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]2−

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−Co(CN)5]6−

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:

2[Co(CN)5]3− + C6H5CH2Cl → [Co(CN)5CH2C6H5]3− + [Co(CN)5Cl]3−