Cyanide Complexes of the Transition Metals

Abstract Cyano metal complexes contain one or more bound cyanide ligands, CN − , and constitute one of the largest and longest‐known classes of compounds in inorganic chemistry. Like carbon monoxide, the cyanide ion can function as a π‐acid ligand, but because of its negative charge, the cyanide ion can also form strong sigma bonds. This bifunctional behavior allows CN − to stabilize both high and low oxidation states of metals. The cyanide ion can bind to metals in both terminal and bridging (MCNM′) modes; the bridges are commonly linear and are present in Prussian Blue and many other polymeric metal cyanides. Framework solids with high degrees of void space can be formed with bridging cyanide ligands. Cyano metal complexes undergo a variety of oxidation–reduction reactions, but owing to the strong sigma‐donor properties of CN − , only a few ligands are capable of directly replacing metal‐bound cyanide. For most substitution reactions, photochemically induced dissociation of one or more of the CN − ligands is needed. Cyano complexes are used as pigments and dyes and are being increasingly employed as electroactive, zeolitic, and chemical sensing materials.