Redetermination of MgCrO4·5H2O

Weil, M.

doi:10.1107/S1600536813018588

inorganic compounds

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ISSN: 2056-9890

Volume 69| Part 8| August 2013| Pages i48-i49

doi:10.1107/S1600536813018588

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Redetermination of MgCrO₄·5H₂O

Matthias Weil ^a ^*

^aInstitute for Chemical Technologies and Analytics, Division of Structural Chemistry, Vienna University of Technology, Getreidemarkt 9/164-SC, A-1060 Vienna, Austria
^*Correspondence e-mail: mweil@mail.zserv.tuwien.ac.at

(Received 2 July 2013; accepted 4 July 2013; online 10 July 2013)

The CCD-data based redetermination of the crystal structure of the title compound, magnesium chromate(VI) pentahydrate, confirms in principle the previous study based on precession film data [Bertrand et al. (1971 ). C. R. Hebd. Seances Acad. Sci. Serie C, 272, 530–533.], but with all atoms refined with anisotropic displacement parameters and with all H atoms localized. This allowed an unambiguous assignment of the hydrogen-bonding pattern. MgCrO₄·5H₂O adopts the MgSO₄·5H₂O structure type. It contains two Mg²⁺ sites on special positions with site symmetry -1, one tetrahedral CrO₄ group and five water molecules. Four of them coordinate to the Mg²⁺ cation, and one is an uncoordinating lattice water molecule. The octahedral environment of the Mg²⁺ cation is completed by two axial O atoms of CrO₄ tetrahedra. This arrangement leads to the formation of chains parallel to [011]. Adjacent chains are linked through O—H⋯O hydrogen bonds (one of them bifurcated), involving both the coordinating and lattice water molecules, into a three-dimensional network.

Related literature

For the original structure determination of the title compound, see: Bertrand et al. (1971). For hydrogen-bonding pattern in the structures of MXO₄·5H₂O compounds (M = Mg, Cu; X = S, Cr), see: Baur & Rolin (1972 ). For Cr—O bond length distributions in chromates(VI), see: Pressprich et al. (1988 ). For bond lengths and angles in the related structure of MgCrO₄·11H₂O, see: Fortes et al. (2013 ). For standardization of structure data, see: Gelato & Parthé (1987 ).

Experimental

Crystal data

MgCrO₄·5H₂O
M_r = 230.39
Triclinic, $[P \overline 1]$
a = 6.1467 (3) Å
b = 6.3742 (4) Å
c = 10.7048 (6) Å
α = 75.919 (4)°
β = 81.603 (3)°
γ = 71.134 (3)°
V = 383.92 (4) Å³
Z = 2
Mo Kα radiation
μ = 1.59 mm⁻¹
T = 296 K
0.10 × 0.08 × 0.01 mm

Data collection

Bruker APEXII CCD diffractometer
Absorption correction: multi-scan (SADABS; Bruker, 2011 ) T_min = 0.594, T_max = 0.748
10569 measured reflections
4019 independent reflections
3340 reflections with I > 2σ(I)
R_int = 0.031

Refinement

R[F² > 2σ(F²)] = 0.031
wR(F²) = 0.069
S = 1.02
4019 reflections
143 parameters
10 restraints
H atoms treated by a mixture of independent and constrained refinement
Δρ_max = 1.17 e Å⁻³
Δρ_min = −0.72 e Å⁻³

Table 1
Selected bond lengths (Å)

Mg1—OW3	2.0505 (9)
Mg1—OW2ⁱ	2.0656 (9)
Mg1—O4ⁱⁱ	2.0952 (9)
Mg2—OW5	2.0265 (10)
Mg2—OW1	2.0467 (8)
Mg2—O1	2.1099 (9)
Cr1—O3ⁱⁱⁱ	1.6357 (9)
Cr1—O4ⁱⁱⁱ	1.6554 (9)
Cr1—O1	1.6568 (9)
Cr1—O2	1.6579 (8)
Symmetry codes: (i) x, y-1, z; (ii) -x+1, -y, -z; (iii) -x+1, -y, -z+1.

