Chromium(III) chloride

Chromium(III) chloride

Anhydrous

Hexahydrate
Names
IUPAC names
Chromium(III) chloride
Chromium trichloride
Other names
Chromic chloride
Identifiers
10025-73-7 YesY
10060-12-5 (hexahydrate) N
3D model (Jmol) Interactive image
ChEBI CHEBI:53351 YesY
ChEMBL ChEMBL1200528 N
ChemSpider 4954736 YesY
ECHA InfoCard 100.030.023
PubChem 6452300
RTECS number GB5425000
UNII Z310X5O5RP YesY
Properties
CrCl3
Molar mass 158.36 g/mol (anhydrous)
266.45 g/mol (hexahydrate)[1]
Appearance purple when anhydrous, dark green when hexahydrate
Density 2.87 g/cm3 (anhydrous)
1.760 g/cm3 (hexahydrate)
Melting point 1,152 °C (2,106 °F; 1,425 K) (anhydrous)
83 °C (hexahydrate)
Boiling point 1,300 °C (2,370 °F; 1,570 K) decomposes
slightly soluble (anhydrous)
585 g/L (hexahydrate)
Solubility insoluble in ethanol
insoluble in ether, acetone
Acidity (pKa) 2.4 (0.2M solution)
Structure
YCl3 structure
Octahedral
Hazards
Safety data sheet ICSC 1316 (anhydrous)
ICSC 1532 (hexahydrate)
Not listed
NFPA 704
Flammability code 0: Will not burn. E.g., water Health code 3: Short exposure could cause serious temporary or residual injury. E.g., chlorine gas Reactivity code 0: Normally stable, even under fire exposure conditions, and is not reactive with water. E.g., liquid nitrogen Special hazards (white): no codeNFPA 704 four-colored diamond
0
3
0
Flash point Non-flammable
Lethal dose or concentration (LD, LC):
1870 mg/kg (oral, rat)[2]
US health exposure limits (NIOSH):
PEL (Permissible)
 TWA 1 mg/m3[3]
REL (Recommended)
 TWA 0.5 mg/m3[3]
IDLH (Immediate danger)
 250 mg/m3[3]
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 otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
N verify (what is YesYN ?)
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 finds uses 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.[4][5]

Chromium(III) chloride hydrates

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 [CrCl3−n(H2O)n]z+. The main hexahydrate can be more precisely described as [CrCl2(H2O)4]Cl•2H2O. It consists of the cation trans-[CrCl2(H2O)4]+ and additional molecules of water and a chloride anion in the lattice.[6] Two other hydrates are known, pale green [CrCl(H2O)5]Cl2•H2O and violet [Cr(H2O)6]Cl3. Similar behaviour occurs 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[7][8]

Cr2O3 + 3 C + 3 Cl2 → 2 CrCl3 + 3 CO

It may also be prepared by treating the hexahydrate with thionyl chloride:[9]

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

The hydrated chlorides 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

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.

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.

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:[10]

CrCl3.(H2O)6 + 12 (CH3)3SiCl + 3 THF → CrCl3(THF)3 + 6 ((CH3)3Si)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.[11][12]

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.[13]

Dyestuffs

A number of chromium-containing dyes are used commercially for wool. Typical dyes are triarylmethanes consisting of ortho-hydroxylbenzoic acid derivatives.[14]

Precautions

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

References

  1. ↑ http://www.sigmaaldrich.com/catalog/product/aldrich/200050?lang=en&region=AU
  2. ↑ "Chromium(III) compounds [as Cr(III)]". Immediately Dangerous to Life and Health. National Institute for Occupational Safety and Health (NIOSH).
  3. 1 2 3 "NIOSH Pocket Guide to Chemical Hazards #0141". National Institute for Occupational Safety and Health (NIOSH).
  4. ↑ Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. p. 1020. ISBN 0-08-037941-9.
  5. ↑ A. F. Wells, 'Structural Inorganic Chemistry, 5th ed., