Chromium(III) chloride

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Chromium(III) chloride

Anhydrous

hydrate
IUPAC name

Chromium(III) chloride
Chromium trichloride

Other names

Chromic chloride

Identifiers
CAS number 10025-73-7 YesY, 
10060-12-5 (hexahydrate)
PubChem 6452300
ChemSpider 4954736 YesY
UNII Z310X5O5RP YesY
ChEBI CHEBI:53351 YesY
ChEMBL CHEMBL1200528 N
RTECS number GB5425000
Jmol-3D images {{#if:[Cr+2].[Cl-].[Cl-].[Cl-][Cr+3].[Cl-].[Cl-].[Cl-]|Image 1
Image 2
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)
Melting point 1152 °C (anhydrous)
83 °C (hexahydrate)
Boiling point 1300 °C decomp.
Solubility in water slightly soluble (anhydrous)
585 g/L (hexahydrate)
Solubility insoluble in ethanol
insoluble in ether, acetone
Acidity (pKa) 2.4 (0.2M solution)
Structure
Crystal structure YCl3 structure
Coordination
geometry		 Octahedral
Hazards
MSDS ICSC 1316 (anhydrous)
ICSC 1532 (hexahydrate)
EU classification Not listed
NFPA 704
0
3
0
Flash point Non-flammable
LD50 440 mg/kg
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
 N (verify) (what is: YesY/N?)
Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
Infobox references

Chromium(III) chloride (also called chromic chloride) describes any of several compounds of with the formula CrCl3(H2O)x, where x 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", CrCl3.6H2O. Chromium chloride find some use as catalysts and as precursors to dyes for wool.

Structure

Anhydrous chromium(III) chloride adopts the YCl3 structure, with Cr3+ occupying two thirds 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.[1]

Hydrates

Chromium(III) chlorides display the somewhat unusual property of existing in a number of distinct hydrates, forming a series of [CrCl3−n(H2O)n]z+. The main hexahydrate can be more precisely described as [CrCl2(H2O)4]Cl.2H2O. It consists of the cation [CrCl2(H2O)4]+ and additional molecules of water and a chloride anion in the lattice. Two other hydrates are known, pale green [CrCl(H2O)5]Cl2.H2O and violet [Cr(H2O)6]Cl3.

Preparation

Anhydrous

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 650–800 °C, with carbon monoxide as a side-product:[2]

Cr2O3 + 3 C + 3 Cl2 → 2 CrCl3 + 3 CO

It may also be prepared by treating the hexahydrate with thionyl chloride:[3]

CrCl3·6H2O + 6 SOCl2 → CrCl3 + 6 SO2 + 12 HCl

Hydrates

Industrially the hydrated halides are prepared by treatment of chromate with hydrochloric acid and methanol. In laboratory the hydrates are usually prepared by dissolving the chromium metal or chromium(III) oxide in hydrochloric acid.

Reactions and uses

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.

Role of Cr(II)-catalysis in substitution 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 [CrCl2(H2O)4]+ are accelerated by chromium(II) catalysts.

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 [CrCl3L3]z, where L is a Lewis base. For example, it reacts with pyridine (C
5H
5N) to form an adduct:

CrCl3 + 3 C5H5N → CrCl3(C5H5N)3

Treatment with trimethylsilylchloride in THF gives the anhydrous THF complex:[4]

CrCl3.(H2O)6 + 12 (CH3)SiCl + 3 THF → CrCl3(THF)3 + 6 ((CH3)Si)2O + 12 HCl

Precursor to organochromium complexes

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

Phosphine complexes derived from CrCl3 catalyse the trimerization of ethylene to 1-hexene.[5][6]

Use in organic synthesis

One niche 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.[7]

Dyestuffs

A number of chromium-containing dyes are used commercially for wool. Typical dyes are triarylmethanes consisting of ortho-hydroxylbenzoic acid derivatives.[8]

Precautions

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

References

  1. ↑ Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. p. 1020. ISBN 0080379419. 
  2. ↑ D. Nicholls, Complexes and First-Row Transition Elements, Macmillan Press, London, 1973.
  3. ↑ Pray, A. P. "Anhydrous Metal Chlorides" Inorganic Syntheses, 1990, vol 28, 321–2. doi:10.1002/9780470132401.ch36
  4. ↑ Philip Boudjouk, Jeung-Ho So "Solvated and Unsolvated Anhydrous Metal Chlorides from Metal Chloride Hydrates" Inorganic Syntheses, 2007, vol. 29, p. 108-111. doi:10.1002/9780470132609.ch26
  5. ↑ John T. Dixon, Mike J. Green, Fiona M. Hess, David H. Morgan “Advances in selective ethylene trimerisation – a critical overview” Journal of Organometallic Chemistry 2004, Volume 689, Pages 3641-3668. doi:10.1016/j.jorganchem.2004.06.008
  6. ↑ Feng Zheng, Akella Sivaramakrishna, John R. Moss “Thermal studies on metallacycloalkanes” Coordination Chemistry Reviews 2007, Volume 251, 2056-2071. doi:10.1016/j.ccr.2007.04.008
  7. ↑ Calvet, G.; Dussaussois, M.; Blanchard, N.; Kouklovsky, C. (2004). "Lewis Acid-Promoted Hetero Diels-Alder Cycloaddition of α-Acetoxynitroso Dienophiles". Organic Letters 6 (14): 2449–2451. doi:10.1021/ol0491336. PMID 15228301. 
  8. ↑ Thomas Gessner and Udo Mayer "Triarylmethane and Diarylmethane Dyes" in Ullmann's Encyclopedia of Industrial Chemistry 2002, Wiley-VCH, Weinheim.doi:10.1002/14356007.a27_179

Further reading

  • N. N. Greenwood, A. Earnshaw, Chemistry of the Elements, 2nd ed., Butterworth-Heinemann, Oxford, UK, 1997.
  • 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.
  • A. F. Wells, 'Structural Inorganic Chemistry, 5th ed., Oxford University Press, Oxford, UK, 1984.
  • 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.

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