Table 2
Hydrogen-bond geometry (Å, °)

D—H⋯A	D—H	H⋯A	D⋯A	D—H⋯A
OW1—HW1A⋯O2^iv	0.81 (2)	1.96 (2)	2.7702 (13)	174 (2)
OW1—HW1B⋯O3	0.80 (2)	1.97 (2)	2.7598 (13)	169 (2)
OW2—HW2A⋯O1^v	0.79 (2)	2.19 (2)	2.9014 (13)	150 (2)
OW2—HW2A⋯OW1	0.79 (2)	2.52 (2)	3.1008 (14)	132 (2)
OW2—HW2B⋯OW4	0.80 (2)	2.02 (2)	2.8173 (12)	171 (2)
OW3—HW3A⋯O2ⁱⁱⁱ	0.82 (2)	1.97 (2)	2.7891 (13)	170 (2)
OW3—HW3B⋯OW4^vi	0.81 (2)	1.99 (2)	2.7906 (14)	172 (2)
OW4—HW4B⋯O1^iv	0.83 (2)	2.32 (2)	3.1205 (14)	163 (2)
OW4—HW4A⋯O4	0.80 (2)	2.06 (2)	2.8535 (14)	170 (2)
OW5—HW5B⋯O2ⁱⁱⁱ	0.80 (2)	1.93 (2)	2.7262 (13)	173 (2)
OW5—HW5A⋯O3^vii	0.79 (2)	1.96 (2)	2.7409 (12)	174 (2)
Symmetry codes: (iii) -x+1, -y, -z+1; (iv) -x+1, -y+1, -z+1; (v) -x, -y+1, -z+1; (vi) -x+1, -y+1, -z; (vii) x-1, y, z.

Data collection: APEX2 (Bruker, 2011); cell refinement: SAINT (Bruker, 2011); data reduction: SAINT; program(s) used to solve structure: SHELXS97 (Sheldrick, 2008 ); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: ATOMS for Windows (Dowty, 2008 ); software used to prepare material for publication: publCIF (Westrip, 2010 ).

Supporting information

Crystal structure: contains datablocks I, global. DOI: 10.1107/S1600536813018588/br2229sup1.cif

Structure factors: contains datablock I. DOI: 10.1107/S1600536813018588/br2229Isup2.hkl

checkCIF report

Comment top

In the current study MgCrO₄.5H₂O was prepared as a precursor for preparation of anhydrous MgCrO₄. The structure of MgCrO₄.5H₂O has already been determined from film data (precession camera) by Bertrand et al. (1971), revealing isotypism with MgSO₄.5H₂O, CuSO₄.5H₂O and other MXO₄.5H₂O structures (M = Mg or a divalent first row transition metal; X = S, Se, Cr). The original refinement of MgCrO₄.5H₂O did not report any standard uncertainties on lattice parameters and atomic coordinates. It converged with a R value of 0.095, with the displacement parameters of all atoms refined isotropically and with no H-atoms localized. The geometric parameters (bond lengths, bond angles, hydrogen bonding pattern) of the crystal structures of the three isotypic salts MgSO₄.5H₂O, CuSO₄.5H₂O and MgCrO₄.5H₂O were compared by Baur & Rolin (1972), using the original MgCrO₄.5H₂O data by Bertrand et al. (1971) under assumption of geometrically calculated hydrogen positions for the water molecules. Therefore a redetermination of the MgCrO₄.5H₂O structure based on modern CCD-based intensity data seemed appropriate. The current study revealed all non-H atoms with anisotropic displacement parameters and with all H atoms localized, allowing an unambiguous assignment of the hydrogen-bonding pattern, together with more accurate bond lengths.

In a crystal chemical sense, MgCrO₄.5H₂O is better represented by the formula [Mg(H₂O)₄]CrO₄.H₂O. Its structure contains two Mg²⁺ cations, each located on an inversion centre, one CrO₄^2- anion and five water molecules. The Mg²⁺ cations are octahedrally surrounded by four water molecules in equatorial sites and by O atoms of CrO₄ tetrahedra in axial sites. The bridging character of the CrO₄ tetrahedra leads to the formation of chains extending parallel to [011] (Fig. 1). The two [MgO₂(H₂O)₄] octahedra are slightly distorted (Table 1), with average Mg—O bond lengths of 2.070 for Mg1 and 2.061 for Mg2. The CrO₄ tetrahedron is likewise slightly distorted and has a mean Cr—O bond lengths of 1.658 Å, typical for chromates(VI) with isolated CrO₄ anions (1.646 (25) Å; Pressprich et al. 1988). The values of bond lengths and angles of the MgO₆ octahedron and the CrO₄ tetrahedron in the title structure are in the same range as in the related undecahydrate MgCrO₄.11H₂O (Fortes et al., 2013).

