Cobalt(III) hexammine chloride

Chembox new
Name = Cobalt(III) hexammine chloride
ImageFile = CoA6Cl3.png ImageName =
IUPACName = Hexaamminecobalt(III) chloride
OtherNames = Cobalt hexammine chloride
Section1 = Chembox Identifiers
CASNo = 10534-89-1
RTECS =
Section2 = Chembox Properties
Formula = H₁₈N₆Cl₃Co
MolarMass = 267.48 g/mol
Appearance = yellow or orange crystals
Density = ? g/cm³, ?
Solubility = 0.26M (20 °C)
tribromide: 0.04M (18 °C)
Solvent = other solvents
SolubleOther = soluble in NH₃
MeltingPt = decomposes
pKa =
Wavelength Absorbtion = ?
Section3 = Chembox Structure
Coordination = octahedral
CrystalStruct =
Dipole = 0 D
Section7 = Chembox Hazards
ExternalMSDS =
MainHazards = poison
RPhrases = 36/37/38
SPhrases = none
Section8 = Chembox Related
OtherAnions = [Co(NH₃)₆] Br₃
[Co(NH₃)₆] (OAc)₃
OtherCations = [Cr(NH₃)₆] Cl₃
[Ni(NH₃)₆] Cl₂
Function = compounds
OtherFunctn = [Co(H₂NCH₂CH₂NH₂)₃] Cl₃
[Co(NH₃)₅(H₂O)] Cl₃
[Co(NH₃)₅Cl] Cl₂

Cobalt(III) hexammine 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. It is also known as "leuto," a classical name rarely used now outside of teaching laboratories, where it is used along with violeo, praseo, and other cobalt complexes for teaching basic coordination chemistry.

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. [Bjerrum, J.; McReynolds, J. P. "Hexamminecobalt(III) Salts" Inorganic Syntheses, 1946, volume II, pages 216-221.] This salt appears to have been first reported by Fremy. [cite journal
author = M. E. Fremy
journal = Annales de chimie et de physique
year = 1852
volume = 35
pages = 257-312
url = http://gallica.bnf.fr/ark:/12148/bpt6k34776q/f255.table
title = Recherches sur le cobalt]

The acetate salt can be prepared by aerobic oxidation of cobalt(II) acetate, ammonium acetate, and ammonia in methanol. [Lindholm, R. D. "Hexamminecobalt(III) Salts" Inorganic Syntheses, 1978, volume XVIII, pages 67-69.] The acetate salt is highly water-soluble to the level of 1.9M (20 °C), vs. 0.26M for the trichloride.

[Co(NH₃)₆] ³⁺ is a component of some protein crystallization methods to help solve their structures by X-ray crystallography.

References