Ferrous oxalate (iron(II) oxalate) refers to

inorganic compound An inorganic compound is typically a chemical compound that lacks carbon–hydrogen bonds⁠that is, a compound that is not an organic compound. The study of inorganic compounds is a subfield of chemistry known as ''inorganic chemistry''. Inorgan ...

s with the formula FeC₂O₄(H₂O)_x where x is 0 or 2. These are yellow compounds. Characteristic of

metal oxalate complex Transition metal oxalate complexes are coordination complexes with oxalate (C2O42−) ligands. Some are useful commercially, but the topic has attracted regular scholarly scrutiny. Oxalate (C2O42-) is a kind of dicarboxylate ligand. As a small, sym ...

es, these compounds tend to be

polymeric A polymer () is a substance or material that consists of very large molecules, or macromolecules, that are constituted by many repeating subunits derived from one or more species of monomers. Due to their broad spectrum of properties, bot ...

, hence their low solubility in water.

Structure and reactions

Like other iron oxalates, ferrous oxalates feature octahedral Fe centers. The dihydrate FeC₂O₄(H₂O)_x is a

coordination polymer Coordination may refer to: * Coordination (linguistics), a compound grammatical construction * Coordination complex, consisting of a central atom or ion and a surrounding array of bound molecules or ions ** A chemical reaction to form a coordinat ...

, consisting of chains of oxalate-bridged ferrous centers, each with two aquo ligands.
When heated to 120 °C, the dihydrate dehydrates, and the anhydrous ferrous oxalate decomposes near 190 °C. The products of thermal decomposition is a mixture of iron oxides and

pyrophoric A substance is pyrophoric (from , , 'fire-bearing') if it ignites spontaneously in air at or below (for gases) or within 5 minutes after coming into contact with air (for liquids and solids). Examples are organolithium compounds and triethylb ...

iron metal, as well as released

carbon dioxide Carbon dioxide is a chemical compound with the chemical formula . It is made up of molecules that each have one carbon atom covalent bond, covalently double bonded to two oxygen atoms. It is found in a gas state at room temperature and at norma ...

carbon monoxide Carbon monoxide (chemical formula CO) is a poisonous, flammable gas that is colorless, odorless, tasteless, and slightly less dense than air. Carbon monoxide consists of one carbon atom and one oxygen atom connected by a triple bond. It is the si ...

, and water. Ferrous oxalates are precursors to

iron phosphate Iron phosphate may refer to: * Iron(II) phosphate * Iron(III) phosphate See also * Strengite Strengite is a relatively rare iron phosphate mineral with the formula: FePO4·2H2O. The mineral is named after the German mineralogist (1830–1 ...

s, which are of value in batteries.

Natural occurrence

Anhydrous iron(II) oxalate is unknown among minerals as of 2020. However, the dihydrate is known as

humboldtine Humboldtine is a rarely occurring mineral from the mineral class of "organic compounds" with the chemical composition FeC2O4•2H2O and is therefore a water-containing iron(II) oxalate or the iron salt of oxalic acid. Humboldtine crystallizes in ...

. A related, although much more complex mineral is stepanovite.
Na g(H₂O)₆Fe(III)(C₂O₄)₃]·3H₂O is an unusual example of a naturally-occurring

ferrioxalate Ferrioxalate or trisoxalatoferrate(III) is a trivalent anion with formula . It is a transition metal complex consisting of an iron atom in the +3 oxidation state and three bidentate oxalate ions anions acting as ligands. The ferrioxalate anio ...

References

{{Oxalates Iron(II) compounds Oxalates Inorganic compounds