Neighbouring chains are linked through O—H···O hydrogen bonds, involving the coordinating water molecules as well as the lattice water molecule (OW4). The strength of most of the hydrogen bonds can be considered as medium-strong, with O···O separations between 2.7262 (13) and 2.7906 (14) Å. Somewhat weaker hydrogen bonds are also present, with O···O separations > 2.80 Å, and the longest O···O separation being 3.1205 (14) Å. It is interesting to note that HW2A protons are involved in a bifurcated hydrogen bond (Table 1).

The experimentally determined hydrogen bonding scheme of [Mg(H₂O)₄]CrO₄.H₂O is in good agreement with the one calculated and discussed previously by Baur & Rolin (1972).

Related literature top

For the original structure determination of the title compound, see: Bertrand et al. (1971). For hydrogen-bonding pattern in the structures of MXO₄.5H₂O compounds (M = Mg, Cu; X = S, Cr), see: Baur & Rolin (1972). For Cr—O bond length distributions in chromates(VI), see: Pressprich et al. (1988). For bond lengths and angles in the related structure of MgCrO₄.11H₂O, see: Fortes et al. (2013). For standardization of structure data, see: Gelato & Parthé (1987).

Experimental top

Half-concentrated chromic acid, prepared by dissolving CrO₃ in water, was neutralized with MgCO₃. This solution was evaporated until dryness and the resulting solid recrystallized in water. Yellow crystals with a platy habit and edge length up to 1 mm were obtained.

Computing details top

Data collection: APEX2 (Bruker, 2011); cell refinement: SAINT (Bruker, 2011); data reduction: SAINT (Bruker, 2011); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: ATOMS for Windows (Dowty, 2008); software used to prepare material for publication: publCIF (Westrip, 2010).

Figures top

Fig. 1. Projection of the structure of MgCrO₄.5H₂O along [110]. MgO₆ octahedra are blue, CrO₄ tetrahedra are red, O atoms are colourless. For non-H atoms, displacement parameters are given at the 90% probability level. O—H···O hydrogen bonds are indicated by green lines.

Magnesium chromate(VI) pentahydrate top

Crystal data top

MgCrO₄·5H₂O	Z = 2
M_r = 230.39	F(000) = 236
Triclinic, P1	D_x = 1.993 Mg m⁻³
Hall symbol: -P 1	Mo Kα radiation, λ = 0.71073 Å
a = 6.1467 (3) Å	Cell parameters from 3623 reflections
b = 6.3742 (4) Å	θ = 3.5–39.7°
c = 10.7048 (6) Å	µ = 1.59 mm⁻¹
α = 75.919 (4)°	T = 296 K
β = 81.603 (3)°	Plate, yellow
γ = 71.134 (3)°	0.10 × 0.08 × 0.01 mm
V = 383.92 (4) Å³

Data collection top

Bruker APEXII CCD diffractometer	4019 independent reflections
Radiation source: fine-focus sealed tube	3340 reflections with I > 2σ(I)
Graphite monochromator	R_int = 0.031
ω– and ϕ–scans	θ_max = 37.5°, θ_min = 3.5°
Absorption correction: multi-scan (SADABS; Bruker, 2011)	h = −10→10
T_min = 0.594, T_max = 0.748	k = −10→10
10569 measured reflections	l = −17→18

Refinement top

Refinement on F²	Primary atom site location: isomorphous structure methods
Least-squares matrix: full	Hydrogen site location: difference Fourier map
R[F² > 2σ(F²)] = 0.031	H atoms treated by a mixture of independent and constrained refinement
wR(F²) = 0.069	w = 1/[σ²(F_o²) + (0.0263P)² + 0.1663P] where P = (F_o² + 2F_c²)/3
S = 1.02	(Δ/σ)_max < 0.001
4019 reflections	Δρ_max = 1.17 e Å⁻³
143 parameters	Δρ_min = −0.72 e Å⁻³
10 restraints

