Pentacyanocobaltate

In chemistry, pentacyanocobaltate is the coordination complex with the formula [Co(CN)5](3-). 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.

Synthesis and structure
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 (quat)3[Co(CN)5]. According to X-ray crystallography, the salt features square pyamidal [Co(CN)5](3-).

Reactions
Solutions of [Co(CN)5](3-) 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:
 * 2[Co(CN)5](3-) + H2  ->   2 [Co(CN)5H](3-)

When allowed to stand as a dilute solution for several minutes, the complex reacts with water to give two Co(III) derivatives:
 * 2[Co(CN)5](3-) + H2O  ->   [Co(CN)5H](3-)  +  [Co(CN)5OH](3-)

In concentrated solution, the complex dimerizes:
 * 2[Co(CN)5](3-) ->  [(NC)5Co\sCo(CN)5](6-)

With benzyl chloride and related alkylating agents, Co(III) alkyls are formed:
 * 2[Co(CN)5](3-) + C6H5CH2Cl  ->   [Co(CN)5CH2C6H5](3-)  +  [Co(CN)5Cl](3-)