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) Aa (2), b = 6.6580(9) (1) and 6.6695(3) Aa (2), c = 12.930(2) (1) and 12.8435(6) Aa (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. (Copyright copyright 2015 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim)