hexamminecobalt(III) chloride

Hexaamminecobalt(III) chloride is the chemical compound with the formula o(NH₃)₆l₃. It is the chloride salt of the coordination complex o(NH₃)₆sup>3+, which is considered an archetypal "Werner complex", named after the pioneer of coordination chemistry, Alfred Werner. The cation itself is a metal ammine complex with six ammonia ligands attached to the cobalt(III) ion.

Originally salts of o(NH₃)₆sup>3+ were described as the ''luteo'' (Latin: yellow) complex of cobalt. 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

o(NH₃)₆sup>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, o(NH₃)₆l₃ 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 i(NH₃)₆l₂, 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 o(NH₃)₆l₃ can be exchanged with a variety of other anions such as nitrate, bromide, iodide, sulfamate to afford the corresponding o(NH₃)₆₃ derivative. Such salts are orange or bright yellow and display varying degrees of water solubility. The chloride ion can be also exchanged with more complex anions such as the hexathiocyanatochromate(III), yielding a pink compound with formula o(NH₃)₆Cr(SCN)₆], or the Ferricyanide, ferricyanide ion.

Preparation

o(NH₃)₆l₃ is prepared by treating cobalt(II) chloride 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.

Uses

o(NH₃)₆sup>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 ) provide anomalous scattering to solve the phase problem and produce an electron-density map of the structure.

o(NH₃)₆sup>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, such as interactions with and between nucleic acids.

Related compounds

*Tris(ethylenediamine)cobalt(III) chloride

References

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Cobalt complexes
Cobalt(III) compounds
Inorganic compounds
Chlorides
Metal halides
Octahedral compounds
Ammine complexes