Cobalt(III) hexammine chloride

Cobalt(III) hexammine chloride is the chemical compound with the formula [Co(NH₃)₆]Cl₃. This coordination compound is considered an archetypal "Werner complex", named after the pioneer of coordination chemistry, Alfred Werner. This salt consists of [Co(NH₃)₆]³⁺ trications with three Cl^- anions. The term "ammine" refers to ammonia in its metal complexes.

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Properties and structure

[Co(NH₃)₆]³⁺ is diamagnetic, with a low-spin octahedral Co(III) center. The cation obeys the 18-electron rule and is considered to be a classic example of an exchange inert metal complex. As a manifestation of its inertness, [Co(NH₃)₆]Cl₃ can be recrystallized unchanged from concentrated hydrochloric acid: the NH₃ is so tightly bound to the Co(III) centers that it does not dissociate to allow its protonation. In contrast, labile metal ammine complexes, such as [Ni(NH₃)₆]Cl₂, react rapidly with acids reflecting the lability of the Ni(II)-NH₃ bonds. Upon heating, hexamminecobalt(III) begins to lose some of its ammine ligands, eventually producing a stronger oxidant.

The chlorides in [Co(NH₃)₆]Cl₃ can be exchanged with a variety of other anions such as nitrate, bromide, and iodide to afford the corresponding [Co(NH₃)₆]X₃ derivative. Such salts are bright yellow and display varying degrees of water solubility.

Preparation

Since CoCl₃ is not available, [Co(NH₃)₆]Cl₃ is prepared from cobalt(II) chloride. The latter is treated with ammonia and ammonium chloride followed by oxidation. Oxidants include hydrogen peroxide or oxygen in the presence of charcoal catalyst.^[1] This salt appears to have been first reported by Fremy.^[2]

The acetate salt can be prepared by aerobic oxidation of cobalt(II) acetate, ammonium acetate, and ammonia in methanol.^[3] The acetate salt is highly water-soluble to the level of 1.9M (20 °C), vs. 0.26M for the trichloride.

[Co(NH₃)₆]³⁺ is a component of some protein crystallization methods to help solve their structures by X-ray crystallography.

References

1. ^ Bjerrum, J.; McReynolds, J. P. "Hexamminecobalt(III) Salts" Inorganic Syntheses, 1946, volume II, pages 216-221.
2. ^ Fremy Annales de chimie et de physique, 1852, volume 35, page 257ff.
3. ^ Lindholm, R. D. "Hexamminecobalt(III) Salts" Inorganic Syntheses, 1978, volume XVIII, pages 67-69.