Hexamminecobalt(III) chloride

Hexaamminecobalt(III) chloride

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Names
IUPAC name Hexaamminecobalt(III) chloride
Other names Cobalt hexammine chloride, hexaamminecobalt(III) chloride
Identifiers
CAS Number	10534-89-1 Y
Properties
Chemical 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	soluble in NH3
Structure
Coordination geometry	octahedral
Dipole moment	0 D
Vapor pressure	{{{value}}}
Related compounds
Other anions	[Co(NH3)6]Br3 [Co(NH3)6](OAc)3
Other cations	[Cr(NH3)6]Cl3 [Ni(NH3)6]Cl2
Related compounds	[Co(H2NCH2CH2NH2)3]Cl3 [Co(NH3)5(H2O)]Cl3 [Co(NH3)5Cl]Cl2
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Infobox references

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⁺ trications with three Cl⁻ anions. The term "ammine" refers to ammonia in its metal complexes, and the prefix hex (Greek: six) indicates that there are six ammonias per cation.

Originally this compound was described as a "luteo" (Latin: yellow) complex, 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 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 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.

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 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.