Hexacyanoferrate

Hexacyanoferrate refers to coordination complexes containing the hexacyanoferrate ion, [Fe(CN)6]4− or [Fe(CN)6]3−, and the salts in which these anions are paired with cations such as potassium, sodium, or ammonium. The two common oxidation states are iron(II) in ferrocyanide [Fe(CN)6]4− and iron(III) in ferricyanide [Fe(CN)6]3−. Cyanide is a strong-field ligand, producing octahedral coordination at iron with low-spin, terminal cyanide ligands.

Ferrocyanide and ferricyanide exhibit distinct colors in solution—ferrocyanide is typically colorless or pale yellow, while ferricyanide is often orange or yellow-brown. Common salts include K4[Fe(CN)6], K3[Fe(CN)6], Na4[Fe(CN)6], and Na3[Fe(CN)6]. These complexes are highly stable due to strong Fe–CN bonds and the inert nature of the octahedral framework.

In redox chemistry, the ferrocyanide/ferricyanide couple [Fe(CN)6]4−/[Fe(CN)6]3− is a classic, reversible one-electron system with a standard potential near +0.36 V vs. SHE under neutral conditions. This couple is widely used as a redox mediator in electrochemical experiments and as a model for electron-transfer processes in analytical and bioanalytical methods. The chemistry of hexacyanoferrates is also linked to Prussian blue, a mixed-valence pigment formed from iron and hexacyanoferrate units.

Safety and handling considerations reflect their cyanide ligands: while these complexes are comparatively less toxic than free cyanide, appropriate precautions are still advised in laboratory settings.