metal-organic compounds
Acta Crystallographica Section C
Crystal Structure
studied by Lagier et al. (1969) and Deyrieux et al. (1973) using
powder diffraction.
Communications
The atomic coordinates of humboldtine have been used as
starting values for a least-squares re®nement based on ®tting
the calculated and observed intensity pro®les of a powder
ISSN 0108-2701
diffraction pattern of microcrystalline [Mn(C2O4)]Á2D2O
Â
(Sledzinska et al., 1987). These results, and diffraction studies
catena-Poly[[diaquacobalt(II)]-
l-oxalato]
Â
on single crystals of [Zn(C2O4)]Á2H2O (Bacsa, 1995), con®rm
that these oxalates are isostructural and isomorphous. A new
hydrated phase of cobalt oxalate crystallizes in the triclinic
space group P1 with similar cell parameters to the ꢀ-forms
(Castillo et al., 2004). We report here the crystal structure of
the title compound, (I), prepared by hydrothermal methods.
John Bacsa,a* Desmond Eveb and Kim R. Dunbara
aDepartment of Chemistry, Texas A&M University, PO Box 30012, College Station,
Texas 77842-3012, USA, and bDepartment of Chemistry, Rhodes University, PO Box
94, Grahamstown, 6140, South Africa
Correspondence e-mail: jbacsa@mail.chem.tamu.edu
Received 16 August 2004
Accepted 22 November 2004
Online 6 January 2005
Single crystals of the title compound, [Co(C2O4)(H2O)2]n,
have been prepared by hydrothermal methods and character-
ized by X-ray diffraction analysis. The crystal structure
consists of in®nite one-dimensional chains of diaquacobalt(II)
units bridged by oxalate groups. These chains lie on twofold
symmetry axes parallel to the b axis, and the [Co(C2O4)]n
system is nearly planar within experimental error. The
cobalt(II) coordination polyhedra are irregular octahedra,
with oxalate O atoms at the equatorial positions and water
molecules at the axial positions. The chains are linked by
hydrogen bonds via the water molecules.
The structure of (I) consists of in®nite chains of [Co-
(C2O4)(H2O)2] units which lie on twofold symmetry axes
parallel to the b axis (Fig. 1). The neighbouring chains are
Ê
parallel but displaced by 0.787 (1) A relative to each other
along the b axis. In this arrangement, the H atoms of the water
molecules are in favourable positions for donating normal
hydrogen bonds to oxalate O atoms (Fig. 2). This network of
hydrogen bonds is in an up±down arrangement along these
chains, i.e. when an oxalate O atom accepts a hydrogen bond
above the [Co(C2O4)]n plane, the next O atom on the same
side of the chain accepts a bond below the plane of the oxalate
group. The separations of the CoII atoms along the chains
coincide with the repeat distance of the b axis. The closest
Comment
Polymeric divalent metal oxalates tend to form microcrystal-
line precipitates that are dif®cult to characterize by single-
crystal X-ray diffraction. Rare prismatic crystals of whewellite
{[Ca(C2O4)]ÁH2O} up to 100 mm long have been discovered in
Kladno, Czech Republic (Korbel & Novak, 1999), and
diffraction quality crystals of humboldtine {[Fe(C2O4)]Á2H2O}
can be found as hydrothermal deposits in coal basins.
The mineral humboldtine crystallizes in the space group
C2/c and consists of one-dimensional chains of [FeII(C2O4)-
(H2O)2] units with bridging oxalate ions (Caric, 1959). One-
dimensional coordination polymers are important for studying
fundamental theoretical aspects of magnetism in the solid
state.
II
Ê
interchain spacing is 4.9 A, and the Co atoms are located on
crystallographic planes with indices (200) and with a repeat
Ê
distance of 5.853 (1) A. The 200 re¯ection in the powder
diffraction patterns of the ꢀ-oxalates is the most intense, and
Metal oxalates have also been subjected to detailed thermal
analyses and the results used in establishing theoretical
concepts applicable to solid-state reactions (see, for example,
Coetzee et al., 1994). Two different forms of ferrous oxalate
dihydrate (ꢀ and ꢁ) have been characterized (Deyrieux &
Peneloux, 1969), but the ꢀ form, humboldtine, is the more
common. Powder diffraction patterns of the ꢀ-oxalates
[M(C2O4)]Á2H2O (with M = Mg, Mn, Fe, Co, Ni and Zn) have
been indexed in the monoclinic space group C2/c with nearly
identical cell parameters. The crystal structures of ꢀ-oxalates
[M(C2O4)]Á2H2O (with M = Mn, Co, Ni and Zn) have been
Figure 1
A view of one of the linear chains of (I). Displacement ellipsoids are
drawn at the 50% probability level and H atoms are shown as small circles
of arbitrary radii.
m58 # 2005 International Union of Crystallography
DOI: 10.1107/S0108270104030409
Acta Cryst. (2005). C61, m58±m60
Bacsa, John
