Magnesium selenate hexahydrate, MgSeO₄·6H₂O

MgSeO₄·6H₂O is isostructural with its sulfate analogue and the corresponding cobalt(II) salt. The structure is based on alternating layers of Mg(H₂O)₆ octahedra and SeO₄ tetrahedra parallel to (001), connected via hydrogen bonds. All atoms are on general positions except Mg1 and Mg2 (site symmetries 2 and , respectively). The average Mg-O and Se-O bond lengths are 2.066 and 1.639 Å, respectively.

Key indicators

• Single-crystal X-ray study
• T = 293 K
• Mean [sigma](Se-O) = 0.001 Å
• R factor = 0.019
• wR factor = 0.055
• Data-to-parameter ratio = 18.3

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S1. Comment

Magnesium selenate(VI) hexahydrate, MgSeO₄·6H₂O, crystallizes in the monoclinic structure type of its sulfate analogue MgSO₄·6H₂O (C2/c; Zalkin et al., 1964), known in nature as the mineral hexahydrite. It is also isostructural with the selenates Co^IISeO₄·6H₂O (Ojkova et al., 1992) and Fe^IISeO₄·6H₂O (ICDD-PDF 51–1819). In contrast, both Ni^IISeO₄·6H₂O (Snyman & Pistorius, 1964; Fuess, 1970; Ptasiewicz-Bak et al., 1993) and ZnSeO₄·6H₂O (Hajek & Cepelak, 1965; Stadnicka et al., 1988; Koleva & Stoilova, 1995) are tetragonal with space group P4₁2₁2. Interestingly, the substitution of only about 4at% Ni^II for Mg in MgSeO₄·6H₂O causes the structure to become tetragonal (Stoilova et al., 1995). No data are available on hypothetical M^IISeO₄·6H₂O compounds where M = V, Cr, Mn, Cu, Ca, Ru or Cd.

The occurrence of both monoclinic and tetragonal modifications has also been reported for the M^II–sulfate hexahydrates (e.g. Kutoglu, 1973; Angel & Finger, 1988; Gerkin & Reppart, 1988). In nature, monoclinic and tetragonal Ni^IISO₄·6H₂O are known as the minerals nickelhexahydrite and retgersite, respectively (Mandarino, 1999). The monoclinic structure type is apparently also found for the fluoroberyllate hexahydrates M^IIBeF₄·6H₂O where M = Co, Ni, Zn (Crouzet & Aleonard, 1969; Tedenac et al., 1969).

The unit-cell volume of the title compound is about 5.2% larger than that of its sulfate analogue (Zalkin et al., 1964). This percentage compares favourably with the volume difference of 5.1% between the isostructural oxysalt pentahydrates CuSeO₄·5H₂O and CuSO₄·5H₂O (Kolitsch, 2001).

The polyhedral arrangement in the structure of MgSeO₄·6H₂O is characterized by alternating layers of Mg(H₂O)₆ octahedra and SeO₄ tetrahedra oriented parallel to the (001) plane (Figs. 1 and 2). The average Mg—O and Se—O bond lengths of 2.066 and 1.639 Å, respectively, are close to expected values. The connection between adjacent polyhedra is achieved via medium-strong to weak hydrogen bonds accepted by the oxygen ligands of the SeO₄ group (Table 2). Atom H62, which is involved in the longest hydrogen bond [OW6···OW7ⁱⁱ = 3.0671 (19) Å], shows the largest displacement parameter (if freely refined) of all hydrogen atoms, and it is also the only H atom which donates a hydrogen bond to a water molecule. A determination of the single-crystal unit-cell parameters at 120 K indicated no structural change in the title compound.

The dehydration of MgSeO₄·6H₂O has been studied by Stoilova & Koleva (1995a,b) who observed the penta-, tetra-, di- and monohydrate, as well as anhydrous magnesium selenate, and found that all compounds are isostructural with their sulfate analogues.

S2. Experimental

The title compound was prepared by controlled evaporation at room temperature of an aqueous solution containing selenic acid and magnesium carbonate. Colourless crude columnar crystals up to several cm in length formed. A single large crystal of the sulfite MgSeO₃·6H₂O (Andersen & Lindqvist, 1984) accompanied these crystals.

