Hexacyanometalates

Hexacyanometalates are coordination compounds featuring a metal center M octahedrally coordinated by six cyanide ligands, giving the discrete anions [M(CN)6]n−. In these complexes, CN− ligands bind through the carbon atom to the metal, creating strong M–C≡N motifs. The most familiar species are ferrocyanide [Fe(CN)6]4− and ferricyanide [Fe(CN)6]3−, but hexacyanometalates of many other metals (such as Mn, Co, Ni, Cr, Ru, and Os) are known, yielding a family of [M(CN)6]n− anions with varying oxidation states and charges. Common salts include K4[Fe(CN)6] and K3[Fe(CN)6].

Properties and reactivity: The cyanide ligands are strong-field ligands, imparting high stability and notable redox activity. The ferrocyanide/ferricyanide couple is a classic, reversible one-electron redox system widely used in electrochemistry and analytical chemistry. Many hexacyanometalates are air-stable in solid form and in solution, and their chemistry is often governed by the metal center's oxidation state.

Applications: They serve as redox mediators and calibration standards, and their salts are used to synthesize coordination polymers and Prussian blue–like materials. Prussian blue itself is an extended network formed when hexacyanometalate anions couple with Fe3+ centers to yield Fe4[Fe(CN)6]3, a historic pigment and nanoporous material with electrochemical applications.

Safety and handling: Although some ferrocyanide salts are relatively non-toxic because CN− is tightly bound, cyanide-containing compounds require careful handling and proper disposal.