Hexamminecobalt(III) chloride

Properties Molecular formula H18N6Cl3Co Molar mass 267.48 g/mol Appearance yellow or orange crystals Density 1.71 g/cm3, Melting point decomposes Solubility in water 0.26M (20 °C) tribromide: 0.04M (18 °C) Solubility in other solvents soluble in NH3

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.

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.