S3. Refinement

All OW—H distances were restrained to a length of 0.90 (2) Å, and H atoms were constrained to have a fixed U_iso of 0.06 Ų. (Note: freely refined O—H distances ranged between 0.68 and 0.85 Å; the largest isotropic displacement parameters were shown by the H atoms bonded to OW6.)

Computing details

Data collection: COLLECT (Nonius, 2001); cell refinement: HKL SCALEPACK (Otwinowski & Minor. 1997); data reduction: HKL SCALEPACK and DENZO (Otwinowski & Minor, 1997); program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: ATOMS (Shape Software, 1999) and ORTEP-3 for Windows (Farrugia, 1997); software used to prepare material for publication: SHELXL97.

Figures

	Fig. 1. Polyhedral view of MgSeO4·6H2O along [010]. Layers of Mg(H2O)6 octahedra (turquois) parallel to (001) alternate with layers of SeO4 tetrahedra (yellow and marked with crosses). The unit cell is outlined.
	Fig. 2. Anisotropic displacement ellipsoids (50% probability level) of the atoms in the two Mg(H2O)6 octahedra and the SeO4 tetrahedron.

'magnesium selenate(VI) hexahydrate'

Crystal data

MgSeO4·6H2O	F(000) = 1104
Mr = 275.37	Dx = 1.974 Mg m^−3
Monoclinic, C2/c	Mo Kα radiation, λ = 0.71073 Å
a = 10.224 (1) Å	Cell parameters from 2899 reflections
b = 7.370 (1) Å	θ = 2.0–30.0°
c = 24.866 (2) Å	µ = 4.15 mm^−1
β = 98.41 (1)°	T = 293 K
V = 1853.5 (3) Å^3	Fragment, colourless
Z = 8	0.15 × 0.08 × 0.08 mm

Data collection

Nonius KappaCCD diffractometer	2705 independent reflections
Radiation source: fine-focus sealed tube	2510 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.010
ψ and ω scans	θmax = 30.0°, θmin = 3.3°
Absorption correction: multi-scan (HKL SCALEPACK; Otwinowski & Minor, 1997)	h = −14→14
Tmin = 0.575, Tmax = 0.733	k = −10→10
5214 measured reflections	l = −35→34

Refinement

Refinement on F^2	Secondary atom site location: difference Fourier map
Least-squares matrix: full	Hydrogen site location: difference Fourier map
R[F^2 > 2σ(F^2)] = 0.019	H atoms treated by a mixture of independent and constrained refinement
wR(F^2) = 0.055	w = 1/[σ^2(Fo^2) + (0.027P)^2 + 1.70P] where P = (Fo^2 + 2Fc^2)/3
S = 1.06	(Δ/σ)max = 0.004
2705 reflections	Δρmax = 0.48 e Å^−3
148 parameters	Δρmin = −0.39 e Å^−3
12 restraints	Extinction correction: SHELXL97, Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
Primary atom site location: structure-invariant direct methods	Extinction coefficient: 0.0023 (2)

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 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 (Ų)

