Ferrous oxalate (iron(II) oxalate) are inorganic compound with the formula FeC2O4(H2O)x where x is 0 or 2. These are orange compounds, poorly soluble in water.

Structure and reactions

Like other iron oxalates, ferrous oxalates feature octahedral Fe centers. The dihydrate FeC2O4(H2O)x is a coordination polymer, 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 iron metal, as well as released carbon dioxide, carbon monoxide, and water. Ferrous oxalates are precursors to iron phosphates, 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. A related, though much more complex mineral is stepanovite, Na[Mg(H2O)6] [Fe3+(C2O4)3]·3H2O - an example of trioxalatoferrate(III).