Orientational disorder of [Mg(H₂O)₆] octa­hedra in the novel magnesium selenite hydrate Mg(SeO₃)·7.5H₂O

The crystal structure of magnesium selenite 7.5-hydrate, Mg(SeO₃)·7.5H₂O (space group P6₃/mmc), is characterized by two crystallographically distinct [Mg(H₂O)₆]²⁺ octa­hedra, one of which is disordered over two different orientations. The selenite groups and water mol­ecules (with partially disordered H atoms) bridge the octa­hedra via hydrogen bonds. All the atoms are located on special positions, except for one water mol­ecule.

S1. Comment

In the course of our studies of kroenkite-like compounds (Fleck & Kolitsch, 2003, Kolitsch & Fleck, 2006) and Tutton's salts (Fleck & Kolitsch, 2002a,b), we have investigated the system Rb–Mg–selenic acid. Instead of the expected binary selenate compounds, the syntheses yielded small, white crystals which turned out to be a novel Mg selenite hydrate species, namely MgSeO₃·7.5H₂O. A search of the literature showed that a total number of six Mg selenite and hydrogenselenite structures have been published (Table 3). However, we found no indication that the present compound has been encountered before. Nevertheless, this structure is remarkable because it is unique in several ways among Mg selenite and hydrogenselenite compounds.

The principal building units of the structure are two crystallographically distinct [Mg(H₂O)₆]²⁺ octahedra, both of which are located on special positions. Although both octahedra are virtually undistorted, they show significant differences. The Mg1 atoms are located at Wyckoff position 2a and the O1 atoms at the 12k positions, thus building nearly perfect octahedra. The Mg2 atoms, however, are located at the 2d position with the coordinating O2 atoms on the 24l positions. Because of the site symmetry, there are 12 symmetry-related O atoms around the Mg2 atoms, which is chemically impossible. However, the refinement indicated that this position is only half-occupied, which means that there are two possible orientations of the Mg2 octahedra in the crystal structure. The refinement gave an occupation of 0.5134 (s.u.?) for atom O2, which was then fixed at 1/2, as well as for atoms H2A and H2B. The selenite groups (on threefold axes) and two crystallographically distinct water molecules are located in the interstices between the [Mg(H₂O)₆]²⁺ octahedra. The O atoms of these water molecules are also located on special positions, viz. 4e for O4 and 2c for O5. The H atoms of these molecules could be found in a difference Fourier map in partially occupied positions. Atom O4 is surrounded by four and the O5 atom by six possible H-atom positions. Around O4, one H atom is located on position 4e, which is fully occupied. Three more symmetry-equivalent H atoms are on position 12k. For chemical reasons, the occupancy of these H-atom positions was fixed at one-third. The O5 position 2c is surrounded by six symmetry-equivalent H atoms on position 12k, all of which were also assumed to be one-third occupied.

Although the refinement of hydrogen site occupancies is something to be considered with greatest doubt, it was attempted in this case. Most surprisingly, the refined values matched reasonably well with the assumed values for all H atoms. Of course, the refined H-atom occupancies can only be taken as an indication that the model seems to be correct. Thus, the ideal formula of the new selenite is given as Mg SeO₃·7.5H₂O.

Naturally, the presence of disorder in the structure may lead to the conclusion that the symmetry of the crystal was chosen incorrectly. Therefore, the refinement of a structure model with lower symmetry (P31c, P31c and P3) was attempted. However, the disordered Mg2 octahedra were found in all space groups, with a refined occupancy for the O2 position about 50%. Also, the O atoms of the non-coordinating water molecules were found to have the same occupancy as in the P6₃/mmc model. Furthermore, the lower the symmetry, the harder the refinement of the H atoms. In all cases, the final R indices were worse than in the P6₃/mmc case: 0.0303, 0.0328 and 0.0342 for the space groups P31c, P31c and P3, respectively. Finally, all these structure models were checked with the program PLATON (Spek, 2000), which detected higher symmetry corresponding to P6₃/mmc in all cases. All these points lead to the conclusion that the higher symmetry is correct.

The presence of disordered [Mg(H₂O)₆]²⁺ octahedra in Mg(SeO₃)·7.5(H₂O) is unique among all the known Mg selenites and hydrogenselenites (see Table 3). The only member of this group where disorder occurs is Mg(HSeO₃)₂·3(H₂O) (Boldt et al., 1999), which is also hexagonal (space group P62c). However, in this case, the disorder involves the hydrogenselenite groups.

S2. Experimental

An equimolar mixture of Rb₂CO₃ (47.54 mg) and MgCO₃ (16.22 mg) was dissolved in dilute selenic acid. The solutions were evaporated slowly at a temperature of approximately 295 K over a period of several weeks. The syntheses yielded small, colourless crystals of the title compound of up to 0.15 mm in size and crystals of Rb₂CO₃.

Computing details

Data collection: COLLECT (Nonius, 2003); cell refinement: COLLECT; data reduction: COLLECT; program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: DIAMOND (Pennington, 1999); software used to prepare material for publication: SHELXL97.

