Preparation and Crystal Structures of Mg(NH3)2Cl2, Mg(NH3)2Br2, and Mg(NH3)2I2

Diammine magnesium halides Mg(NH3)2X2 with X=Cl, Br, I have been prepared from magnesium metal and the corresponding ammonium halide in steel autoclaves at 350°C. Mg(NH3)2Cl2 crystallizes in the Cd (NH3)2Cl2-structure type: Rietveld X-ray analysis, Cmmm, Z=2; a=8.1810(2) ?, b=8.2067(2) ?, c=3.7550(1) ?, R(F2)Bragg=5.9%. The bromide and the iodide are isotypic and crystallize in a new structure type: single crystal X-ray analysis, Pbam, Z=2; Mg(NH3)2Br2: a=5.944(2) ?, b= 11.876(3) ?, c=3.983(1) ?, R1 (F)=0.045; Mg(NH3)2I2: a=6.285(1) ?, b=12.559 (3) ?, c=4.302(1) ?, R1(F)=0.080. Both structure types contain chains of edge-sharing octahedra 1infinity[MgX4/2(NH3)2] running along c. These are arranged in different ways relative to one another in the two structure types.