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 °C followed by annealing at 330 °C, quenching to room temperature, and subsequent annealing at 120 and 100 °C for days to weeks. The compound crystallizes in the monoclinic space group P2₁/c (No. 14), with Z = 4, a = 10.132(1) Å, b = 22.323(2) Å, c = 13.376(1) Å, and β = 93.14(1)°, and consists of Tl₇^7- clusters embedded in a matrix of potassium ions. The cluster is an axially compressed pentagonal bipyramid close to D_5h symmetry. The apex−apex bond distance (3.462(1) Å) is little longer than the bonds in the pentagonal waist (3.183(1)−3.247(1) Å). Structurally the compound is not electron-precise:  K₁₀Tl₇ has three extra electrons per Tl₇ cluster and is Pauli paramagnetic (χ₃₀₀ = 2.25 × 10^-4 emu/mol).