Tribarium tetrahedro ‐Tetragermanide Acetylenide, Ba 3 [Ge 4 ][C 2 ]: Synthesis, structure, and properties

Ba 3 Ge 4 C 2 is formed at 1530 K from the elements or by reaction of BaC 2 with BaGe 2 (corundum crucible; steel ampoule). The compound is a semiconductor (grey colour; E g = 1.1 eV), brittle, very sensitive to moisture, and reacts with NH 4 Cl at about 400 K forming acetylene and germanes up to Ge 4 H n . The new Ba 3 Ge 4 C 2 structure type (space group I4/mcm , No. 140; a = 8.840(1) Å, c = 12.466(1) Å; Z = 4, Pearson code tI 36), contains two kinds of isolated polyanions, namely tetrahedro ‐tetragermanide [Ge 4 ] 4− and acetylenide [C 2 ] 2− anions. The bond lengths are d(Ge‐Ge) = 2.517 Å (4×) and 2.641 Å (2×), and d(CC) = 1.20 Å. The Ba 3 [Ge 4 ][C 2 ] structure is a hierarchical derivative of the perovskite (CaTiO 3 ) generated by a partial atom/cluster replacement ([Ge 4 ] for Ca, [C 2 ] for Ti and Ba for O). The Raman spectrum shows bands at 168, 199 and 280 cm −1 , and at 1796cm −1 characteristic for [Ge 4 ] 4− and [C 2 ] 2− polyanions, respectively.