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Acta Crystallogr Sect E Struct Rep Online. 2008 July 1; 64(Pt 7): m863.
Published online 2008 June 7. doi:  10.1107/S1600536808015870
PMCID: PMC2961852

A new polymorph of magnesium oxalate dihydrate


In the asymmetric unit of the title compound, catena-poly[[diaqua­magnesium(II)]-μ-oxalato], [Mg(C2O4)(H2O)2]n, there is one Mg atom in an octa­hedral coordination with site symmetry 222, a unique C atom of the oxalate anion lying on a twofold axis, an O atom of the anion in a general position and a water O atom at a site with imposed twofold rotation symmetry. The Mg2+ ions are ligated by water mol­ecules and bridged by the anions to form chains that are held together by O—H(...)O hydrogen bonds. The structure of the title compound has already been reported in a different space group [Lagier, Pezerat & Dubernat (1969 [triangle]). Rev. Chim. Miner. 6, 1081–1093; Levy, Perrotey & Visser (1971 [triangle]). Bull. Soc. Chim. Fr. pp. 757–761].

Related literature

For related literature, see: Basso et al. (1997 [triangle]); Caric (1959 [triangle]); Deyrieux et al. (1973 [triangle]); Echigo et al. (2005 [triangle]); Huang & Mak (1990 [triangle]); Lagier et al. (1969 [triangle]); Le Page (1987 [triangle]); Lethbridge et al. (2003 [triangle]); Levy et al. (1971 [triangle]); Neder et al. (1997 [triangle]); Schefer & Grube (1995 [triangle]); Tazzoli & Domeneghetti (1980 [triangle]); Vanhoyland, Bouree et al. (2001 [triangle]); Vanhoyland, Van Bael et al. (2001 [triangle]).

An external file that holds a picture, illustration, etc.
Object name is e-64-0m863-scheme1.jpg


Crystal data

  • [Mg(C2O4)(H2O)2]
  • M r = 148.36
  • Orthorhombic, An external file that holds a picture, illustration, etc.
Object name is e-64-0m863-efi11.jpg
  • a = 5.3940 (11) Å
  • b = 12.691 (3) Å
  • c = 15.399 (3) Å
  • V = 1054.1 (4) Å3
  • Z = 8
  • Mo Kα radiation
  • μ = 0.29 mm−1
  • T = 290 K
  • 0.30 × 0.20 × 0.15 mm

Data collection

  • Rigaku AFC-7R diffractometer
  • Absorption correction: ψ scan (Kopfmann & Huber, 1968 [triangle]) T min = 0.915, T max = 0.962
  • 1110 measured reflections
  • 483 independent reflections
  • 321 reflections with I > 2σ(I)
  • R int = 0.054
  • 3 standard reflections every 150 reflections intensity decay: 1.1%


  • R[F 2 > 2σ(F 2)] = 0.034
  • wR(F 2) = 0.110
  • S = 0.97
  • 483 reflections
  • 27 parameters
  • All H-atom parameters refined
  • Δρmax = 0.89 e Å−3
  • Δρmin = −0.48 e Å−3

Data collection: Rigaku/AFC Diffractometer Control Software (Rigaku, 1994 [triangle]); cell refinement: Rigaku/AFC Diffractometer Control Software; data reduction: Rigaku/AFC Diffractometer Control Software; program(s) used to solve structure: SHELXS97 (Sheldrick, 2008 [triangle]); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008 [triangle]); molecular graphics: ATOMS (Dowty, 1999 [triangle]); software used to prepare material for publication: SHELXL97.

Table 1
Hydrogen-bond geometry (Å, °)

Supplementary Material

Crystal structure: contains datablocks global, I. DOI: 10.1107/S1600536808015870/pv2083sup1.cif

Structure factors: contains datablocks I. DOI: 10.1107/S1600536808015870/pv2083Isup2.hkl

Additional supplementary materials: crystallographic information; 3D view; checkCIF report


This work was supported by the Funding Project for Academic Human Resources Development in Institutions of Higher Learning under the jurisdiction of Beijing Municipality.

supplementary crystallographic information


Oxalates are of considerable interest because many of them are natural minerals and in addition, the oxalate anion can adopt different coordination modes to bind metals to form infinite chains, sheets and networks, leading to the rich structural chemistry. For instance, in the system of MO (M = alkali-earth metal)–H2C2O4–H2O, at least eight phases have been structurally characterized, including Mg(C2O4).2H2O (Lagier et al., 1969; Levy et al., 1971), Ca(C2O4).H2O (Echigo et al., 2005), Ca(C2O4).2.375H2O (Tazzoli & Domeneghetti, 1980), Ca(C2O4).3H2O (Basso et al., 1997), SrH(C2O4)(C2O4)0.5 (Vanhoyland, Van Bael et al., 2001), SrH(C2O4)(C2O4)0.5.H2O (Vanhoyland, Bouree et al., 2001), Ba(C2O4).H2O (Huang & Mak, 1990), and Ba(C2O4).3.5H2O (Neder et al., 1997). Among them, Mg(C2O4).2H2O (which we call α-Mg(C2O4).2H2O later) was reported to have one-dimensional (1D) magnesium oxalate chains, Ca(C2O4).3H2O has a layered structure, and others contain three-dimensional metal oxalate frameworks. During our exploratory syntheses of novel hydrated borate materials, we have unexpectedly obtained a new Mg(C2O4).2H2O polymorph [called β-Mg(C2O4).2H2O in this work], (I), as a byproduct. It crystallizes in a space group different from those of other oxalates of similar stoichiometry. We describe its synthesis and crystal structure here for the first time.

