Iron(II) oxalate dihydrate Fe(C2O4)∙2H2O naturally occurs as a mineral humboldtine. The mineral displays two crystalline modifications that are very similiar to each other and cannot be differentiated by the means of spectroscopic methods. The only difference is found in reordering of H-bonds in the structures . The most stable form is α-Fe(C2O4)∙2H2O. It crystallizes in monoclinic crystal system. The structure can be described as an infinite linear chain of iron(II) ions bridged by oxalate anions. The other coordination places in distorted octahedra are occupied by oxygen atoms from aditional water molecules.
Iron(III) oxalate tetrahydrate Fe2(C2O4)3∙4H2O is commercially available chemical. It was widely used in iron imaging systems (Kallitype, Argenotype, etc.). When exposed to UV light it undergoes self-redox process in which iron(III) is reduced to iron(II) and oxalate anion is turned into carbon dioxide. The structure of the compound has been lately described based on the powder diffraction analysis . The structure consists of zig-zag chains of oxalate bridged octahedrons. One oxalate anion binds in monodentate manner. The structure is stabilized through inter-chain H-bonds along with uncoordinated water molecules.
Procedure (Iron(II) oxalate):
Two solutions were prepared.
Oxalic acid dihydrate (12 g, 95,19 mmol) was dissolved in 30 mL of distilled water. Potassium hydroxide (10,5 g, 178,24 mmol) was dissolved in 20 mL of disstilled water. The solution of potassium hydroxide was carefully poured to the oxalic acid solution which was in little excess. After that, an excess of iron(II) sulfate heptahydrate was added to the solution and stirred. Yellow Iron(II) oxalate has immediately precipitated. It was decanted several times and filtered. The filtrate was dried and later used in preparation of pyrophoric iron. Yield: 58,4%.
Procedure (Iron(III) oxalate):
Iron(III) chloride hexahydrate (1 g, 3.70 mmol) was dissolved in 20 mL of distilled water. A little excess of potassium hydroxide solution was added dropwise under constant stirring. The red-brown preciptate of Iron(III) hydroxide was after 10 minutes filtered off and washed with water. The precipitate was then tranfered to a hot solution of oxalic acid (0.70 g, 5.55 mmol) in 20 mL of water. Light green solution resulted that was filtered and let freely to crystallize.
 D’Antonio, M.C. et al. J. Braz. Chem. Soc. 2009, Vol. 20, No. 3 p. 445-450
 Ahouari, H. et al. Chem. Mater. 2015, Vol. 27, p. 1631-1639