Crystal data top

MgCrO₄·5H₂O	γ = 71.134 (3)°
M_r = 230.39	V = 383.92 (4) Å³
Triclinic, P1	Z = 2
a = 6.1467 (3) Å	Mo Kα radiation
b = 6.3742 (4) Å	µ = 1.59 mm⁻¹
c = 10.7048 (6) Å	T = 296 K
α = 75.919 (4)°	0.10 × 0.08 × 0.01 mm
β = 81.603 (3)°

Data collection top

Bruker APEXII CCD diffractometer	4019 independent reflections
Absorption correction: multi-scan (SADABS; Bruker, 2011)	3340 reflections with I > 2σ(I)
T_min = 0.594, T_max = 0.748	R_int = 0.031
10569 measured reflections

Refinement top

R[F² > 2σ(F²)] = 0.031	10 restraints
wR(F²) = 0.069	H atoms treated by a mixture of independent and constrained refinement
S = 1.02	Δρ_max = 1.17 e Å⁻³
4019 reflections	Δρ_min = −0.72 e Å⁻³
143 parameters

Special details top

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

Refinement. Refinement of F² against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F², conventional R-factors R are based on F, with F set to zero for negative F². The threshold expression of F² > σ(F²) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F² are statistically about twice as large as those based on F, and R- factors based on ALL data will be even larger.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å²) top

	x	y	z	U_iso*/U_eq
Mg1	0.0000	0.0000	0.0000	0.00672 (11)
Mg2	0.0000	0.5000	0.5000	0.00587 (10)
Cr1	0.35414 (3)	0.02843 (3)	0.709355 (18)	0.00508 (4)
O1	0.17472 (14)	0.28496 (15)	0.66131 (9)	0.00919 (15)
O2	0.61762 (13)	0.04795 (15)	0.70554 (9)	0.00825 (15)
O3	0.64389 (14)	0.14163 (16)	0.38351 (9)	0.00978 (16)
O4	0.72867 (14)	0.07678 (16)	0.14081 (9)	0.00919 (15)
OW1	0.30276 (14)	0.54057 (17)	0.40541 (10)	0.01038 (16)
HW1A	0.331 (4)	0.660 (3)	0.378 (2)	0.029 (6)*
HW1B	0.413 (3)	0.435 (3)	0.397 (2)	0.032 (6)*
OW2	0.16925 (14)	0.70426 (17)	0.12175 (9)	0.01025 (16)
HW2A	0.118 (4)	0.675 (4)	0.1938 (16)	0.027 (6)*
HW2B	0.305 (3)	0.669 (4)	0.130 (2)	0.026 (5)*
OW3	0.16025 (16)	0.18082 (17)	0.06898 (10)	0.01161 (17)
HW3A	0.240 (3)	0.115 (3)	0.1307 (17)	0.024 (5)*
HW3B	0.221 (4)	0.263 (4)	0.016 (2)	0.038 (7)*
OW4	0.65171 (16)	0.54576 (17)	0.13300 (10)	0.01237 (17)
HW4A	0.689 (4)	0.414 (3)	0.130 (2)	0.036 (7)*
HW4B	0.714 (4)	0.560 (4)	0.1924 (18)	0.030 (6)*
OW5	0.03331 (14)	0.23596 (17)	0.41734 (10)	0.01081 (16)
HW5A	−0.076 (3)	0.201 (4)	0.413 (2)	0.023 (5)*
HW5B	0.143 (3)	0.156 (3)	0.384 (2)	0.029 (6)*

