Preparation and Crystal Structures of Ni(NH3)2Cl2 and of Two Modifications of Ni(NH3)2Br2 and Ni(NH3)2I2

Abstract

Diammine nickel(II) halides, Ni(NH3)2X2 (X=Cl, Br, I), were prepared by decomposition of the corresponding hexaammines at 120°C in dynamical vacuum. Their crystal structures are of the Cd(NH3)2Cl2 type (“β-type”, space group Cmmm, Z=2): for Ni(NH3)2Cl2, a=8.019 Å, b=8.013 Å, c=3.661 Å; for β-Ni(NH3)2Br2, a=8.273 Å, b=8.297 Å, c=3.851 Å. Both were obtained by Rietveld refinement of diffractometer data with standard deviations <0.001 Å. For β-Ni(NH3)2I2, a=b=8.753(3) Å, c=4.127(1) Å, obtained by Guinier film data. In the case of the bromide and the iodide, annealing at 300°C leads to an irreversible structural rearrangement. A new modification is formed which is of the Mg(NH3)2Br2 type (“α-type”, space group Pbam, Z=2): for α-Ni(NH3)2Br2, a=5.865 Å, b=11.723 Å, c=3.856 Å, obtained by Rietveld refinement of diffractometer data with standard deviations <0.001; for α-Ni(NH3)2I2, 2a=b=12.359(3) Å, c=4.126(1) Å, obtained by Guinier film data. Both types of structures contain infinite chains of edge-sharing octahedra 1∞[NiX4/2(NH3)2]. These run parallel to the c-axes of the unit cells of the corresponding structure types and are arranged in different ways relative to one another. The decomposition of hexaammine nickel(II) halides to the corresponding diammines is analyzed in terms of structural analogies.