Crystal structures of ZnCl2·2.5H2O, ZnCl2·3H2O and ZnCl2·4.5H2O

The formation of different complexes in aqueous solutions is an important step in understanding the behavior of zinc chloride in water. The structure of concentrated ZnCl2 solutions is governed by coordination competition of Cl− and H2O around Zn2+. According to the solid–liquid phase diagram, the title compounds were crystallized below room temperature. The structure of ZnCl2·2.5H2O contains Zn2+ both in a tetrahedral coordination with Cl− and in an octahedral environment defined by five water molecules and one Cl− shared with the [ZnCl4]2− unit. Thus, these two different types of Zn2+ cations form isolated units with composition [Zn2Cl4(H2O)5] (pentaaqua-μ-chlorido-trichloridodizinc). The trihydrate {hexaaquazinc tetrachloridozinc, [Zn(H2O)6][ZnCl4]}, consists of three different Zn2+ cations, one of which is tetrahedrally coordinated by four Cl− anions. The two other Zn2+ cations are each located on an inversion centre and are octahedrally surrounded by water molecules. The [ZnCl4] tetrahedra and [Zn(H2O)6] octahedra are arranged in alternating rows parallel to [001]. The structure of the 4.5-hydrate {hexaaquazinc tetrachloridozinc trihydrate, [Zn(H2O)6][ZnCl4]·3H2O}, consists of isolated octahedral [Zn(H2O)6] and tetrahedral [ZnCl4] units, as well as additional lattice water molecules. O—H...O hydrogen bonds between the water molecules as donor and ZnCl4 tetrahedra and water molecules as acceptor groups leads to the formation of a three-dimensional network in each of the three structures