is the chemical compound with the formula [Co(NH3
. This coordination compound is considered an archetypal "Werner complex", named after the pioneer of coordination chemistry, Alfred Werner
Alfred Werner was a Swiss chemist who was a student at ETH Zurich and a professor at the University of Zurich. He won the Nobel Prize in Chemistry in 1913 for proposing the octahedral configuration of transition metal complexes. Werner developed the basis for modern coordination chemistry...
. This salt consists of [Co(NH3
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
is diamagnetic, with a low-spin octahedral
In chemistry, octahedral molecular geometry describes the shape of compounds where in six atoms or groups of atoms or ligands are symmetrically arranged around a central atom, defining the vertices of an octahedron...
Co(III) center. The cation obeys the 18-electron rule
The 18-electron rule is a rule of thumb used primarily for predicting formulas for stable metal complexes. The rule rests on the fact that valence shells of a transition metal consists of nine valence orbitals, which collectively can accommodate 18 electrons either as nonbinding electron pairs or...
and is considered to be a classic example of an exchange inert metal complex. As a manifestation of its inertness, [Co(NH3
can be recrystallized unchanged from concentrated hydrochloric acid
Hydrochloric acid is a solution of hydrogen chloride in water, that is a highly corrosive, strong mineral acid with many industrial uses. It is found naturally in gastric 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
, 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
can be exchanged with a variety of other anions such as nitrate
The nitrate ion is a polyatomic ion with the molecular formula NO and a molecular mass of 62.0049 g/mol. It is the conjugate base of nitric acid, consisting of one central nitrogen atom surrounded by three identically-bonded oxygen atoms in a trigonal planar arrangement. The nitrate ion carries a...
A bromide is a chemical compound containing bromide ion, that is bromine atom with effective charge of −1. The class name can include ionic compounds such as caesium bromide or covalent compounds such as sulfur dibromide.-Natural occurrence:...
, and iodide
An iodide ion is the ion I−. Compounds with iodine in formal oxidation state −1 are called iodides. This page is for the iodide ion and its salts. For information on organoiodides, see organohalides. In everyday life, iodide is most commonly encountered as a component of iodized salt,...
to afford the corresponding [Co(NH3
derivative. Such salts are bright yellow and display varying degrees of water solubility.
is not available, [Co(NH3
is prepared from cobalt(II) chloride
Cobalt chloride is an inorganic compound of cobalt and chloride, with the formula CoCl2. It is usually supplied as the hexahydrate CoCl2·6H2O, which is one of the most commonly used cobalt compounds in the laboratory. The hexahydrate is deep purple in color, whereas the anhydrous form is sky blue...
. The latter is treated with ammonia
Ammonia is a compound of nitrogen and hydrogen with the formula . It is a colourless gas with a characteristic pungent odour. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to food and fertilizers. Ammonia, either directly or...
and ammonium chloride
Ammonium chloride NH4Cl is an inorganic compound with the formula NH4Cl. It is a white crystalline salt that is highly soluble in water. Solutions of ammonium chloride are mildly acidic. Sal ammoniac is a name of natural, mineralogical form of ammonium chloride...
followed by oxidation. Oxidants include hydrogen peroxide
Hydrogen peroxide is the simplest peroxide and an oxidizer. Hydrogen peroxide is a clear liquid, slightly more viscous than water. In dilute solution, it appears colorless. With its oxidizing properties, hydrogen peroxide is often used as a bleach or cleaning agent...
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
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
Cobalt acetate is the cobalt salt of acetic acid. It may also be found as the tetrahydrate.It may be formed by the reaction between cobalt oxide or hydroxide and acetic acid:...
, ammonium acetate
Ammonium acetate is a chemical compound with the formula CH3COONH4 . It is a white solid, which can be derived from the reaction of ammonia and acetic acid...
, 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.
is a component of some protein crystallization methods to help solve their structures by X-ray crystallography
X-ray crystallography is a method of determining the arrangement of atoms within a crystal, in which a beam of X-rays strikes a crystal and causes the beam of light to spread into many specific directions. From the angles and intensities of these diffracted beams, a crystallographer can produce a...