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.

Structure

Tetrahydrate

Structure of hydrated ferric oxalate Color code: red=O, white = H, blue = Fe, gray = C.
Room temperature Mössbauer spectrum of Fe2(C2O4)3·4H2O

According to X-ray crystallography of the tetrahydrate Fe2(C2O4)3 · 4 H2O, iron is octahedrally coordinated. 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.

The Mössbauer spectrum of Fe2(C2O4)3 · 4 H2O exhibits a doublet with an isomer shift of 0.38 mm/s, and a quadrupole splitting of 0.40 mm/s, suggesting a high spin Fe3+ in distorted 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

Uses

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 (cathode) for lithium-ion batteries. It can intercalate lithium ions at an average potential of 3.35 V, and has shown a sustainable capacity of 98 mAh/g. Additionally, ferric oxalate is one synthetic precursor to lithium iron phosphate, a popular Li-ion battery cathode.

Organic synthesis

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

See also

A number of other iron oxalates are known:-