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.

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. This salt appears to have been first reported by Fremy.

The acetate salt can be prepared by aerobic oxidation of cobalt(II) acetate, ammonium acetate, and ammonia in methanol. 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.