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

The crystal structures of ZnCl2·xH2O (x = 2.5, 3 and 4.5) consist of Zn2+ ions both in an octahedral and tetrahedral environment. O—H⋯O hydrogen bonds between water molecules and tetrahedral ZnCl4 units lead to the formation of a three-dimensional network in each of the structures.

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 ZnCl 2 solutions is governed by coordination competition of Cl À and H 2 O around Zn 2+ . According to the solid-liquid phase diagram, the title compounds were crystallized below room temperature. The structure of ZnCl 2 Á2.5H 2 O contains Zn 2+ both in a tetrahedral coordination with Cl À and in an octahedral environment defined by five water molecules and one Cl À shared with the [ZnCl 4 [ZnCl 4 ] units, as well as additional lattice water molecules. O-HÁ Á ÁO hydrogen bonds between the water molecules as donor and ZnCl 4 tetrahedra and water molecules as acceptor groups leads to the formation of a three-dimensional network in each of the three structures.

Chemical context
Zinc chloride solutions, especially at lower temperatures, are helpful in the understanding of the formation of different complex ion species in solution. The solubility of zinc chloride in water has been investigated by several authors in different concentration areas and at different temperatures (Haghighi et al., 2008;Mylius & Dietz, 1905;Jones & Getman, 1904;Chambers & Frazer, 1900;Biltz, 1902;Dietz, 1899;Etard, 1894). In the literature (Mylius & Dietz, 1905), the 4-, 3-, and 2.5-hydrates have been reported at lower temperatures. We have also found the 2.5-hydrate, the trihydrate and the 4.5hydrate as stable phases along the equilibrium crystallization curves. The 4.5-hydrate crystallizes below 240 K. The crystal structure of the trihydrate reported herein has also been determined by Wilcox (2009)

Structural commentary
Within the crystal structure of the 2.5-hydrate, there are two crystallographic different Zn 2+ cations, as shown in Fig. 1. The Zn1 cation is octahedrally coordinated by five water molecules and one chloride anion. The Zn2 cation is coordinated by four chloride anions, one shared with the Zn1 cation, leading to the formation of isolated [Zn 2 Cl 4 (H 2 O) 5 ] units. Since the bond lengths of the bridging Cl atom of the tetrahedron are shorter than to that of the octahedron, the latter becomes more distorted. The crystal structure of zinc chloride trihydrate consists of three crystallographically different Zn 2+ cations (Fig. 2a). Two (Zn2 and Zn3) are located about an inversion centre and are coordinated octahedrally by six water molecules, forming [Zn(H 2 O) 6 ] 2+ cations. The third one (Zn1) is tetrahedrally coordinated by chlorine anions, [ZnCl 4 ] 2À . The polyhedra are not connected by sharing a single atom like in the 2.5-hydrate, but they are linked by hydrogen bonds (Fig. 2b). The octahedra and tetrahedra are arranged in a CsCl-like arrangement with eight tetrahedra located around one octahedron (Fig. 3a). As shown in Fig. 4a The asymmetric unit of ZnCl 2 Á2.5H 2 O. Displacement ellipsoids are drawn at the 50% probability level.       Symmetry codes: (i) x þ 1; y; z; (ii) Àx þ 3 2 ; y À 1 2 ; Àz þ 1 2 ; (iii) Àx þ 3 2 ; y þ 1 2 ; Àz þ 1 2 ; (iv) x À 1 2 ; Ày þ 3 2 ; z À 1 2 ; (v) x þ 1 2 ; Ày þ 3 2 ; z À 1 2 .

Figure 5
The trihydrate from an aqueous solution of 69.14 wt% ZnCl 2 at 263 K after 2 d and zinc chloride 4.5 hydrate from an aqueous solution of 53.98 wt% ZnCl 2 at 223K after 2 d. For preparing these solutions, zinc chloride (Merck, 99%) was used. The content of Zn 2+ was analysed by complexometric titration with EDTA. The crystals are stable in their saturated solutions over a period of at least four weeks. The samples were stored in a freezer or a cryostat at low temperatures. The crystals were separated and embedded in perfluorinated ether for X-ray diffraction analysis.

Refinement
Crystal data, data collection and structure refinement details are summarized in  Integration (Coppens, 1970) Integration (Coppens, 1970) Integration (Coppens, 1970)   Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.

Special details
Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq  (8) Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.