Cobalt(II) bromide (CoBr
2) is an inorganic compound. In its anhydrous form, it is a green solid that is soluble in water, used primarily as a
catalyst
Catalysis () is the process of increasing the rate of a chemical reaction by adding a substance known as a catalyst (). Catalysts are not consumed in the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recyc ...
in some processes.
Properties
When
anhydrous, cobalt(II) bromide appears as green crystals. It is
hygroscopic
Hygroscopy is the phenomenon of attracting and holding water molecules via either absorption or adsorption from the surrounding environment, which is usually at normal or room temperature. If water molecules become suspended among the substan ...
and forms the hexahydrate in air, which appears as red-purple crystals. The hexahydrate loses four water of crystallization molecules at 100 °C forming the dihydrate:
:CoBr
2·6H
2O → CoBr
2·2H
2O + 4 H
2O
Further heating to 130 °C produces the anhydrous form:
:CoBr
2·2H
2O → CoBr
2 + 2 H
2O
The anhydrous form melts at 678 °C. At higher temperatures, cobalt(II) bromide reacts with
oxygen
Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as ...
, forming
cobalt(II,III) oxide
Cobalt(II,III) oxide is an inorganic compound with the formula Co3O4. It is one of two well characterized cobalt oxides. It is a black antiferromagnetic solid. As a mixed valence compound, its formula is sometimes written as CoIICoIII2O4 and ...
and
bromine
Bromine is a chemical element with the symbol Br and atomic number 35. It is the third-lightest element in group 17 of the periodic table ( halogens) and is a volatile red-brown liquid at room temperature that evaporates readily to form a simi ...
vapor.
Preparation
Cobalt(II) bromide can be prepared as a hydrate by the reaction of
cobalt hydroxide with
hydrobromic acid
Hydrobromic acid is a strong acid formed by dissolving the diatomic molecule hydrogen bromide (HBr) in water. "Constant boiling" hydrobromic acid is an aqueous solution that distills at and contains 47.6% HBr by mass, which is 8.77 mol/L. ...
:
:Co(OH)
2(s) + 2HBr(aq) → CoBr
2·6H
2O(aq)
Reactions and uses
The classical
coordination compound bromopentaamminecobalt(III) bromide is prepared by oxidation of a solution of cobalt(II) bromide in aqueous ammonia.
:2 CoBr
2 + 8 NH
3 + 2 NH
4Br + H
2O
2 → 2
o(NH3)5Brr
2 + 2 H
2O
Triphenylphosphine
Triphenylphosphine (IUPAC name: triphenylphosphane) is a common organophosphorus compound with the formula P(C6H5)3 and often abbreviated to P Ph3 or Ph3P. It is widely used in the synthesis of organic and organometallic compounds. PPh3 exists ...
complexes of cobalt(II) bromide have been used as a
catalyst
Catalysis () is the process of increasing the rate of a chemical reaction by adding a substance known as a catalyst (). Catalysts are not consumed in the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recyc ...
s in organic synthesis.
Safety
Exposure to large amounts of cobalt(II) can cause
cobalt poisoning.
Bromide is also mildly toxic.
References
Cobalt(II) compounds
Bromides
Metal halides
{{Inorganic-compound-stub