1. Properties and structure

2. Preparation

3. Uses

4. References

[Co(NH₃)₅(H₂O)]Cl₃

[Co(NH₃)₅Cl]Cl₂

Hexaamminecobalt(III) chloride is the chemical compound with the formula [Co(NH₃)₆]Cl₃. It is the chloride salt of the coordination complex [Co(NH₃)₆]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 [Co(NH₃)₆]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.^[1]

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, iodide, sulfamate to afford the corresponding [Co(NH₃)₆]X₃ 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 [Co(NH₃)₆][Cr(SCN)₆], or the ferricyanide ion.

Preparation

[Co(NH₃)₆]Cl₃ 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.^[2] This salt appears to have been first reported by Fremy.^[3]

The acetate salt can be prepared by aerobic oxidation of cobalt(II) acetate, ammonium acetate, and ammonia in methanol.^[4] The acetate salt is highly water-soluble to the level of 1.9 M (20 °C), versus 0.26 M for the trichloride.

Uses

[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^[5] or by nuclear magnetic resonance.^[6] 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.^[7]

[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.

References

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