Chromium(III) Chloride

Chromium(III) chloride (also called chromic chloride) describes any of several chemical compounds with the formula CrCl₃, where can be 0, 5, and 6. The anhydrous compound with the formula CrCl₃ is a violet solid. The most common form of the trichloride is the dark green hexahydrate, CrCl₃. Chromium chlorides find use as catalysts and as precursors to dyes for wool.

Structure

Anhydrous chromium(III) chloride adopts the YCl₃ structure, with Cr³⁺ occupying one third of the octahedral interstices in alternating layers 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 (micaceous) appearance of samples of chromium(III) chloride. If pressurized to 9.9 GPa it goes under a phase transition.

File:Chromium(III)-chloride-sheet-from-monoclinic-xtal-3D-balls-SF-overlay.png, Space-filling model of cubic close packing of chloride ions in the crystal structure of CrCl₃
File:Chromium(III)-chloride-sheet-from-monoclinic-xtal-3D-balls.png, Ball-and-stick model of part of a layer
File:Chromium(III)-chloride-layers-stacking-from-monoclinic-xtal-3D-balls.png, Stacking of layers

Chromium(III) chloride hydrates

The hydrated chromium(III) chlorides display the somewhat unusual property of existing in a number of distinct chemical forms (isomers), which differ in terms of the number of chloride anions that are coordinated to Cr(III) and the water of crystallization. The different forms exist both as solids and in aqueous solutions. Several members are known of the series of rCl_3−''z''(H₂O)_''n''sup>''z''+. The common hexahydrate can be more precisely described as rCl₂(H₂O)₄l. It consists of the cation ''trans''- rCl₂(H₂O)₄sup>+ and additional molecules of water and a chloride anion in the lattice. Two other hydrates are known, pale green rCl(H₂O)₅l₂ and violet r(H₂O)₆l₃. Similar isomerism is seen with other chromium(III) compounds.

Preparation

Anhydrous chromium(III) chloride may be prepared by chlorination of chromium metal directly, or indirectly by carbothermic chlorination of chromium(III) oxide at 650–800 °C

:Cr₂O₃ + 3 C + 3 Cl₂ → 2 CrCl₃ + 3 CO

The hydrated chlorides are prepared by treatment of chromate with hydrochloric acid and aqueous methanol.

Reactions

Slow reaction rates are common with chromium(III) complexes. 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.

With the presence of some chromium(II), however, solid CrCl₃ dissolves rapidly in water. Similarly, ligand substitution reactions of solutions of rCl₂(H₂O)₄sup>+ are accelerated by chromium(II) catalysts.

With molten alkali metal chlorides such as potassium chloride, CrCl₃ gives salts of the type M₃CrCl₆ and K₃Cr₂Cl₉, which is also octahedral but where the two chromiums are linked via three chloride bridges.

The hexahydrate can also be dehydrated with thionyl chloride:
:CrCl₃ + 6 SOCl₂ → CrCl₃ + 6 SO₂ + 12 HCl

Complexes with organic ligands

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 rCl₃L₃sup>''z'', where L is a Lewis base. For example, it reacts with pyridine () to form the pyridine complex:
:CrCl₃ + 3 C₅H₅N → CrCl₃(C₅H₅N)₃

Treatment with trimethylsilylchloride in THF gives the anhydrous THF complex:
:CrCl₃ + 12 Me₃SiCl → CrCl₃(THF)₃ + 6 (Me₃Si)₂O + 12 HCl

Precursor to organochromium complexes

Chromium(III) chloride is used as the precursor to many organochromium compounds, for example bis(benzene)chromium, an analogue of ferrocene:

:

Phosphine complexes derived from CrCl₃ catalyse the trimerization of ethylene to 1-hexene.

Use in organic synthesis

One niche use of CrCl₃ in organic synthesis is for the ''in situ'' preparation of chromium(II) chloride, a reagent for the reduction of alkyl halides and for 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.

Dyestuffs

A number of chromium-containing dyes are used commercially for wool. Typical dyes are triarylmethanes consisting of ortho-hydroxylbenzoic acid derivatives.Thomas Gessner and Udo Mayer "Triarylmethane and Diarylmethane Dyes" in Ullmann's Encyclopedia of Industrial Chemistry 2002, Wiley-VCH, Weinheim.

Precautions

Although trivalent chromium is far less poisonous than hexavalent, chromium salts are generally considered toxic.

References

Further reading

*''Handbook of Chemistry and Physics'', 71st edition, CRC Press, Ann Arbor, Michigan, 1990.
*''The Merck Index'', 7th edition, Merck & Co, Rahway, New Jersey, USA, 1960.
*J. March, ''Advanced Organic Chemistry'', 4th ed., p. 723, Wiley, New York, 1992.
*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.

External links

International Chemical Safety Card 1316
(anhydr. CrCl₃)

(CrCl₃·6H₂O)

* ttps://www.cdc.gov/niosh/npg/npgd0140.html NIOSH Pocket Guide to Chemical Hazardsbr>IARC Monograph "Chromium and Chromium compounds"

{{Chlorides

Chromium(III) compounds
Chlorides
Metal halides
Coordination complexes