Cobalt hexammine chloride, hexaamminecobalt(III) chloride
|Molar mass||267.48 g/mol|
|Appearance||yellow or orange crystals|
|0.26 M (20 °C)|
tribromide: 0.04 M (18 °C)
|Solubility||soluble in NH3|
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
|what is ?)(|
Hexaamminecobalt(III) 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+ cations, each accompanied by three Cl− anions. The cation itself is a metal ammine complex with six ammonia molecules as ligands on the cobalt atom.
Originally this compound was described as the luteo (Latin: yellow) complex of cobalt, but this name has been discarded as modern chemistry considers color less important than molecular structure. Other similar complexes also had color names, such as purpureo (Latin: purple) for a cobalt pentammine complex, and praseo (Greek: green) and violeo (Latin: violet) for two isomeric tetrammine complexes. 
Properties and structure
[Co(NH3)6]3+ is diamagnetic, with a low-spin 3d6 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 chloride ions 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.
Since CoCl3 is not available (the Co3+ would oxidise chloride to chlorine gas), [Co(NH3)6]Cl3 is prepared from cobalt(II) chloride using a redox synthesis. 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 1.9 M (20 °C), versus 0.26 M for the trichloride.
[Co(NH3)6]3+ is a component of some structural biology methods (especially for DNA or RNA, where positive ions stabilize tertiary structure of the phosphate backbone), to help solve their structures by X-ray crystallography or by nuclear magnetic resonance. In the biological system, the counterions would more probably be Mg2+, but the heavy atoms of cobalt (or sometimes iridium, as in PDB file 2GIS) provide anomalous scattering to solve the phase problem and produce an electron-density map of the structure.
[Co(NH3)6]3+ is an unusual example of a water-soluble trivalent metal complex and is of utility for charge-shielding applications such as the stabilization of highly negatively charged complexes e.g. of interactions with and between nucleic acids.
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