Cyanometalate

Cyanometallates or cyanometalates are a class of coordination compounds, most often consisting only of cyanide ligands. Most are anions. 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. Homoleptic cyanometallates are complexes where the only ligand is cyanide. For transition metals, well known homoleptic cyanometallates are the hexacyanides. Hexacyanometalates are known for Ti(III), V(III), Cr(III), Cr(II), Mn(IV), Mn(III), Mn(II), Fe(II), Fe(III), Co(III), Ru(III), Ru(II), Os(III), and Os(II). Other more labile derivatives are also known. The Cr(II), Mn(III), Mn(II), Fe(II), Fe(III), and Co(III) derivatives are low-spin, reflecting the strong binding of cyanide, i.e. cyanide ranks highly in the spectrochemical series when significant backbonding can occur. With low d-electron counts, inversion of cyanometallates to nitrile complexes can occur. Lower metal oxidation states can be achieved with binding of Lewis acids to the terminal nitrogen lone pairs. Pentacyanocobaltate () is produced by the addition of five or more equivalents of a cyanide to a solution of a cobalt(II) salt. It is square pyramidal. Solutions of undergo a variety of reactions, such as hydrogenation: Several tetracyanometalates are also known, the best known being those of the d8 metals, Ni(II), Pd(II), and Pt(II). These species are square-planar and diamagnetic. In addition to [Ni(CN)4]4−, nickel also forms [Ni2(CN)6]4-, with a Ni(I)-Ni(I) bond. The coinage metals form stable dicyanometallates, [Cu(CN)2]−, [Ag(CN)2]−, and [Au(CN)2]−. For heavier metals, other stoichiometries are known such as K4Mo(CN)8 and Potassium heptacyanorhenate. Some cyanometallates are clusters featuring metal-metal bonds, such as [Mo2(CN)8]4−. Mixed ligand cyanometallates with anywhere from one to five cyanide ligands have been prepared.