Chromium(III) bromide

Chromium(III) bromide is an inorganic compound with the chemical formula CrBr3. It is a dark colored solid that appears green in transmitted light but red with reflected light. It is used as a precursor to catalysts for the oligomerization of ethylene.

Chromium(III) bromide
Names
IUPAC name
Chromium(III) bromide
Other names
Chromium tribromide, Chromium bromide, Chromic bromide
Identifiers
3D model (JSmol)
ChemSpider
ECHA InfoCard 100.030.068
EC Number
  • 233-088-6
UNII
  • InChI=1S/3BrH.Cr/h3*1H;/q;;;+3/p-3
    Key: UZDWIWGMKWZEPE-UHFFFAOYSA-K
  • [Cr+3].[Br-].[Br-].[Br-]
Properties
CrBr3
Molar mass 291.71 g/mol
Appearance Black lustrous crystals; green in transmitted light, reddish in reflected light[1]
Density 4.25 g/cm3[2]
Melting point 1,130 °C (2,070 °F; 1,400 K) (anhydrous)[2]
79 °C (hexahydrate)
insoluble in cold water, soluble with addition of Chromium(II) ion salts,[1] soluble in hot water[2]
Structure
trigonal
Hazards
NIOSH (US health exposure limits):
PEL (Permissible)
 TWA 1 mg/m3[3]
REL (Recommended)
 TWA 0.5 mg/m3[3]
IDLH (Immediate danger)
 250 mg/m3[3]
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Infobox references

Synthesis

The compound is prepared in a tube furnace by the reaction of bromine vapor and chromium powder at 1000 °C. It is purified by extracting with absolute diethyl ether to remove any CrBr2, and is subsequently washed with absolute diethyl ether and absolute ethanol.[1]

Analogous to the behavior of related chromium(III) halides, the tribromide dissolves in water to give CrBr3(H2O)3 only upon the addition of catalytic amounts of a reducing agent, which generates CrBr2.[1] The reducing agent generates chromous bromide on the surface of the solid, which dissolves and re-oxidizes to Cr(III).

Reactions

Chromium(III) bromide is reduced by hydrogen gas at 350-400 °C to give chromium(II) bromide:[1]

2 CrBr3 + H2 → 2 CrBr2 + 2 HBr

References
  1. Brauer, Georg (1965) [1962]. Handbuch Der Präparativen Anorganischen Chemie [Handbook of Preparative Inorganic Chemistry] (in German). Vol. 2. Stuttgart; New York, New York: Ferdinand Enke Verlag; Academic Press, Inc. p. 1340. ISBN 978-0-32316129-9. Retrieved 2014-01-10.
  2. Perry, Dale L. (2011). Handbook of Inorganic Compounds, Second Edition. Boca Raton, Florida: CRC Press. p. 122. ISBN 978-1-43981462-8. Retrieved 2014-01-10.
  3. NIOSH Pocket Guide to Chemical Hazards. "#0141". National Institute for Occupational Safety and Health (NIOSH).
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