Ferric oxalate

Ferric oxalate, also known as iron(III) oxalate, refers to inorganic compounds with the formula Fe2(C2O4)3(H2O)x but could also refer to salts of [Fe(C2O4)3]3-. Fe2(C2O4)3(H2O)x are coordination polymers with varying degrees of hydration. The coordination complex with the formula [Fe(C2O4)3]3- forms a variety of salts, a well-known example being potassium ferrioxalate. This article emphasizes the coordination polymers.

Tetrahydrate
According to X-ray crystallography of the tetrahydrate Fe2(C2O4)3 * 4 H2O, iron is octahedral. The oxalate ligands are bridging. Some through all four oxygen atoms, some with two oxygen atoms. Half of the water is lattice water, being situated between chains of Fe oxalates. Mössbauer spectrum of  Fe2(C2O4)3 * 4 H2O exhibits an isomer shift of 0.38 mm/s and a quadrupole splitting of 0.40 mm/s, suggesting a high spin Fe(3+) in octahedral coordination.

Production
Ferric oxalate may be produced by reaction of iron(III) hydroxide and solution of oxalic acid:


 * 2Fe(OH)3 + 3H2C2O4 -> Fe2(C2O4)3 + 6H2O

Dentistry
Like many oxalates, ferric oxalate has been investigated as a short-term treatment for dentin hypersensitivity. It is used in certain toothpaste formulations; however, its effectiveness has been questioned.

Photography
Ferric oxalate is used as the light-sensitive element in the Kallitype photographic printing process; and the platinotype process Platinum/Palladium Printing.

Batteries
Ferric oxalate tetrahydrate has been investigated as a possible cheap material for the positive electrode of lithium-iron batteries. It can intercalate lithium ions at an average potential of 3.35 V, and has shown a sustainable capacity of 98 mAh/g.

Organic synthesis
Ferric oxalate hexahydrate is used with sodium borohydride for radical Markovnikov hydrofunctionalization reactions of alkenes.