Figures

Fig. 1. : The connectivity in the title compound, with displacement ellipsoids at the 50% probability level. For clarity, the symmetry-equivalent O2 and H atoms of the water molecules around the Mg atoms are not shown. [Symmetry codes: (i) −x + y, −x, z; (ii) −y, x − y, z; (iii) y, x, −z; (iv) x − y, −y, −z; (v) −x, −x − y, −z; (vi) 1 − y, x − y, z, (vii) 1 − x + y, 1 − x, z; (viii) −x + y, 1 − x, z; (ix) 1 − y, 1 + x-y, z; (x) x, 1 + x − y, 1/2 − z; (xi) −x + y, y, 1/2 − z; (xii) 1 − y, 1 − x, 1/2 − z.]

Fig. 2. : The packing of the structure of the title compound, viewed along [110]. Note the two different Mg(H2O)6 octahedra, one of which is shown disordered between two positions at z = ±1/4.

(I)

Crystal data

Mg(SeO3)·7.5H2O	Dx = 1.854 Mg m^−3
Mr = 286.39	Mo Kα radiation, λ = 0.71073 Å
Hexagonal, P63/mmc	Cell parameters from 2894 reflections
Hall symbol: -P 6c 2c	θ = 4.5–28.4°
a = 7.262 (1) Å	µ = 3.75 mm^−1
c = 22.470 (5) Å	T = 293 K
V = 1026.2 (3) Å^3	Prism, colourless
Z = 4	0.10 × 0.06 × 0.06 mm
F(000) = 580

Data collection

Nonius KappaCCD diffractometer	655 independent reflections
Radiation source: fine-focus sealed tube	629 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.066
Detector resolution: 9 pixels mm^-1	θmax = 30.5°, θmin = 3.2°
ϕ and ω scans	h = −10→10
Absorption correction: multi-scan (Otwinowski & Minor, 1997)	k = −8→8
Tmin = 0.706, Tmax = 0.807	l = −31→32
3869 measured reflections

Refinement

Refinement on F^2	Secondary atom site location: difference Fourier map
Least-squares matrix: full	Hydrogen site location: inferred from neighbouring sites
R[F^2 > 2σ(F^2)] = 0.023	All H-atom parameters refined
wR(F^2) = 0.053	w = 1/[σ^2(Fo^2) + (0.0164P)^2 + 0.425P] where P = (Fo^2 + 2Fc^2)/3
S = 1.21	(Δ/σ)max < 0.001
655 reflections	Δρmax = 0.48 e Å^−3
55 parameters	Δρmin = −0.50 e Å^−3
5 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: 0.044 (2)

Special details

Experimental. Single-crystal X-ray intensity data were collected at 293 K on a Nonius Kappa diffractometer with CCD-area detector, using 348 frames with phi- and omega-increments of 1 degree and a counting time of 200 s per frame. The crystal- to-detector-distance was 32 mm. The whole Ewald sphere was measured. The reflection data were processed with the Nonius program suite DENZO-SMN and corrected for Lorentz, polarization, background and absorption effects (Otwinowski and Minor, 1997). The crystal structure was determined by direct methods (SHELXS97, Sheldrick, 1997) and subsequent Fourier and difference Fourier syntheses, followed by full-matrix least-squares refinements on F² (SHELXL97, Sheldrick, 1997). The O—H-distances were fixed to 0.96 (2) Å (except for O1, the site occupation parameters for O2, H2a, H2b, H4a, H4b and H5 were fixed at the ideal values. Using anisotropic treatment of the non-H atoms and unrestrained isotropic treatment of the H atoms, the refinement converged to R = 0.023.

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	Occ. (<1)
Mg1	0.0000	0.0000	0.0000	0.0205 (3)
O1	0.13314 (14)	0.2663 (3)	0.05284 (8)	0.0393 (4)
H1	0.074 (5)	0.352 (4)	0.0636 (11)	0.092 (10)*
Mg2	0.6667	0.3333	0.2500	0.0272 (4)
O2	0.4333 (4)	0.0968 (4)	0.19699 (10)	0.0409 (5)	0.50
H2A	0.462 (9)	0.058 (10)	0.1592 (13)	0.10 (2)*	0.50
H2B	0.300 (6)	0.079 (10)	0.185 (3)	0.12 (3)*	0.50
Se	0.6667	0.3333	0.050245 (14)	0.02134 (14)
O3	0.54585 (10)	0.45415 (10)	0.08411 (6)	0.0259 (3)
O4	0.0000	0.0000	0.17652 (14)	0.0411 (7)
H4A	0.0000	0.0000	0.1343 (9)	0.08 (2)*
H4B	0.067 (3)	0.134 (6)	0.195 (3)	0.04 (2)*	0.33
O5	0.3333	0.6667	0.2500	0.0560 (14)
H5	0.392 (4)	0.784 (8)	0.222 (2)	0.04 (2)*	0.33