The title structure contains Mg2+ cations, [C2O4]2- anions, and H2O molecules as the fundamental structural building units (Fig. 1). The anions are bridged by octahedral Mg2+ centers to generate a 1D infinite polymeric chain, and H2O molecules are located on the two sides of the chains and coordinated to the Mg2+ centers to complete the octahedral coordination sphere (Fig. 2). The [Mg(C2O4)(H2O)2] chains extend along the [100] direction, and are held together via O—H···O hydrogen bonds (Table 1).

The Mg atom occupies one crystallographically distinct octahedral site with site symmetry 222. Each Mg2+ is coordinated by six O atoms, four of which are from two oxalate ions and the others from two H2O molecules. The Mg—O distances are very reasonable when compared with those observed in Mg(NO3)2.6H2O, where octahedrally coordinated Mg2+ is also found (Schefer & Grube, 1995). The unique C atom of the anion lies on a 2-fold axis and an O-atom on a general position. The unique O atom of H2O lies on a 2-fold axis. The oxalate ion is nearly planar, with a mean deviation of 0.0134 Å, and the bond geometries of [C2O4]2- are in accord with those observed in other oxalate compounds (Lethbridge et al., 2003).

In the previously reported oxalates, no one is exactly isotypic with the title compound. Several compounds including M(C2O4).2H2O (M = Mg, Fe, Co, Ni, Zn) (Levy et al., 1971; Caric, 1959; Deyrieux et al., 1973) also contain topologically identical [M(C2O4)(H2O)2] chains, but crystallize in the monoclinic space group C2/c. An examination of positional parameters of these compounds using the program MISSYM (Le Page, 1987) did not show potential additional symmetry. In fact, the space group C2/c of α-Mg(C2O4).2H2O as well as the isostructural analogs is a "translationengleiche" subgroup (index 2) of the group Fddd adopted by β-Mg(C2O4).2H2O. The lattice vectors of α-Mg(C2O4).2H2O (a1, a2 and a3) are related to those of its β-form (a, b and c) in the following manner: a1 = b, a2 = a, and a3 = -0.5b - 0.5c. The other compound, Mn(C2O4).2H2O, was also reported to exist in two forms. The α-phase (Deyrieux et al., 1973) is isostructural with α-Mg(C2O4).2H2O, while the β-phase crystallizes in the space group P212121 (Lethbridge et al., 2003). The crystal structure of β-Mn(C2O4).2H2O also consists of chains of oxalate-bridged Mn2+ centers, but MnO6 octahedra in these chains are interconnected through sharing O corners and each oxalate ion acts as a tri-dentate ligand. This is different from the situation in other members of the M(C2O4).2H2O family of compounds, where MO6 octahedra are separated from each other and the oxalate ions act as tetra-dentate ligands. It is the difference in the coordination modes of the oxalate ions that is responsible for the structural versatility of M(C2O4).2H2O.


β-Mg(C2O4).2H2O was first obtained from a hydrothermal reaction in an attempt to prepare novel hydrated borates. For the preparation of MgB6O10, a stoichiometric mixture of MgO and B2O3 was heated at 873 K for two weeks with several intermediate re-mixings and the resulting product was identified to be the pure phase of MgB6O10 based on the powder XRD analysis. A 0.300 g (3.376 mmol) sample of MgB6O10, 3 ml pyridine, 0.5 ml 14.5 M (65%) HNO3, and 0.5 ml H2O were sealed in an 15-ml Teflon-lined autoclave and subsequently heated at 453 K for one week, then cooled slowly to room temperature. The product consisted of colorless, block-like crystals with the largest having dimensions of 0.6 × 0.6 × 0.8 mm3 in pale yellow mother liquor. The final pH of the reaction system was about 1.0. The crystals were isolated in about 30% yield (based on Mg) by washing the reaction product with deionized water and anhydrous ethanol followed by drying with anhydrous acetone. X-ray structural analysis indicated that the formula of this compound may be Mg(C2O4).2H2O. It is unclear how the oxalate groups are formed.

Subsequently, a separate set of experiments was conducted, in which the starting materials were: 0.2718 g (6.7403 mmol) MgO, 0.8497 g (6.7400 mmol) H2(C2O4).2H2O, and 3 ml H2O, and the heating and isolation procedures were the same as those described above. The reaction resulted in pure colorless crystals. The powder XRD pattern of the ground crystals in this experiment was in good agreement with that calculated from the single-crystal data of Mg(C2O4).2H2O from the former experiment, confirming that the same phase had been obtained.


H-atom positions were located in a difference Fourier map and all associated parameters were refined freely.


