pentacyanocobaltate
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| ImageFile = Co(CN)5 3-.svg
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| CASNo = 14971-18-7
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| PubChem = 22141881
| StdInChI=1S/5CN.Co/c5*1-2;/q5*-1;+2
| StdInChIKey = YOZCCRZCYZQCAC-UHFFFAOYSA-N
| SMILES = [N-]#C[Co+2](C#[N-])(C#[N-])(C#[N-])C#[N-]
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| Section2 = {{Chembox Properties
| Co=1|C=5|N=5|Formula_Charge = 3−
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In chemistry, pentacyanocobaltate is the coordination complex with the formula {{chem2|[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 {{chem2|(quat)3[Co(CN)5]}}. According to X-ray crystallography, the salt features square pyamidal {{chem2|[Co(CN)5](3-)}}.{{cite journal |doi=10.1021/ic50153a002|title=Structural Characterization of the Pentacyanocobaltate(II) Anion in the Salt Tris(diethyldiisopropylammonium) Pentacyanocobaltate(II) |year=1975 |last1=Brown |first1=Leo D. |last2=Raymond |first2=Kenneth N. |journal=Inorganic Chemistry |volume=14 |issue=11 |pages=2590–2594 }}
Reactions
Solutions of {{chem2|[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:{{cite journal |doi=10.1080/01614946808064700|title=Reactions Catalyzed by Pentacyanocobaltate(II) |year=1968 |last1=Kwiatek |first1=Jack |journal=Catalysis Reviews |volume=1 |pages=37–72 }}
:{{chem2|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:
:{{chem2|2[Co(CN)5](3-) + H2O -> [Co(CN)5H](3-) + [Co(CN)5OH](3-)}}
In concentrated solution, the complex dimerizes:
:{{chem2|2[Co(CN)5](3-) -> [(NC)5Co\sCo(CN)5](6-)}}
With benzyl chloride and related alkylating agents, Co(III) alkyls are formed:{{cite journal |doi=10.1021/ja01031a010|title=Reactions of Pentacyanocobaltate(II) with Some Organic Halides |year=1969 |last1=Chock |first1=Pwen Boon |last2=Halpern |first2=Jack |journal=Journal of the American Chemical Society |volume=91 |issue=3 |pages=582–588 }}
:{{chem2|2[Co(CN)5](3-) + C6H5CH2Cl -> [Co(CN)5CH2C6H5](3-) + [Co(CN)5Cl](3-)}}