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Mg1	0.0190 (4)	0.0190 (4)	0.0234 (7)	0.0095 (2)	0.000	0.000
O1	0.0242 (5)	0.0358 (9)	0.0617 (10)	0.0179 (4)	−0.0123 (4)	−0.0245 (8)
Mg2	0.0246 (5)	0.0246 (5)	0.0324 (8)	0.0123 (3)	0.000	0.000
O2	0.0343 (12)	0.0379 (12)	0.0363 (10)	0.0074 (10)	0.0052 (9)	−0.0100 (9)
Se	0.01786 (14)	0.01786 (14)	0.0283 (2)	0.00893 (7)	0.000	0.000
O3	0.0219 (4)	0.0219 (4)	0.0376 (6)	0.0137 (5)	0.0004 (3)	−0.0004 (3)
O4	0.0441 (10)	0.0441 (10)	0.0350 (14)	0.0221 (5)	0.000	0.000
O5	0.0475 (19)	0.0475 (19)	0.073 (4)	0.0237 (9)	0.000	0.000

Geometric parameters (Å, º)

Mg1—O1^i	2.0528 (15)	Mg2—O2^xiii	2.081 (2)
Mg1—O1	2.0528 (16)	Mg2—O2^xiv	2.081 (2)
Mg1—O1^ii	2.0528 (16)	Mg2—O2^xv	2.081 (2)
Mg1—O1^iii	2.0528 (16)	Mg2—O2^xvi	2.081 (2)
Mg1—O1^iv	2.0528 (16)	O2—O2^vi	1.672 (5)
Mg1—O1^v	2.0528 (16)	O2—O2^xiv	1.741 (5)
O1—H1	0.95 (2)	O2—H2A	0.948 (19)
Mg2—O2^vi	2.081 (2)	O2—H2B	0.95 (2)
Mg2—O2^vii	2.081 (2)	Se—O3^xv	1.6995 (13)
Mg2—O2	2.081 (2)	Se—O3^x	1.6995 (13)
Mg2—O2^viii	2.081 (2)	Se—O3	1.6995 (13)
Mg2—O2^ix	2.081 (2)	O4—H4A	0.95 (2)
Mg2—O2^x	2.081 (2)	O4—H4B	0.94 (2)
Mg2—O2^xi	2.081 (2)	O5—H5	0.96 (2)
Mg2—O2^xii	2.081 (2)
O1^i—Mg1—O1	180.00 (10)	O2^vi—O2—Mg2	66.32 (7)
O1^i—Mg1—O1^ii	89.90 (8)	O2^xiv—O2—Mg2	65.27 (7)
O1—Mg1—O1^ii	90.10 (8)	O2^vi—O2—H2A	69 (3)
Mg1—O1—H1	127.6 (17)	O2^xiv—O2—H2A	114 (4)
O2^vi—Mg2—O2^vii	90.29 (13)	Mg2—O2—H2A	123 (4)
O2^vi—Mg2—O2	47.36 (13)	O2^vi—O2—H2B	163 (4)
O2^vii—Mg2—O2	131.59 (4)	O2^xiv—O2—H2B	68 (4)
O2^vi—Mg2—O2^viii	69.83 (13)	Mg2—O2—H2B	129 (4)
O2^vii—Mg2—O2^viii	49.46 (14)	H2A—O2—H2B	94 (5)
O2—Mg2—O2^viii	88.73 (14)	O3^xv—Se—O3^x	101.50 (6)
O2—Mg2—O2^ix	178.90 (14)	O3^xv—Se—O3	101.50 (6)
O2^viii—Mg2—O2^ix	90.49 (9)	O3^x—Se—O3	101.50 (6)
O2^vi—O2—O2^xiv	120.000 (1)	H4A—O4—H4B	116 (5)

Symmetry codes: (i) −x, −y, −z; (ii) x−y, x, −z; (iii) −x+y, −x, z; (iv) −y, x−y, z; (v) y, −x+y, −z; (vi) −x+y+1, y, z; (vii) −y+1, x−y, −z+1/2; (viii) −x+y+1, y, −z+1/2; (ix) −y+1, −x+1, −z+1/2; (x) −y+1, x−y, z; (xi) x, y, −z+1/2; (xii) −y+1, −x+1, z; (xiii) −x+y+1, −x+1, −z+1/2; (xiv) x, x−y, z; (xv) −x+y+1, −x+1, z; (xvi) x, x−y, −z+1/2.

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
O1—H1···O3^xvii	0.95 (2)	1.76 (2)	2.6923 (12)	167 (3)
O2—H2A···O3^x	0.95 (2)	1.77 (3)	2.671 (3)	158 (5)
O2—H2B···O4	0.95 (2)	1.97 (2)	2.897 (2)	167 (6)
O4—H4A···O1	0.95 (2)	2.48 (2)	3.245 (3)	138 (1)
O4—H4B···O2^xiv	0.94 (2)	2.31 (2)	2.897 (2)	120 (2)
O5—H5···O2^xviii	0.96 (2)	2.21 (3)	3.071 (3)	148 (4)

Symmetry codes: (x) −y+1, x−y, z; (xiv) x, x−y, z; (xvii) −x+y, −x+1, z; (xviii) −x+y+1, y+1, z.