Ferricyanide should not be confused with ferrocyanide.
Ferricyanide is the anion [Fe(CN)<sub>6</sub>]3−. It is also called hexacyanoferrate(III) and in rare, but systematic nomenclature, hexacyanidoferrate(III). The most common salt of this anion is potassium ferricyanide, a red crystalline material that is used as an oxidant in organic chemistry.
[Fe(CN)<sub>6</sub>]3− consists of a Fe3+ center bound in octahedral geometry to six cyanide ligands. The complex has Oh symmetry. The iron is low spin and easily reduced to the related ferrocyanide ion [Fe(CN)<sub>6</sub>]4−, which is a ferrous (Fe2+) derivative. This redox couple is reversible and entails no making or breaking of Fe–C bonds:
[Fe(CN)<sub>6</sub>]3− + e− ⇌ [Fe(CN)<sub>6</sub>]4−This redox couple is a standard in electrochemistry.
Compared to main group cyanides like potassium cyanide, ferricyanides are much less toxic because of the strong bond between the cyanide ion (CN-) and the Fe3+. They do react with mineral acids, however, to release highly toxic hydrogen cyanide gas.
Treatment of ferricyanide with iron(II) salts affords the brilliant, long-lasting pigment Prussian blue, the traditional color of blueprints.