New Ternary Hafniumhalides: Cs[Hf2Br9] and Rb[Hf2Br9]

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.