassium ions. The cluster is an axially compressed pentagonal bipyramid close to D5h symmetry. The apex-apex bond distance (3.462(1) A) is little longer than the bonds in the pentagonal waist (3.183(1)-3.247(1) A). Structurally the compound is not electron-precise: K10Tl7 has three extra electrons per Tl7 cluster and is Pauli paramagnetic (chi300 = 2.25 x 10(-4) emu/mol).