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

Abstract

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 Zn(2+). According to the solid-liquid phase diagram, the title compounds were crystallized below room temperature. The structure of ZnCl2·2.5H2O contains Zn(2+) both in a tetra-hedral coordination with Cl(-) and in an octa-hedral environment defined by five water mol-ecules and one Cl(-) shared with the [ZnCl4](2-) unit. Thus, these two different types of Zn(2+) cations form isolated units with composition [Zn2Cl4(H2O)5] (penta-aqua-μ-chlorido-tri-chlorido-di-zinc). The trihydrate {hexa-aqua-zinc tetra-chlorido-zinc, [Zn(H2O)6][ZnCl4]}, consists of three different Zn(2+) cations, one of which is tetra-hedrally coordinated by four Cl(-) anions. The two other Zn(2+) cations are each located on an inversion centre and are octa-hedrally surrounded by water mol-ecules. The [ZnCl4] tetra-hedra and [Zn(H2O)6] octa-hedra are arranged in alternating rows parallel to [001]. The structure of the 4.5-hydrate {hexa-aqua-zinc tetra-chlorido-zinc trihydrate, [Zn(H2O)6][ZnCl4]·3H2O}, consists of isolated octa-hedral [Zn(H2O)6] and tetra-hedral [ZnCl4] units, as well as additional lattice water mol-ecules. O-H⋯O hydrogen bonds between the water mol-ecules as donor and ZnCl4 tetra-hedra and water mol-ecules as acceptor groups leads to the formation of a three-dimensional network in each of the three structures.

Keywords: chloride hydrates; crystal structure; low-temperature salt hydrates; zinc salts.

Figure 1

The asymmetric unit of ZnCl₂·2.5H₂O. Displacement ellipsoids are drawn at the 50% probability level.

Figure 2

(a) The mol­ecular units and (b) the unit cell in the structure of ZnCl₂·3H₂O. Displacement ellipsoids are drawn at the 50% probability level. Dashed lines indicate hydrogen bonds. [Symmetry codes: (i) 1 − x, 1 − y, 2 − z; (ii) 1 − x, 1 − y, 1 − z.]

Figure 3

(a) Arrangement of [ZnCl₄]²⁻-anions and [Zn(H₂O)₆]²⁺ cations in a CsCl-like structure and (b) formation of chains by alternation of different coordination polyhedra in ZnCl₂·3H₂O. Dashed lines indicate hydrogen bonds. Only hydrogen bonds in one chain are shown.

Figure 4

(a) The mol­ecular units in the structure of ZnCl₂·4.5H₂O and (b) formation of a second coordination shell. Displacement ellipsoids are drawn at the 50% probability level. Dashed lines indicate hydrogen bonds.

Figure 5

The connection of individual [Zn₂Cl₄(H₂O)₅] units through hydrogen bonds (dashed lines) in the structure of ZnCl₂·2.5H₂O.