Fig. 1.
The coordination geometry of Mg1 in (I) with displacement ellipsoids drawn at the 50% probability level. Symmetry codes: (i) 3/4 - x, 3/4 - y, z; (ii) x, 3/4 - y, 3/4 - z; (iii) 3/4 - x, y, 3/4 - z; (iv) 7/4 - x, 3/4 - y, z; (v) 7/4 - x, y, 3/4 - z; (vii) ...
Fig. 2.
The crystal structure of (I) projected approximately along the [100] direction (a) as well as the single chain of [Mg(C2O4)(H2O)2] (b); the H2···O1 contacts are shown as dashed lines.

Crystal data

[Mg(C2O4)(H2O)2]F000 = 608
Mr = 148.36Dx = 1.870 Mg m3
Orthorhombic, FdddMo Kα radiation λ = 0.71073 Å
Hall symbol: -F 2uv 2vwCell parameters from 25 reflections
a = 5.3940 (11) Åθ = 13.0–19.6º
b = 12.691 (3) ŵ = 0.29 mm1
c = 15.399 (3) ÅT = 290 K
V = 1054.1 (4) Å3Block, colourless
Z = 80.30 × 0.20 × 0.15 mm

Data collection

Rigaku AFC-7R diffractometerRint = 0.054
Radiation source: fine-focus sealed tubeθmax = 32.5º
Monochromator: graphiteθmin = 4.2º
T = 290 Kh = 0→8
2θ/ω scansk = 0→19
Absorption correction: ψ scan(Kopfmann & Huber, 1968)l = 0→23
Tmin = 0.915, Tmax = 0.9623 standard reflections
1110 measured reflections every 150 reflections
483 independent reflections intensity decay: 1.1%
321 reflections with I > 2σ(I)


Refinement on F2Secondary atom site location: difference Fourier map
Least-squares matrix: fullHydrogen site location: difference Fourier map
R[F2 > 2σ(F2)] = 0.034All H-atom parameters refined
wR(F2) = 0.110  w = 1/[σ2(Fo2) + (0.0683P)2] where P = (Fo2 + 2Fc2)/3
S = 0.97(Δ/σ)max < 0.001
483 reflectionsΔρmax = 0.89 e Å3
27 parametersΔρmin = −0.48 e Å3
Primary atom site location: structure-invariant direct methodsExtinction correction: none

Special details

Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds 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 > 2sigma(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)

Mg10.37500.37500.37500.0163 (2)
C10.87500.37500.32406 (10)0.0153 (3)
O10.66783 (15)0.37630 (11)0.28779 (5)0.0202 (3)
O20.37500.53689 (11)0.37500.0343 (4)
H20.399 (6)0.578 (2)0.3335 (16)0.050 (7)*

Atomic displacement parameters (Å2)

Mg10.0117 (4)0.0237 (4)0.0136 (4)0.0000.0000.000
C10.0149 (6)0.0199 (6)0.0110 (6)−0.0001 (8)0.0000.000
O10.0145 (4)0.0342 (5)0.0120 (4)0.0012 (4)−0.0014 (3)−0.0018 (4)
O20.0631 (11)0.0228 (7)0.0169 (6)0.0000.0086 (10)0.000

Geometric parameters (Å, °)

Mg1—O2i2.0546 (15)Mg1—O12.0734 (9)
Mg1—O22.0546 (15)C1—O11.2494 (11)
Mg1—O1i2.0734 (9)C1—O1iv1.2494 (11)
Mg1—O1ii2.0734 (9)C1—C1v1.569 (3)
Mg1—O1iii2.0734 (9)O2—H20.84 (3)
O2i—Mg1—O2180.0O2i—Mg1—O190.45 (4)
O2i—Mg1—O1i89.55 (4)O2—Mg1—O189.55 (4)
O2—Mg1—O1i90.45 (4)O1i—Mg1—O199.26 (5)
O2i—Mg1—O1ii89.55 (4)O1ii—Mg1—O180.75 (5)
O2—Mg1—O1ii90.45 (4)O1iii—Mg1—O1179.09 (8)
O1i—Mg1—O1ii179.09 (7)O1—C1—O1iv126.89 (14)
O2i—Mg1—O1iii90.45 (4)O1—C1—C1v116.56 (7)
O2—Mg1—O1iii89.55 (4)O1iv—C1—C1v116.56 (7)
O1i—Mg1—O1iii80.75 (5)C1—O1—Mg1113.06 (8)
O1ii—Mg1—O1iii99.26 (5)Mg1—O2—H2128.7 (19)

Symmetry codes: (i) −x+3/4, −y+3/4, z; (ii) x, −y+3/4, −z+3/4; (iii) −x+3/4, y, −z+3/4; (iv) −x+7/4, −y+3/4, z; (v) −x+7/4, y, −z+3/4.

Hydrogen-bond geometry (Å, °)

O2—H2···O1vi0.84 (3)1.97 (2)2.761 (1)158 (2)

Symmetry codes: (vi) x−1/4, y+1/4, −z+1/2.


Supplementary data and figures for this paper are available from the IUCr electronic archives (Reference: PV2083).


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