Atomic displacement parameters (Å²) top

	U¹¹	U²²	U³³	U¹²	U¹³	U²³
Mg1	0.0071 (2)	0.0069 (3)	0.0059 (3)	−0.00218 (18)	−0.00012 (17)	−0.0010 (2)
Mg2	0.0065 (2)	0.0051 (3)	0.0058 (3)	−0.00195 (18)	−0.00013 (17)	−0.0007 (2)
Cr1	0.00505 (6)	0.00478 (8)	0.00506 (8)	−0.00132 (5)	−0.00015 (5)	−0.00072 (6)
O1	0.0101 (3)	0.0067 (4)	0.0085 (4)	−0.0003 (3)	−0.0012 (3)	−0.0001 (3)
O2	0.0072 (3)	0.0092 (4)	0.0093 (4)	−0.0038 (3)	−0.0002 (3)	−0.0021 (3)
O3	0.0102 (3)	0.0092 (4)	0.0113 (4)	−0.0021 (3)	−0.0015 (3)	−0.0054 (3)
O4	0.0100 (3)	0.0084 (4)	0.0071 (4)	−0.0023 (3)	0.0017 (3)	0.0004 (3)
OW1	0.0078 (3)	0.0071 (4)	0.0147 (4)	−0.0024 (3)	0.0022 (3)	−0.0010 (3)
OW2	0.0094 (3)	0.0116 (4)	0.0074 (4)	−0.0020 (3)	−0.0003 (3)	0.0006 (3)
OW3	0.0143 (4)	0.0125 (4)	0.0099 (4)	−0.0075 (3)	−0.0038 (3)	0.0007 (3)
OW4	0.0140 (4)	0.0081 (4)	0.0141 (5)	−0.0029 (3)	0.0001 (3)	−0.0018 (3)
OW5	0.0078 (3)	0.0105 (4)	0.0162 (5)	−0.0024 (3)	0.0002 (3)	−0.0075 (3)

Geometric parameters (Å, º) top

Mg1—OW3ⁱ	2.0505 (9)	Mg2—OW1	2.0467 (8)
Mg1—OW3	2.0505 (9)	Mg2—OW1^vi	2.0467 (8)
Mg1—OW2ⁱⁱ	2.0656 (9)	Mg2—O1	2.1099 (9)
Mg1—OW2ⁱⁱⁱ	2.0656 (9)	Mg2—O1^vi	2.1099 (9)
Mg1—O4^iv	2.0952 (9)	Cr1—O3^vii	1.6357 (9)
Mg1—O4^v	2.0952 (9)	Cr1—O4^vii	1.6554 (9)
Mg2—OW5	2.0265 (10)	Cr1—O1	1.6568 (9)
Mg2—OW5^vi	2.0265 (10)	Cr1—O2	1.6579 (8)

OW3ⁱ—Mg1—OW3	180.00 (5)	OW5^vi—Mg2—O1^vi	92.35 (4)
OW3ⁱ—Mg1—OW2ⁱⁱ	90.97 (4)	OW1—Mg2—O1^vi	88.90 (4)
OW3—Mg1—OW2ⁱⁱ	89.03 (4)	OW1^vi—Mg2—O1^vi	91.10 (4)
OW3ⁱ—Mg1—OW2ⁱⁱⁱ	89.03 (4)	O1—Mg2—O1^vi	180.00 (3)
OW3—Mg1—OW2ⁱⁱⁱ	90.97 (4)	O3^vii—Cr1—O4^vii	109.01 (5)
OW2ⁱⁱ—Mg1—OW2ⁱⁱⁱ	180.00 (7)	O3^vii—Cr1—O1	111.25 (5)
OW3ⁱ—Mg1—O4^iv	91.55 (4)	O4^vii—Cr1—O1	108.90 (5)
OW3—Mg1—O4^iv	88.45 (4)	O3^vii—Cr1—O2	109.30 (4)
OW2ⁱⁱ—Mg1—O4^iv	88.19 (4)	O4^vii—Cr1—O2	109.31 (4)
OW2ⁱⁱⁱ—Mg1—O4^iv	91.81 (4)	O1—Cr1—O2	109.05 (4)
OW3ⁱ—Mg1—O4^v	88.45 (4)	Cr1—O1—Mg2	142.08 (6)
OW3—Mg1—O4^v	91.55 (4)	Cr1^vii—O4—Mg1^viii	140.12 (5)
OW2ⁱⁱ—Mg1—O4^v	91.81 (4)	Mg2—OW1—HW1A	126.4 (16)
OW2ⁱⁱⁱ—Mg1—O4^v	88.19 (4)	Mg2—OW1—HW1B	121.9 (17)
O4^iv—Mg1—O4^v	180.00 (5)	HW1A—OW1—HW1B	111 (2)
OW5—Mg2—OW5^vi	180.00 (5)	Mg1^ix—OW2—HW2A	119.3 (16)
OW5—Mg2—OW1	90.91 (4)	Mg1^ix—OW2—HW2B	122.2 (15)
OW5^vi—Mg2—OW1	89.09 (4)	HW2A—OW2—HW2B	102 (2)
OW5—Mg2—OW1^vi	89.09 (4)	Mg1—OW3—HW3A	118.2 (15)
OW5^vi—Mg2—OW1^vi	90.91 (4)	Mg1—OW3—HW3B	117.0 (18)
OW1—Mg2—OW1^vi	180.0	HW3A—OW3—HW3B	111 (2)
OW5—Mg2—O1	92.35 (4)	HW4A—OW4—HW4B	109 (2)
OW5^vi—Mg2—O1	87.65 (4)	Mg2—OW5—HW5A	119.5 (16)
OW1—Mg2—O1	91.10 (4)	Mg2—OW5—HW5B	131.6 (16)
OW1^vi—Mg2—O1	88.90 (4)	HW5A—OW5—HW5B	109 (2)
OW5—Mg2—O1^vi	87.65 (4)

