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 Chromic bromide Chromium bromide Chromium tribromide
Identifiers
CAS Number	10031-25-1 Y 13478-06-3 (hexahydrate)
3D model (JSmol)	Interactive image
ChemSpider	74280
ECHA InfoCard	100.030.068
EC Number	233-088-6
PubChem CID	82309
UNII	WZB719DDER Y
CompTox Dashboard (EPA)	DTXSID40905343
InChI InChI=1S/3BrH.Cr/h3*1H;/q;;;+3/p-3 Key: UZDWIWGMKWZEPE-UHFFFAOYSA-K
SMILES [Cr+3].[Br-].[Br-].[Br-]
Properties
Chemical formula	CrBr3
Molar mass	291.708 g·mol−1
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)
Solubility in water	anhydrous: insoluble in cold water, soluble with addition of Chromium(II) ion salts,[1] soluble in hot water;[2] hexahydrate: highly soluble[2]
Structure
Crystal 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]

2Cr + 3Br₂ → 2CrBr₃

The effect of bromine on a highly heated mixture of chromium(III) oxide with coal:

Cr₂O₃ + 3Br₂ + 3C → 2CrBr₃ + 3CO

Chemical properties

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).^{[citation needed]}

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

Oxidizes when heated in air:

2 CrBr₃ + 3 O₂ → 2 Cr₂O₃ + 6 Br₂