On the Crystal Structures of the Polymorphs of Manganese(II) Chloride Tetrahydrate: α-MnCl 2 ·4H 2 O and β-MnCl 2 ·4H 2 O

The manganese(II) chloride tetrahydrate MnCl2·4H2O (α-form) crystallizes in the space group P21/n with a = 6.1918(4) A, b = 9.5230(6) A, c = 11.1918(6) A, β = 99.757(3)°, and V = 650.37(7) A3 (Z = 4). A second polymorph of MnCl2·4H2O (β-form) is mentioned in the literature to be isomorphous with FeCl2·4H2O but its crystal structure was never confirmed. In this study, we report for the first time on the crystal structure of this second polymorph of manganese(II) chloride tetrahydrate, β-MnCl2·4H2O, using room temperature single-crystal X-ray diffraction. The β-form crystallizes in space group P21/c with a = 5.9893(5) A, b = 7.2877(6) A, c = 8.5838(7) A, β = 110.952(4)°, and V = 349.89(5) A3 (Z = 2), confirming that β-MnCl2·4H2O is isotype to Fe(II)Cl2·4H2O. To the best of our knowledge this is the first compound reported to crystallize in both MnCl2·4H2O-type (α-form) and FeCl2·4H2O-type (β-form) structures. The crystal structure of both α-form and β-form of MnCl2·4H2O consists of discrete octahedra complexes [MnCl2(H2O)4] linked by O–H⋅⋅⋅Cl− hydrogen bonds. However, chlorine atoms are in trans-configuration in the β-form (trans-[MnCl2(H2O)4] complexes) and in cis-configuration in the α-form (cis-[MnCl2(H2O)4] complexes). Moreover, the coordination polyhedra of the manganese(II) atoms are less distorted in the β-form. This leads to two different hydrogen bond networks and to a slightly more compact [MnCl2(H2O)4] complexes arrangement in the α-form. The title compound is the first representative among divalent metal tetrahydrate halogenides to present both the MnCl2·4H2O-type (α-form) and FeCl2·4H2O-type (β-form) structures, leading to different configuration of complexes [MnCl2(H2O)4], crystal packing, hydrogen bond networks, and possibly magnetic properties.