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

摘要

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.