In chemistry, pentacyanocobaltate is the coordination complex with the formula . When crystallized with a quaternary ammonium cation, it can be obtained as a yellow solid. Pentacyanocobaltate attracted attention as an early example of a metal complex that reacts with hydrogen. It contains low-spin cobalt(II), with a doublet ground state.
Aqueous solutions of pentacyanocobaltate are produced by the addition of five or more equivalents of a cyanide salt to a solution of a cobalt(II) salt. Initially this reaction produces insoluble cobalt dicyanide, but this solid dissolves in the presence of the excess cyanide. Pentacyanocobaltate forms within seconds. When prepared using a quaternary ammonium (quat) cyanide, crystals can be obtained with the formula . According to X-ray crystallography, the salt features square pyamidal .
Solutions of undergo a variety of reactions. The complex attracted attention in the 1940s for its reactivity toward hydrogen, which is now understood to produce a cobalt hydride:
When allowed to stand as a dilute solution for several minutes, the complex reacts with water to give two Co(III) derivatives:
In concentrated solution, the complex dimerizes:
These complexes catalyze the selective reduction of conjugated dienes, probably through an allyl intermediate.
With benzyl chloride and related alkylating agents, Co(III) alkyls are formed: