- Chromium(III) chloride
Chromium trichlorideOther namesChromic chloride
Identifiers CAS number 10025-73-7 ,
PubChem 6452300 ChemSpider 4954736 UNII Z310X5O5RP ChEBI CHEBI:53351 ChEMBL CHEMBL1200528 RTECS number GB5425000 Jmol-3D images Image 1
Properties Molecular formula CrCl3 Molar mass 158.36 g/mol (anhydrous)
266.48 g/mol (hexahydrate)
Appearance purple when anhydrous, dark green when hexahydrate Density 2.87 g/cm3 (anhydrous)
1.760 g/cm3 (hexahydrate)
1152 °C (anhydrous)
83 °C (hexahydrate)
1300 °C decomp.
Solubility in water slightly soluble (anhydrous)
585 g/L (hexahydrate)
Solubility soluble in ethanol
insoluble in ether, acetone
Acidity (pKa) 2.4 (0.2M solution) Structure Crystal structure YCl3 structure Coordination
Octahedral Hazards MSDS ICSC 1316 (anhydrous)
ICSC 1532 (hexahydrate)
EU classification Not listed NFPA 704 Flash point Non-flammable LD50 440 mg/kg Related compounds Other anions Chromium(III) fluoride
Other cations Molybdenum(III) chloride
Related compounds Chromium(II) chloride
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Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Chromium(III) chloride (also called chromic chloride) is a violet coloured solid with the formula CrCl3. The most common form of CrCl3 sold commercially is a dark green hexahydrate with the formula [CrCl2(H2O)4]Cl.2H2O. Two other hydrates are known, pale green [CrCl(H2O)5]Cl2.H2O and violet [Cr(H2O)6]Cl3. This unusual feature of chromium(III) chlorides, having a series of [CrCl3−n(H2O)n]z+, each of which is isolable, is also found with other chromium(III) compounds.
Anhydrous chromium(III) chloride adopts the YCl3 structure, with Cr3+ occupying two thirds of the octahedral interstices in every other layer of a pseudo-cubic close packed lattice of Cl− ions. The absence of cations in alternate layers leads to weak bonding between adjacent layers. For this reason, crystals of CrCl3 cleave easily along the planes between layers, which results in the flaky appearance of samples of chromium(III) chloride.
Space-filling model of cubic close packing of chloride ions in the crystal structure of CrCl3
Ball-and-stick model of part of a layer
Commercially anhydrous chromium(III) chloride may be prepared by chlorination of chromium metal directly, or indirectly by chlorination of chromium(III) oxide in the presence of carbon at 800 °C, with carbon monoxide as a side-product:
- Cr2O3 + 3 C + 3 Cl2 → 2 CrCl3 + 3 CO
It may also be prepared by treating the hexahydrate with thionyl chloride.
In laboratory it's usually prepared by dissolving the elemental chromium or chromium(III) oxide in hydrochloric acid.
However, it is also a chloro complex which is quite inert to substitution, so in fact it is ordinarily quite unreactive. Slow reaction rates are common with chromium(III) complexes in general. The low reactivity of the d3 Cr3+ ion can be explained using crystal field theory. One way of opening CrCl3 up to substitution in solution is to reduce even a trace amount to CrCl2, for example using zinc in hydrochloric acid. This chromium(II) compound undergoes substitution easily, and it can exchange electrons with CrCl3 via a chloride bridge, allowing all of the CrCl3 to react quickly.
Reactions and uses
CrCl3 is a Lewis acid, classified as "hard" according to the Hard-Soft Acid-Base theory. It forms a variety of adducts of the type [CrCl3L3]z, where L is a Lewis base. For example, it reacts with pyridine (C5H5N) to form an adduct:
- CrCl3 + 3 C5H5N → CrCl3(C5H5N)3
Like most chromium(III) compounds, CrCl3 is chemically inert but its neighboring redox state chromium(II) is reactive. Thus, CrCl3 and its complexes generally react only sluggishly in the absence of a reducing agent capable of reducing Cr3+ to Cr2+. With the presence of some chromium(II), however, solid CrCl3 dissolves rapidly in water. Similarly, ligand substitution reactions of solutions of [CrCl2(H2O)4]+ are accelerated by chromium(II) catalysts.
With molten alkali metal chlorides such as potassium chloride, CrCl3 gives octahedral complexes of the type K3CrCl6, as well as K3Cr2Cl9, which is also octahedral but where the two chromiums are linked via three chloride bridges. Chromium(III) chloride is used as the precursor to many inorganic compounds of chromium, for example dibenzenechromium(0), an analogue of ferrocene:
One use of CrCl3 in organic synthesis is for the in situ preparation of chromium(II) chloride, a popular reagent for (A) reduction of alkyl halides and for (B) the synthesis of (E)-alkenyl halides. The reaction is usually performed using two moles of CrCl3 per mole of lithium aluminium hydride, although if aqueous acidic conditions are appropriate zinc and hydrochloric acid may be sufficient.
Chromium(III) chloride has also been used as a Louis acid in organic reactions, for example to catalyse the nitroso Diels-Alder reaction.
Although trivalent chromium is far less poisonous than hexavalent, chromium salts are generally considered toxic.
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- International Chemical Safety Card 1316 (anhydr. CrCl3)
- International Chemical Safety Card 1532 (CrCl3·6H2O)
- National Pollutant Inventory – Chromium (III) compounds fact sheet
- NIOSH Pocket Guide to Chemical Hazards
- IARC Monograph "Chromium and Chromium compounds"
- Chromium compounds
- Metal halides
- Coordination compounds
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