As all of you probably know, majority of d-block elements form coordination compounds using their free d-orbitals as acceptors for ligand´s free electron pairs. Copper is well known for its blue colored complexes such as tetraaquacopper(II) ion or dark blue tetraammincopper(II) ion. In the latter complexes the central atom – copper – is 4-coordinated and the complex is thought to square planar. Actually, there is much more into it due to Jahn-Teller theorem. One may usually found one or two additional ligands surrounding copper ions with slightly longer bond distances. Based on this, copper atoms are much more likely being 5- or 6-coordinated.
K2[Cu(C2O4)2] is a complex salt of copper that can be easily prepared in a laboratory by two step synthesis. The compound is sparingly soluble in water . The water solutions are unstable and the compound is subsequently hydrolyzed to leave copper(II) oxalate precipitate. The complex is however readily dissolved in potassium oxalate solution. Depending on the crystallization conditions it can exist both as a dihydrate (well-shaped triclinic crystals) and as a tetrahydrate (needle-like crystals). The tetrahydrate is prepared by cooling a saturated solution of dihydrate at 80 °C. The compound can be prepared by two pathways:
1. Using copper(II) oxide as a reactant:
K2CO3 + 2 H2C2O4 → 2KHC2O4 + H2O + CO2
2KHC2O4 + CuO → K2[Cu(C2O4)2] + H2O
2. Using copper(II) sulfate as a rectant
K2C2O4 + CuSO4 → CuC2O4 + K2SO4
CuC2O4 + 2 K2C2O4 → K2[Cu(C2O4)2] + K2SO4
In the structure of K2[Cu(C2O4)2] two inequivalent copper(II) atoms are linked in chain-like manner by the non-planar oxalate ions (parallel to b axis) . Cu(1) atom is coordinated by four oxygen donor atoms from oxalate anion and two oxygen atoms in apical positions from coordinated water molecules with a bond distance of 2.46 Angstroems. Cu(2) atom is also hexacoordinated, however all six oxygen atoms come from oxalate anions as it can be seen in picture below. A coordination polyhedra has a shape of elongated square bipyramide typical for Cu(II) complexes. Potassium atoms are octacoordinated by oxygen atoms in a shape of distorted squared antiprism .
polymeric chain (-Cu(1)-O-C-O-Cu(2)-) in the structure of K2[Cu(C2O4)2]
elongated square bipyramide polyhedra around copper(II) atoms
In the infrared spectrum of the salt two sharp peaks for O-H bond valence vibrations are present at 3497 cm-1 and 3390 cm-1. The local symmetry of coordinated oxalate anion in the structure is lowered to D2h. For C=O double bond assymetric stretch there are three sharp bands at 1668 cm-1 and 1634 cm-1. Band at 1651 cm-1 is most probably H-O-H deformation vibration of water molecules [Edwards]. Symmetric strech of the carboxylate group is found at 1415 cm-1. CO2 wagging is well visible at 1280cm-1 . The strongest absorption band at 477 cm-1 is assigned to Cu-O coordination bond [Begun].
Oxalic acid dihydrate (3.99 g, 31 mmol) was dissolved in 50 mL of distilled water heated to 50 °C. Under constant stirring stoichiometric amount of anhydrous potassium carbonate (8.76 g, 63.4 mmol) was added to the solution. At this stage carbon dioxide is formed. The solution of potassium oxalate was then heated near to its boiling point and copper(II) oxide (1.26 g, 15.9 mmol) was added. The mixture was stirred and heated for 120 minutes. The hot reaction mixture was then filtrated with suction a cooled in an ice-bath. Crystals of product were filtered off and washed with little amount of ethanol/water (1:1) mixture. Molar ratio of copper(II) atom to oxalate anion (1:2,3) was determined by manganometric and chelatometric titrations. Little excess of oxalate anions was observed.
Oxalic acid (3.00 g, 24 mmol) was dissolved in 50mL of demineralized water. Anhydrous potassium carbonate (2.20 g, 16 mmol) was added in small portions and stirred until the evolution of gas stopped. This solution was was heated to 80 °C. Copper(II) oxide (0.64 g, 8 mmol) was added while the solution was stirred. The solution was condensed by evaporation in hot water bath to 40mL and then it was let to crystallize.
K2[Cu(C2O4)2] dihydrate crystals [photo: Michal Hegedus]
K2[Cu(C2O4)2] tetrahydrate [photo: Juraj Kmotorka]
 EDWARDS, H. G. M. J. Mol. Struc. 249 (1991) 233-243.
 WEICHERT, T. LOHN, J. Z. Kristallogr. Cryst. Mater. 139 (1974) 223-235.