Single crystals of AlBr 3 • NH 3 and AlI 3 • NH 3 sufficient in size for X-ray structure determinations were obtained by evaporation/sublimation of the respective compound from its melt. The ammoniates were synthesized by the reaction of the pure halide with NH 3 at Ϫ78°C and following homogenization by slowly heating the reaction mixture up to the melting points of the ammoniates (124°C and 126°C, respectively). The X-ray structure determinations for both monoammoniates were successfully carried out for the heavy atom positions (no hydrogen atoms): AlBr 3 • NH 3 : Pbca, Z ϭ 16, a ϭ 11.529 ( 5) A ˚, b ϭ 12.188 (2) A ˚, c ϭ 19.701 (4) A First fundamental investigations on the ammoniates of aluminium halides were done by Klemm and co-workers in 1931 [1, 2]. These authors were mainly interested in the number of ammoniates, their thermal stability and predicted structural features. A first structure determination by X-ray methods was done on AlCl 3 • NH 3 in 1978 [3]. Nearly tetrahedral molecules of Al(NH 3 )Cl 3 are linked by hydrogen bridge bonds N-H•••Cl three-dimensionally.
Later on we reported about syntheses and crystal structure determinations on AlCl 3 5,6] veryfied by [7] and AlI 3 • 7NH 3 ϵ [Al(NH 3 ) 6 ] 3ϩ I 3 • NH 3 [8]. As the ammoniates AlX 3 • nNH 3 with e.g. X ϭ Cl and n ϭ 1, 2, 3, 5, 6, 7, 14 are spezies of a solvation series they can serve as models for an understanding of a stepwise solvation of cations in polar solvents. We have performed further experiments [9] to grow single crystals from ammoniates for X-ray structure determinations. Here we report results obtained with the monoammoniates of AlBr 3 and AlI 3 .