The syntheses and crystal structures of two new ternary hafnium compounds, Cs[Hf2Br9] (1) and Rb[Hf2Br9] (2), are described. Both compounds are obtained in high yield from the chemical reaction of HfBr4 and CsBr or RbBr, respectively, in the presence of a small amount of elemental Al at 450 °C in sealed silica tubes. They crystallize isostructurally in the monoclinic space group P2/n. The lattice parameters are a = 9.946(1) (1) and 9.9388(4) Å (2), b = 6.6580(9) (1) and 6.6695(3) Å (2), c = 12.930(2) (1) and 12.8435(6) Å (2), and β = 112.479(6)° (1) and 112.726(2)° (2). The crystals of the two compounds contain dinuclear tri‐μ‐bromido‐hexabromido‐dihafnate(–) complex anions, [Hf2Br9]–, besides the alkali metal cations. The complex anions can be described as face‐sharing bioctahedral units.

The reduction of HfCl 4 with Li[AlH 4 ] under ionothermal conditions using (BMIm) [AlCl 4 ] as solvent and as reactant gives the compound (BMIm) 2 [Hf 9 Cl 14 H 8 (AlCl 4 ) 6 ] in moderate yield. It contains nona-nuclear metalloid cluster units with an unprecedented arrangement of the metal atoms. This can be described by a polyhedron consisting of two trigonal bipyramids, which are fused in one basal Hf atom. The edges of the Hf 9 polyhedron are bridged by Cl atoms and the apical sites Cl-bonded to [AlCl 4 ] À anions. These bridge the cluster units into 1D cluster strands. The metal atom polyhedron can be constructed by a step-wise distortion of a body centred cube. DFT calculations on [Hf 9 Cl 14 H (8-x) (AlCl 4 ) 6 ] 2À units for different numbers of hydrogen atoms (x = 8-0) at the UPBE-D3/def2-TZVP level of theory indicate significant metal-metal bonding. These calculations supported strongly the presence of hydridic H atoms bridging triangular faces of the metal atom skeleton. The most stable cluster anion is calculated for eight H atoms present in the cluster unit.Recent publications of M. Ruck, C. Feldmann, S. Dehnen, and others have shown that the use of ionic liquids as reaction media allows for the synthesis of unprecedented metal-rich cluster compounds. [1] For example, the reduction of MoCl 5 with elemental Bi in the ionic liquid (BMIm)[AlCl 4 ] at room temperature results in the formation of Mo 5 and Mo 6 cluster compounds. [1m]

The title compounds are prepared in high yield by solid state reaction of 2:1 molar mixtures of HfBr4 and CsBr or RbBr, resp., in the presence of a small amount of Al (evacuated silica tubes, 450 °C, 2 d).