Hexaamminecobalt(III) chloride is the chemical compound with the formula
3)6">o(NH3)6l
3. It is the chloride salt of the
coordination complex 3)6">o(NH3)6sup>3+, which is considered an archetypal "Werner complex", named after the pioneer of coordination chemistry,
Alfred Werner
Alfred Werner (12 December 1866 – 15 November 1919) 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 ...
. The cation itself is a
metal ammine complex with six
ammonia ligands attached to the
cobalt(III) ion.
Originally salts of
3)6">o(NH3)6sup>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.
Properties and structure
3)6">o(NH3)6sup>3+ is diamagnetic, with a
low-spin Spin states when describing transition metal coordination complexes refers to the potential spin configurations of the central metal's d electrons. For several oxidation states, metals can adopt high-spin and low-spin configurations. The ambiguity o ...
3d
6 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,
3)6">o(NH3)6l
3 can be recrystallized unchanged from concentrated
hydrochloric acid: the NH
3 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
3)6">i(NH3)6l
2, react rapidly with acids, reflecting the
lability of the Ni(II)–NH
3 bonds. Upon heating, hexamminecobalt(III) begins to lose some of its ammine ligands, eventually producing a stronger oxidant.
The chloride ions in
3)6">o(NH3)6l
3 can be exchanged with a variety of other
anions such as
nitrate
Nitrate is a polyatomic ion
A polyatomic ion, also known as a molecular ion, is a covalent bonded set of two or more atoms, or of a metal complex, that can be considered to behave as a single unit and that has a net charge that is not zer ...
,
bromide,
iodide,
sulfamate
Sulfamic acid, also known as amidosulfonic acid, amidosulfuric acid, aminosulfonic acid, sulphamic acid and sulfamidic acid, is a molecular compound with the formula H3NSO3. This colourless, water-soluble compound finds many applications. Sulfami ...
to afford the corresponding
3)6">o(NH3)63 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
3)6">o(NH3)6Cr(SCN)
6], or the
Ferricyanide, ferricyanide ion.
Preparation
3)6">o(NH3)6l
3 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. 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 1.9 M (20 °C), versus 0.26 M for the trichloride.
Uses
3)6">o(NH3)6sup>3+ is a component of some
structural biology methods (especially for
DNA or
RNA
Ribonucleic acid (RNA) is a polymeric molecule essential in various biological roles in coding, decoding, regulation and expression of genes. RNA and deoxyribonucleic acid ( DNA) are nucleic acids. Along with lipids, proteins, and carbohydra ...
, 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 Mg
2+, but the heavy atoms of cobalt (or sometimes
iridium, as in ) provide
anomalous scattering to solve the
phase problem and produce an electron-density map of the structure.
3)6">o(NH3)6sup>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, such as interactions with and between
nucleic acids.
Related compounds
*
Tris(ethylenediamine)cobalt(III) chloride
Tris(ethylenediamine)cobalt(III) chloride is an inorganic compound with the formula o(en)3
O, or o, is the fifteenth Letter (alphabet), letter and the fourth vowel letter in the Latin alphabet, used in the English alphabet, modern English alphabet, the alphabets of other western European languages and others worldwide. Its name in ...
l3 (where "en" is the abbreviation for ethylenediamine). It is the chloride salt of the coordination complex o(en)3
O, or o, is the fifteenth Letter (alphabet), letter and the fourth vowel letter in the Latin alphabet, used in the English alphabet, modern English alphabet, the alphabets of other western European languages and others worldwide. Its name in ...
+. This trication was important in ...
References
{{reflist
Cobalt complexes
Cobalt(III) compounds
Inorganic compounds
Chlorides
Metal halides
Octahedral compounds
Ammine complexes