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.

Pentacyanocobaltate

Identifiers
CAS Number	14971-18-7
3D model (JSmol)	Interactive image
PubChem CID	22141881
InChI InChI=1S/5CN.Co/c5*1-2;/q5*-1;+2 Key: YOZCCRZCYZQCAC-UHFFFAOYSA-N
SMILES [N-]#C[Co+2](C#[N-])(C#[N-])(C#[N-])C#[N-]
Properties
Chemical formula	C5CoN53−
Molar mass	189.025 g·mol−1
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). Infobox references

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.^[1] 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⁻.^[2]

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:^[1]

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⁻

With benzyl chloride and related alkylating agents, Co(III) alkyls are formed:^[3]

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