ArticleStructural and energetic studies on double salts of M(II)Mg2Cl6·12H2O (M  Ca, Mn, Cd) by X-ray diffraction and density functional methods
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AbstractThe crystal structure of the double salt complex MnMg2Cl6·12H2O has been determined by a single crystal X-ray diffraction method. The phase is trigonal, space group P31c, with unit cell dimensions a = 9.953(3) and c = 11.467(3) Å. The crystal structure consists of two kinds of well-separated octahedra, [Mg(H2O)6]2+ and [MnCl6]4−, which is isomorphous with the CdMg2Cl6·12H2O crystal. In order to examine the stability of the double salt crystals, full geometry optimizations have been carried out for several octahedral polyhedra of [M(H2O)6]2+ and [MCl6]4− (MMg2+, Ca2+, Mn2+, Cd2+) by the ab initio density functional method. Comparison of formation energies (ΔE) for [Mg(H2O)6]2+, [M(II)(H2O)6]2+, [MgCl6]4− and [M(II)Cl6]4− independently optimized {(ΔE([Mg(H2O)6]2+) + ΔE([M(II)Cl6]4−)) versus (ΔE([MgCl6]4−) + ΔE([M(II)(H2O6]2+)); M  Ca, Mn, Cd}, reveal that the former combination of polyhedra is significantly stable in comparison with the latter, indicating that the formation energies of the first coordination spheres play a decisive role in determining the constituent polyhedra of the double salt crystals.

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