Ferricyanide
# Ferricyanide

Ferricyanide is the anion $[Fe(CN)_6]^{3−}.$ 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.

## Properties

$[Fe(CN)_6]^{3−}$ consists of a $Fe^{3+}$ center bound in octahedral geometry to six cyanide ligands. The complex has $O_h$ symmetry. The iron is low spin and easily reduced to the related ferrocyanide ion $[Fe(CN)_6]^{4−},$ which is a ferrous $(Fe^{2+})$ derivative. This redox couple is reversible and entails no making or breaking of Fe–C bonds:

    $[Fe(CN)_6]^{3−}$ + $e^−$ ⇌ $[Fe(CN)_6]^{4−}$

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 $Fe^{3+}.$ They do react with mineral acids, however, to release highly toxic hydrogen cyanide gas.

## Uses

Treatment of ferricyanide with iron(II) salts affords the brilliant, long-lasting pigment Prussian blue, the traditional color of blueprints.

