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Chromium(III) chloride



Chromium(III) chloride
IUPAC name Chromium(III) chloride
Chromium trichloride
Other names Chromic chloride
Identifiers
CAS number 10025-73-7
RTECS number GB5425000
Properties
Molecular formula CrCl3
Molar mass 158.35 g/mol (anhydrous)
Appearance see text
Density 2.76 g/cm3, solid
Melting point

1150 °C (anhydrous)

Boiling point

1300 °C decomp.

Structure
Crystal structure YCl3 structure
Coordination
geometry
Octahedral
Hazards
EU classification not listed
Flash point non flammable
Related Compounds
Other anions Chromium(III) fluoride
Chromium(III) bromide
Chromium(III) iodide
Other cations Molybdenum(III) chloride
Tungsten(III) chloride
Related compounds Chromium(II) chloride
Chromium(IV) chloride
Except where noted otherwise, data are given for
materials in their standard state
(at 25 °C, 100 kPa)

Infobox disclaimer and references

Chromium(III) chloride (also called chromic chloride) is a violet coloured solid with the formula CrCl3.

Although it is ionic, the solid state structure is kinetically inert so that anhydrous CrCl3 is surprisingly reluctant to dissolve in water. However, in the presence of a trace of a reducing agent capable of reducing Cr3+ to Cr2+, the CrCl3 dissolves rapidly to form soluble complexes containing hydrated Cr3+ ions. The common commercial form of the hydrate is the dark green complex shown in the picture, [CrCl2(H2O)4]Cl.2H2O, but two other forms are known, viz., pale green [CrCl(H2O)5]Cl2.H2O and violet [Cr(H2O)6]Cl3.

This inertness means that CrCl3 is generally sluggish to react without the presence of a reducing agent. When it does react it undergoes ligand substitution reactions to form other complexes of chromium(III). It reacts as a Lewis acid, forming stable chloro complexes such as [CrCl6]3-.

Additional recommended knowledge

Contents

Chemical properties

Chromium(III) chloride is a Lewis acid, classified as "hard" according to the Hard-Soft Acid-Base theory. However it is also a chloro complex which is quite inert to substitution, so in fact it is ordinarily quite unreactive. 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.

The most common form of CrCl3 sold commercially is a dark green hexahydrate with the structure [CrCl2(H2O)4]Cl.2H2O, and like the anhydrous form it is also very inert towards substitution.

If substitution reactions are performed in the presence of a trace of Cr2+, then CrCl3 can undergo substitution with ligands such as water (giving violet [Cr(H2O)6]3+) or pyridine:

CrCl3 + 3 C5H5N → [CrCl3(C5H5N)3]

Such complexes are usually octahedral.

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.

Preparation

Anhydrous chromium(III) chloride may be prepared from chromium metal and chlorine directly, or indirectly using chromium(III) oxide with carbon and chlorine at 800 °C[4]:

Cr2O3(s) + 3 C(s) + 3 Cl2(g) → 2 CrCl3(s) + 3 CO(g)

It may also be prepared from the hexahydrate, by heating with thionyl chloride which reacts with the water of hydration.

The hydrated chloride may be made by dissolving the metal in hydrochloric acid.

Uses

Chromium(III) chloride is used as the source of chromium for many inorganic compounds of chromium, for example dibenzenechromium(0), an analogue of ferrocene:


A significant 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 Lewis acid in organic reactions, for example to catalyse the nitroso Diels-Alder reaction.8 it has a tae

Precautions

Although trivalent chromium is far less poisonous than hexavalent, chromium salts are generally considered highly toxic. Avoid ingestion and inhalation of dust. Wear gloves and goggles.

References

    1. N. N. Greenwood, A. Earnshaw, Chemistry of the Elements, 2nd ed., Butterworth-Heinemann, Oxford, UK, 1997.
    2. Handbook of Chemistry and Physics, 71st edition, CRC Press, Ann Arbor, Michigan, 1990.
    3. The Merck Index, 7th edition, Merck & Co, Rahway, New Jersey, USA, 1960.
    4. D. Nicholls, Complexes and First-Row Transition Elements, Macmillan Press, London, 1973.
    5. A. F. Wells, 'Structural Inorganic Chemistry, 5th ed., Oxford University Press, Oxford, UK, 1984.
    6. J. March, Advanced Organic Chemistry, 4th ed., p. 723, Wiley, New York, 1992.
    7. K. Takai, in Handbook of Reagents for Organic Synthesis, Volume 1: Reagents, Auxiliaries and Catalysts for C-C Bond Formation, (R. M. Coates, S. E. Denmark, eds.), pp. 206-211, Wiley, New York, 1999.
    8. Calvet, G.; Dussaussois, M.; Blanchard, N.; Kouklovsky, C. Organic Letters 2004, 6, 2449-2451.
     
    This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Chromium(III)_chloride". A list of authors is available in Wikipedia.
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