Symmetry codes: (i) −x, −y, −z; (ii) −x, −y+1, −z; (iii) x, y−1, z; (iv) x−1, y, z; (v) −x+1, −y, −z; (vi) −x, −y+1, −z+1; (vii) −x+1, −y, −z+1; (viii) x+1, y, z; (ix) x, y+1, z.

Hydrogen-bond geometry (Å, º) top

D—H···A	D—H	H···A	D···A	D—H···A
OW1—HW1A···O2^x	0.81 (2)	1.96 (2)	2.7702 (13)	174 (2)
OW1—HW1B···O3	0.80 (2)	1.97 (2)	2.7598 (13)	169 (2)
OW2—HW2A···O1^vi	0.79 (2)	2.19 (2)	2.9014 (13)	150 (2)
OW2—HW2A···OW1	0.79 (2)	2.52 (2)	3.1008 (14)	132 (2)
OW2—HW2B···OW4	0.80 (2)	2.02 (2)	2.8173 (12)	171 (2)
OW3—HW3A···O2^vii	0.82 (2)	1.97 (2)	2.7891 (13)	170 (2)
OW3—HW3B···OW4^xi	0.81 (2)	1.99 (2)	2.7906 (14)	172 (2)
OW4—HW4B···O1^x	0.83 (2)	2.32 (2)	3.1205 (14)	163 (2)
OW4—HW4A···O4	0.80 (2)	2.06 (2)	2.8535 (14)	170 (2)
OW5—HW5B···O2^vii	0.80 (2)	1.93 (2)	2.7262 (13)	173 (2)
OW5—HW5A···O3^iv	0.79 (2)	1.96 (2)	2.7409 (12)	174 (2)

Symmetry codes: (iv) x−1, y, z; (vi) −x, −y+1, −z+1; (vii) −x+1, −y, −z+1; (x) −x+1, −y+1, −z+1; (xi) −x+1, −y+1, −z.

Experimental details

Crystal data
Chemical formula	MgCrO₄·5H₂O
M_r	230.39
Crystal system, space group	Triclinic, P1
Temperature (K)	296
a, b, c (Å)	6.1467 (3), 6.3742 (4), 10.7048 (6)
α, β, γ (°)	75.919 (4), 81.603 (3), 71.134 (3)
V (Å³)	383.92 (4)
Z	2
Radiation type	Mo Kα
µ (mm⁻¹)	1.59
Crystal size (mm)	0.10 × 0.08 × 0.01

Data collection
Diffractometer	Bruker APEXII CCD diffractometer
Absorption correction	Multi-scan (SADABS; Bruker, 2011)
T_min, T_max	0.594, 0.748
No. of measured, independent and observed [I > 2σ(I)] reflections	10569, 4019, 3340
R_int	0.031
(sin θ/λ)_max (Å⁻¹)	0.857

Refinement
R[F² > 2σ(F²)], wR(F²), S	0.031, 0.069, 1.02
No. of reflections	4019
No. of parameters	143
No. of restraints	10
H-atom treatment	H atoms treated by a mixture of independent and constrained refinement
Δρ_max, Δρ_min (e Å⁻³)	1.17, −0.72

Computer programs: APEX2 (Bruker, 2011), SAINT (Bruker, 2011), SHELXS97 (Sheldrick, 2008), SHELXL97 (Sheldrick, 2008), ATOMS for Windows (Dowty, 2008), publCIF (Westrip, 2010).