	x	y	z	Uiso*/Ueq
Se	0.133134 (14)	0.051178 (18)	0.375932 (6)	0.02280 (7)
Mg1	0.0000	0.55734 (9)	0.2500	0.02230 (14)
Mg2	0.0000	0.5000	0.5000	0.02481 (15)
O1	0.00439 (15)	0.05561 (16)	0.32896 (6)	0.0437 (3)
O2	0.20993 (12)	0.24717 (16)	0.38015 (5)	0.0346 (3)
O3	0.23457 (12)	−0.11099 (16)	0.36311 (5)	0.0312 (2)
O4	0.08780 (13)	0.00989 (18)	0.43555 (5)	0.0348 (3)
OW5	0.13620 (16)	0.55724 (16)	0.31969 (6)	0.0397 (3)
OW6	−0.11414 (13)	0.75465 (18)	0.27932 (6)	0.0361 (3)
OW7	−0.11127 (12)	0.35396 (16)	0.28048 (5)	0.0302 (2)
OW8	0.08178 (12)	0.70830 (17)	0.54804 (5)	0.0374 (3)
OW9	−0.03766 (15)	0.6828 (2)	0.43755 (6)	0.0491 (4)
OW10	0.18678 (13)	0.4564 (2)	0.47797 (6)	0.0427 (3)
H51	0.160 (3)	0.657 (3)	0.3357 (10)	0.060*
H52	0.149 (3)	0.465 (3)	0.3421 (10)	0.060*
H61	−0.078 (2)	0.842 (3)	0.2987 (9)	0.060*
H62	−0.1954 (17)	0.771 (4)	0.2739 (10)	0.060*
H71	−0.069 (2)	0.259 (3)	0.2950 (10)	0.060*
H72	−0.159 (2)	0.386 (4)	0.3051 (9)	0.060*
H81	0.140 (2)	0.694 (4)	0.5751 (8)	0.060*
H82	0.037 (2)	0.801 (3)	0.5523 (11)	0.060*
H91	0.003 (2)	0.785 (3)	0.4375 (11)	0.060*
H92	−0.1114 (19)	0.702 (4)	0.4187 (10)	0.060*
H101	0.199 (3)	0.380 (3)	0.4542 (9)	0.060*
H102	0.261 (2)	0.476 (4)	0.4984 (11)	0.060*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Se	0.02295 (9)	0.01662 (9)	0.02709 (9)	0.00057 (5)	−0.00220 (6)	−0.00064 (5)
Mg1	0.0251 (3)	0.0179 (3)	0.0232 (3)	0.000	0.0014 (3)	0.000
Mg2	0.0237 (3)	0.0226 (3)	0.0267 (3)	0.0005 (3)	−0.0013 (3)	−0.0022 (3)
O1	0.0422 (7)	0.0266 (6)	0.0528 (8)	0.0041 (5)	−0.0246 (6)	−0.0035 (5)
O2	0.0346 (6)	0.0206 (5)	0.0454 (7)	−0.0065 (4)	−0.0048 (5)	0.0045 (5)
O3	0.0327 (6)	0.0262 (5)	0.0343 (6)	0.0077 (5)	0.0034 (4)	−0.0029 (5)
O4	0.0406 (7)	0.0297 (6)	0.0359 (6)	0.0016 (5)	0.0111 (5)	−0.0005 (5)
OW5	0.0568 (9)	0.0248 (6)	0.0314 (6)	−0.0049 (5)	−0.0138 (6)	0.0015 (4)
OW6	0.0329 (6)	0.0288 (6)	0.0463 (7)	0.0037 (5)	0.0051 (5)	−0.0110 (5)
OW7	0.0325 (6)	0.0238 (5)	0.0348 (6)	0.0019 (4)	0.0072 (4)	0.0053 (4)
OW8	0.0339 (6)	0.0316 (6)	0.0421 (7)	0.0075 (5)	−0.0097 (5)	−0.0135 (5)
OW9	0.0420 (7)	0.0433 (8)	0.0542 (8)	−0.0176 (6)	−0.0187 (6)	0.0214 (6)
OW10	0.0272 (6)	0.0559 (9)	0.0441 (7)	0.0026 (6)	0.0020 (5)	−0.0197 (6)

Geometric parameters (Å, º)

