hexacyanometalate

Hexacyanometalate refers to a class of coordination compounds in which a central metal atom is bound by six cyanide ligands, forming an octahedral anion [M(CN)6]n−. The cyanide ligands attach through the carbon end, and the metal can be a transition metal in various oxidation states. The overall charge depends on the metal’s oxidation state and the ligand charges, giving salts with alkali, alkaline earth, or ammonium countercations such as K4[Fe(CN)6] or K3[Fe(CN)6].

The most familiar examples are ferrocyanide [Fe(CN)6]4− and ferricyanide [Fe(CN)6]3−, but hexacyanometalates of other metals, including cobalt, chromium, manganese, ruthenium, and nickel, are known. Many of these ions are isolated as salts such as Na3[Co(CN)6] or K3[Ru(CN)6], depending on the metal and oxidation state.

In structure, the hexacyanometalate unit consists of a central metal atom bonded to six cyanide ligands, giving a robust octahedral geometry with strong M–C bonds. Cyanide is a strong-field ligand, which often leads to low-spin configurations for certain metals. Hexacyanometalates can serve as building blocks for extended networks, notably in the formation of Prussian blue and related cyanide-bridged frameworks, where [M(CN)6] units are connected by additional metal centers.

Redox chemistry is a hallmark of this family. The ferrocyanide/ferricyanide couple is a classic, reversible redox system widely used in electrochemistry as a standard reference and mediator. Hexacyanometalates have applications in pigments, catalysis, and materials chemistry.