Ferric oxalate
Ferric oxalate, also known as iron(III) oxalate, is a inorganic compound composed of ferric ions and oxalate ligands. It is a coordination polymer. The anhydrous material is pale yellow; however, it may be hydrated to form several hydrates, such as potassium ferrioxalate, or , which is bright green in colour. Structure

Tetrahydrate

According to X-ray crystallography of the tetrahydrate , iron is octahedral. The oxalate ligand]]s 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   exhibits an isomer shift of 0.38 mm/s and a quadrupole splitting of 0.40 mm/s, suggesting a high spin  in octahedral coordination. 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 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. 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:-
Iron(II) oxalate
Potassium ferrioxalate
Sodium ferrioxalate

References 

Oxalates
Iron(III) compounds