Hexamminecobalt(III) chloride

Hexaamminecobalt(III) chloride

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
Hazards
Main hazards	poison
R-phrases	36/37/38
S-phrases	none
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. ^[1]

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.^[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 1.9M (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^[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]

References