Se—O1	1.6274 (13)	Mg2—OW10	2.0870 (14)
Se—O2	1.6401 (11)	Mg2—OW10^ii	2.0870 (14)
Se—O3	1.6439 (11)	OW5—H51	0.855 (17)
Se—O4	1.6449 (12)	OW5—H52	0.876 (17)
Mg1—OW5^i	2.0582 (13)	OW6—H61	0.855 (17)
Mg1—OW5	2.0582 (13)	OW6—H62	0.832 (17)
Mg1—OW6	2.0632 (13)	OW7—H71	0.872 (17)
Mg1—OW6^i	2.0632 (13)	OW7—H72	0.870 (17)
Mg1—OW7	2.0899 (12)	OW8—H81	0.836 (16)
Mg1—OW7^i	2.0899 (12)	OW8—H82	0.835 (17)
Mg2—OW8^ii	2.0471 (12)	OW9—H91	0.862 (17)
Mg2—OW8	2.0471 (12)	OW9—H92	0.840 (17)
Mg2—OW9^ii	2.0493 (13)	OW10—H101	0.839 (17)
Mg2—OW9	2.0493 (13)	OW10—H102	0.859 (18)
O1—Se—O2	110.95 (6)	OW8^ii—Mg2—OW10	92.45 (5)
O1—Se—O3	110.02 (7)	OW8—Mg2—OW10	87.55 (5)
O2—Se—O3	110.06 (6)	OW9^ii—Mg2—OW10	91.15 (7)
O1—Se—O4	110.22 (8)	OW9—Mg2—OW10	88.85 (7)
O2—Se—O4	107.50 (7)	OW8^ii—Mg2—OW10^ii	87.55 (5)
O3—Se—O4	108.01 (6)	OW8—Mg2—OW10^ii	92.45 (5)
OW5^i—Mg1—OW5	179.96 (8)	OW9^ii—Mg2—OW10^ii	88.85 (7)
OW5^i—Mg1—OW6	87.12 (6)	OW9—Mg2—OW10^ii	91.15 (7)
OW5—Mg1—OW6	92.91 (6)	OW10—Mg2—OW10^ii	180.0
OW5^i—Mg1—OW6^i	92.91 (6)	Mg1—OW5—H51	120.1 (17)
OW5—Mg1—OW6^i	87.12 (6)	Mg1—OW5—H52	123.6 (19)
OW6—Mg1—OW6^i	90.37 (8)	H51—OW5—H52	112 (3)
OW5^i—Mg1—OW7	88.16 (6)	Mg1—OW6—H61	120.9 (18)
OW5—Mg1—OW7	91.81 (5)	Mg1—OW6—H62	131 (2)
OW6—Mg1—OW7	90.65 (5)	H61—OW6—H62	108 (3)
OW6^i—Mg1—OW7	178.56 (5)	Mg1—OW7—H71	117.5 (18)
OW5^i—Mg1—OW7^i	91.81 (5)	Mg1—OW7—H72	117.1 (19)
OW5—Mg1—OW7^i	88.16 (6)	H71—OW7—H72	103 (2)
OW6—Mg1—OW7^i	178.56 (5)	Mg2—OW8—H81	123.6 (19)
OW6^i—Mg1—OW7^i	90.65 (5)	Mg2—OW8—H82	120.2 (19)
OW7—Mg1—OW7^i	88.35 (7)	H81—OW8—H82	110 (2)
OW8^ii—Mg2—OW8	180.00 (6)	Mg2—OW9—H91	122.6 (18)
OW8^ii—Mg2—OW9^ii	87.97 (6)	Mg2—OW9—H92	126.0 (19)
OW8—Mg2—OW9^ii	92.03 (6)	H91—OW9—H92	104 (2)
OW8^ii—Mg2—OW9	92.03 (6)	Mg2—OW10—H101	121.7 (19)
OW8—Mg2—OW9	87.97 (6)	Mg2—OW10—H102	125 (2)
OW9^ii—Mg2—OW9	180.00 (6)	H101—OW10—H102	109 (3)

Symmetry codes: (i) −x, y, −z+1/2; (ii) −x, −y+1, −z+1.

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
OW5—H51···O3^iii	0.86 (2)	1.95 (2)	2.7997 (17)	170 (3)
OW5—H52···O2	0.88 (2)	1.92 (2)	2.7783 (17)	165 (3)
OW6—H61···O1^iii	0.86 (2)	1.89 (2)	2.7339 (18)	169 (3)
OW6—H62···OW7^iv	0.83 (2)	2.31 (2)	3.0671 (19)	152 (2)
OW7—H71···O1	0.87 (2)	1.83 (2)	2.6962 (17)	174 (3)
OW7—H72···O3^v	0.87 (2)	1.93 (2)	2.7784 (17)	165 (3)
OW8—H81···O3^vi	0.84 (2)	1.95 (2)	2.7763 (17)	169 (3)
OW8—H82···O4^ii	0.84 (2)	1.95 (2)	2.7737 (18)	171 (3)
OW9—H91···O4^iii	0.86 (2)	1.87 (2)	2.7346 (19)	178 (3)
OW9—H92···O2^v	0.84 (2)	1.96 (2)	2.8008 (18)	175 (3)
OW10—H101···O2	0.84 (2)	2.10 (2)	2.9190 (19)	164 (3)
OW10—H102···O4^vi	0.86 (2)	2.09 (2)	2.924 (2)	164 (3)

Symmetry codes: (ii) −x, −y+1, −z+1; (iii) x, y+1, z; (iv) −x−1/2, y+1/2, −z+1/2; (v) x−1/2, y+1/2, z; (vi) −x+1/2, −y+1/2, −z+1.