Cyanide is a highly basic and small ligand, hence it readily saturates the coordination sphere of metal ions.
The resulting cyanometallate anions are often used as building blocks for more complex structures called coordination polymers, the best known example of which is Prussian blue, a common dyestuff.
Since cyanide has the largest σ-donation ability at its C-end, most soluble (molecular) metal-cyanide complexes have metal-carbon, rather than metal-nitrogen bonds.
Lower metal oxidation states can be achieved with binding of Lewis acids to the terminal nitrogen lone pairs.
By far the largest application of cyanometalates is the production of [Au(CN)2]− in the extraction of gold from low grade ores.
This conversion involves oxidation of metallic gold into Au+: Because the M-CN bond is strong and delocalizes electron density to the ligands, several cyanometallates exhibit multiple redox states.
The latter tendency is illustrated by the condensation of ferrocyanide salts with other metal ions to give polymers, such as Prussian blue.