Cyanometallates

Cyanometallates are coordination compounds in which cyanide ligands bind to a metal center. The most studied are hexacyanometallate ions [M(CN)6]n−, in which a metal atom M is coordinated by six cyanide ligands in an octahedral geometry. Depending on M and its oxidation state, these anions carry varying negative charges and can be isolated as salts with alkali or alkali earth cations.

In discrete hexacyanometallates, CN− acts as a terminal ligand, donating electrons through the carbon end to the metal. In extended solids, cyanide can bridge metal centers, forming networks such as Prussian blue analogues, where Fe and other metals are connected via Fe–CN–M linkages. This bridging enables electronic communication and magnetic interactions across the material.

Typical examples include ferrocyanide [Fe(CN)6]4− and ferricyanide [Fe(CN)6]3−, as well as hexacyanometalates of cobalt, nickel, ruthenium, and manganese. Salts such as K4[Fe(CN)6] and K3[Fe(CN)6] are common reagents. The well-known pigment Prussian blue corresponds to a mixed-valence iron cyanide solid with the formula Fe4[Fe(CN)6]3·xH2O.

Synthesis commonly involves metathesis reactions between metal salts and ferro-/ferricyanide precursors; alternatively, cyanide-bridged frameworks can be grown from appropriate metal salts and cyanometallate building blocks.

Cyanometallates are valued for redox activity, conductivity, magnetism, and as precursors to porous coordination polymers. They find use in electrochemistry, energy storage, catalysis, and pigment production. Careful handling is advised due to the cyanide component in some materials.