Selected bond lengths (Å) top

Mg1—OW3	2.0505 (9)	Mg2—O1	2.1099 (9)
Mg1—OW2ⁱ	2.0656 (9)	Cr1—O3ⁱⁱⁱ	1.6357 (9)
Mg1—O4ⁱⁱ	2.0952 (9)	Cr1—O4ⁱⁱⁱ	1.6554 (9)
Mg2—OW5	2.0265 (10)	Cr1—O1	1.6568 (9)
Mg2—OW1	2.0467 (8)	Cr1—O2	1.6579 (8)

Symmetry codes: (i) x, y−1, z; (ii) −x+1, −y, −z; (iii) −x+1, −y, −z+1.

Hydrogen-bond geometry (Å, º) top

D—H···A	D—H	H···A	D···A	D—H···A
OW1—HW1A···O2^iv	0.810 (15)	1.963 (16)	2.7702 (13)	174 (2)
OW1—HW1B···O3	0.796 (16)	1.973 (16)	2.7598 (13)	169 (2)
OW2—HW2A···O1^v	0.792 (15)	2.187 (17)	2.9014 (13)	150 (2)
OW2—HW2A···OW1	0.792 (15)	2.517 (19)	3.1008 (14)	131.7 (19)
OW2—HW2B···OW4	0.803 (15)	2.021 (15)	2.8173 (12)	171 (2)
OW3—HW3A···O2ⁱⁱⁱ	0.824 (15)	1.973 (16)	2.7891 (13)	170 (2)
OW3—HW3B···OW4^vi	0.809 (16)	1.988 (16)	2.7906 (14)	172 (2)
OW4—HW4B···O1^iv	0.826 (16)	2.323 (17)	3.1205 (14)	163 (2)
OW4—HW4A···O4	0.803 (16)	2.060 (16)	2.8535 (14)	170 (2)
OW5—HW5B···O2ⁱⁱⁱ	0.798 (16)	1.932 (16)	2.7262 (13)	173 (2)
OW5—HW5A···O3^vii	0.785 (15)	1.960 (15)	2.7409 (12)	174 (2)

Symmetry codes: (iii) −x+1, −y, −z+1; (iv) −x+1, −y+1, −z+1; (v) −x, −y+1, −z+1; (vi) −x+1, −y+1, −z; (vii) x−1, y, z.

Acknowledgements

The X-ray centre of the Vienna University of Technology is acknowledged for financial support and for providing access to the single-crystal diffractometer.

References

Baur, W. H. & Rolin, J. L. (1972). Acta Cryst. B28, 1448–1455. CrossRef CAS IUCr Journals Web of Science Google Scholar
Bertrand, G., Dusausoy, Y., Protas, J. & Watalle-Marion, G. (1971). C. R. Hebd. Seances Acad. Sci. Ser. C, 272, 530–533. CAS Google Scholar
Bruker (2011). APEX2, SAINT and SADABS. Bruker AXS Inc., Madison, Wisconsin, USA. Google Scholar
Dowty, E. (2008). ATOMS for Windows. Shape Software, Kingsport, Tennessee, USA. Google Scholar
Fortes, A. D., Wood, I. G. & Gutmann, M. J. (2013). Acta Cryst. C69, 324–329. Web of Science CrossRef CAS IUCr Journals Google Scholar
Gelato, L. M. & Parthé, E. (1987). J. Appl. Cryst. 20, 139–143. CrossRef Web of Science IUCr Journals Google Scholar
Pressprich, M. R., Willett, R. D., Poshusta, R. D., Saunders, S. C., Davis, H. B. & Gard, H. B. (1988). Inorg. Chem. 27, 260–264. CSD CrossRef CAS Web of Science Google Scholar
Sheldrick, G. M. (2008). Acta Cryst. A64, 112–122. Web of Science CrossRef CAS IUCr Journals Google Scholar
Westrip, S. P. (2010). J. Appl. Cryst. 43, 920–925. Web of Science CrossRef CAS IUCr Journals Google Scholar