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.

Ferric oxalate

Names
Systematic IUPAC name iron(3+) ethanedioate (2:3)
Other names Iron(III) oxalate
Identifiers
CAS Number	2944-66-3 (Anhydrous) Y 166897-40-1 (hexahydrate)
3D model (JSmol)	Interactive image
ChemSpider	147789
ECHA InfoCard	100.019.047
EC Number	220-951-7
PubChem CID	168963
UNII	DTJ9P8AB5L Y
CompTox Dashboard (EPA)	DTXSID10905951
InChI InChI=1S/3C2H2O4.2Fe/c3*3-1(4)2(5)6;;/h3*(H,3,4)(H,5,6);;/q;;;2*+3/p-6
SMILES [Fe+3].[Fe+3].O=C([O-])C([O-])=O.[O-]C(=O)C([O-])=O.[O-]C(=O)C([O-])=O
Properties
Chemical formula	C6Fe2O12
Molar mass	375.747 g/mol
Appearance	Pale yellow solid (anhydrous) Lime green solid (hexahydrate)
Odor	odorless
Melting point	365.1 °C (689.2 °F)
Solubility in water	slightly soluble
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). Infobox references

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 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 Fe³⁺ in octahedral coordination.^[1][2]

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.^[3] It is used in certain toothpaste formulations; however, its effectiveness has been questioned.^[4]

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.^[1]

Organic synthesis

Ferric oxalate hexahydrate is used with sodium borohydride for radical Markovnikov hydrofunctionalization reactions of alkenes.^[5]

See also

A number of other iron oxalates are known:-

• Iron(II) oxalate
• Potassium ferrioxalate
• Sodium ferrioxalate