Pentacyanocobaltate

Pentacyanocobaltate
Identifiers
CAS Number	14971-18-7;
3D model (JSmol)	Interactive image;
PubChem CID	22141881;
	InChI InChI=1S/5CN.Co/c51-2;/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

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.^[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]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−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⁻

References

^ ^a ^b Kwiatek, Jack (1968). "Reactions Catalyzed by Pentacyanocobaltate(II)". Catalysis Reviews. 1: 37–72. doi:10.1080/01614946808064700.
^ Brown, Leo D.; Raymond, Kenneth N. (1975). "Structural Characterization of the Pentacyanocobaltate(II) Anion in the Salt Tris(diethyldiisopropylammonium) Pentacyanocobaltate(II)". Inorganic Chemistry. 14 (11): 2590–2594. doi:10.1021/ic50153a002.
^ Chock, Pwen Boon; Halpern, Jack (1969). "Reactions of Pentacyanocobaltate(II) with Some Organic Halides". Journal of the American Chemical Society. 91 (3): 582–588. doi:10.1021/ja01031a010.

[Kwiatek-1] Kwiatek, Jack (1968). "Reactions Catalyzed by Pentacyanocobaltate(II)". Catalysis Reviews. 1: 37–72. doi:10.1080/01614946808064700.

[2] Brown, Leo D.; Raymond, Kenneth N. (1975). "Structural Characterization of the Pentacyanocobaltate(II) Anion in the Salt Tris(diethyldiisopropylammonium) Pentacyanocobaltate(II)". Inorganic Chemistry. 14 (11): 2590–2594. doi:10.1021/ic50153a002.

[3] Chock, Pwen Boon; Halpern, Jack (1969). "Reactions of Pentacyanocobaltate(II) with Some Organic Halides". Journal of the American Chemical Society. 91 (3): 582–588. doi:10.1021/ja01031a010.

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