Ferric oxalate, also known as iron(III) oxalate, refers to inorganic compounds with the formula (H<sub>2</sub>O)<sub>x</sub> but could also refer to salts of <sup>3-</sup>. (H<sub>2</sub>O)<sub>x</sub> are coordination polymers with varying degrees of hydration. The coordination complex with the formula <sup>3-</sup> forms a variety of salts, a well-known example being potassium ferrioxalate. This article emphasizes the coordination polymers.
According to X-ray crystallography of the tetrahydrate , 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 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 in distorted octahedral coordination.
Ferric oxalate may be produced by reaction of iron(III) hydroxide and solution of oxalic acid:
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.
Ferric oxalate is used as the light-sensitive element in the Kallitype photographic printing process; and the platinotype process Platinum/Palladium Printing.
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.
Ferric oxalate hexahydrate is used with sodium borohydride for radical Markovnikov hydrofunctionalization reactions of alkenes.
A number of other iron oxalates are known:-