Homoleptic cyanometallates refers to complexes where the only ligand is cyanide. For the first row transition metals, well known homoleptic cyanometallates are the hexacyanides. Hexacyanometalates are known for Ti(III), V(III), Cr(III), Mn(III), Fe(II), Fe(III), and Co(III). Other more labile derivatives are known as well. The Mn(III), 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.
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. The coinage metals form stable dicyanometallates, −, −, and −. For heavier metals, other stoichiometries are known such as K4Mo(CN)8. Some cyanometallates are clusters featuring metal-metal bonds, such as 4−.
Mixed ligand cyanometallates have been prepared, one example is the zero-valent −. Only few heteroleptic cyanometallates are rarely of interest outside of the research laboratory, one exception being the drug sodium nitroprusside (Na2FeNO(CN)5).
Because cyanide is a powerful nucleophile and a strong ligand, cyanometallates are generally prepared by the direct reaction of cyanide salts with simple metal salts. If other ligands are present on the metal, these are often displaced by cyanide. By far the largest application of cyanometalates is the production of − in the extraction of gold from low grade ores. This conversion involves oxidation of the gold:
Because the M-CN bond is strong, several cyanometallates can sustain redox. A well known couple is 3−/4−. Few unidentate ligands allow similar redox transformations wherein both members of the redox couple are observable in solution. Another perhaps more dramatic example is the 2e reduction of the square planar tetracyanonickelate to its tetrahedral Ni(0) derivative: