Redetermination of MgCrO4·5H2O

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.

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][531][532][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. MgCrO 4 Á5H 2 O adopts the MgSO 4 Á5H 2 O structure type. It contains two Mg 2+ sites on special positions with site symmetry 1, one tetrahedral CrO 4 group and five water molecules. Four of them coordinate to the Mg 2+ cation, and one is an uncoordinating lattice water molecule. The octahedral environment of the Mg 2+ cation is completed by two axial O atoms of CrO 4 tetrahedra. This arrangement leads to the formation of chains parallel to [011]. Adjacent chains are linked through O-HÁ Á ÁO hydrogen bonds (one of them bifurcated), involving both the coordinating and lattice water molecules, into a three-dimensional network.
The X-ray centre of the Vienna University of Technology is acknowledged for financial support and for providing access to the single-crystal diffractometer.  Baur & Rolin (1972), using the original MgCrO 4 .5H 2 O data by Bertrand et al. (1971) under assumption of geometrically calculated hydrogen positions for the water molecules. Therefore a redetermination of the MgCrO 4 .5H 2 O 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.   Baur & Rolin (1972).

Experimental
Half-concentrated chromic acid, prepared by dissolving CrO 3 in water, was neutralized with MgCO 3 . This solution was evaporated until dryness and the resulting solid recrystallized in water. Yellow crystals with a platy habit and edge length supplementary materials sup-2 Acta Cryst. (2013). E69, i48-i49 up to 1 mm were obtained.

Refinement
In the original study (Bertrand et al., 1971) a non-reduced setting in space group P1 has been used, with lattice parameters a = 6.384, b = 10.702, c = 6.115 Å, α = 81.55, β = 108.75, γ = 104.333 °. For the present study the unit-cell parameters were transformed into the reduced cell using the transformation matrix (0 0 1, 1 0 0, 0 1 0). For refinement, the atomic coordinates of the original determination were used as starting parameters. They were finally standardized with STRUCTURE TIDY (Gelato & Parthé, 1987). All H atoms were discernible from difference maps. Their coordinates were refined with distance restraints of d(O-H) = 0.82 (5) Å, with individual U iso parameters for each H atom. One reflection (0 0 1) was affected by the beam stop and was omitted from the refinement.

Computing details
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). Special details 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 F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.  (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.