inorganic compounds
Acta Crystallographica Section C
Crystal Structure
Communications
ISSN 0108-2701
Magnesium chloride tetrahydrate,
MgCl2ꢀ4H2O
Horst Schmidt,* Erik Hennings and Wolfgang Voigt
Institute of Inorganic Chemistry, TU Bergakademie Freiberg, Leipziger Strasse 29,
D-09596 Freiberg, Germany
Correspondence e-mail: horst.schmidt@chemie.tu-freiberg.de
Received 10 October 2011
Accepted 19 December 2011
Online 23 December 2011
Figure 1
The MgCl2(H2O)4 octahedron, showing displacement ellipsoids at the
50% probability level. The labels ‘T’ and ‘D’ refer to tilted and dipole
orientations of the water molecules, respectively, as discussed in the
Comment. [Symmetry code: (i) ꢂx + 1, y, ꢂz + .]
The title compound, MgCl2ꢀ4H2O, was crystallized at 403 K
and its structure determined at 200 K. The structure is built up
from MgCl2(H2O)4 octahedra with a trans configuration. Each
complex is situated on a crystallographic twofold axis, with the
rotation axis aligned along one H2O—Mg—OH2 axis. They
are connected by a complex network of O—Hꢀ ꢀ ꢀCl hydrogen
bonds. The structure contains two-dimensional sections that
are essentially identical to those in the reported tetrahydrates
of CrCl2, FeCl2, FeBr2 and CoBr2, but they are stacked in a
different manner in MgCl2ꢀ4H2O compared with the transition
metal structures.
1
2
hydrogen bonds between the water molecules of nearest-
neighbour planes. Instead, all the octahedra are inter-
connected by O—Hꢀ ꢀ ꢀCl hydrogen bonds (Table 1 and Fig. 3).
The Mg(H2O)4 coordination planes contain two types of water
molecules: one type (O1 and O3) in a ‘dipole orientation’ and
the other (O2) tilted from the Mg(H2O)4 plane by ca 30ꢁ
(Fig. 1). ‘Dipole orientation’ means that the water molecules
and Mg atoms are located in one plane, and the Mg—O axis
bisects the H—O—H angle. The tilted water molecules form
shorter hydrogen bonds towards adjacent Cl atoms (Table 1
and Fig. 3). Each H atom of an Mg(H2O)4 plane forms one
hydrogen bond to an adjacent Cl atom of a different
MgCl2(H2O)4 octahedron, resulting in connections to eight
octahedra (Fig. 3). Each Cl atom acts as an acceptor for four
Comment
Recently, Sugimoto et al. (2007) reported a crystal structure of
MgCl2ꢀ4H2O from in situ synchrotron powder diffraction
experiments during dehydration of MgCl2ꢀ6H2O. Under these
conditions, the tetrahydrate represents an intermediate phase
between MgCl2ꢀ6H2O and lower hydrates in the dehydration
process, and the degree of equilibration is dependent on the
heating rate and the pressure of the water vapour. A highly
disordered structure was found, where only every second
MgCl2(H2O)4 octahedron is occupied. We report here an
ordered crystal structure for MgCl2ꢀ4H2O, obtained from
single-crystal data under equilibrium conditions.
MgCl2ꢀ4H2O crystallizes as a stable phase between 390 and
454 K, in accordance with the phase diagram of the MgCl2–
¨
H2O system (Fanghanel et al., 1987). The structure is built up
from octahedral MgCl2(H2O)4 units with the Cl atoms in trans
positions, as shown in Fig. 1. The complexes lie on crystal-
lographic twofold axes, aligned along the O1—Mg1—O3 axis.
Although there is only one unique crystallographic position
for the Mg and Cl atoms, two different orientations of the
MgCl2(H2O)4 octahedra exist. Viewing the structure parallel
to the b axis (Fig. 2), adjacent rows of octahedra possess
alternating orientations of their Cl—Mg—Cl axes, giving rise
to a zigzag pattern along the a axis. Consequently, the
Mg(H2O)4 planes also have alternating orientations with an
angle of nearly 90ꢁ between them. However, there are no
Figure 2
A packing diagram, viewed along the b axis, showing the zigzag
arrangement of Cl—Mg—Cl axes along the a axis.
i4 # 2012 International Union of Crystallography
doi:10.1107/S0108270111054709
Acta Cryst. (2012). C68, i4–i6
Schmidt, Horst
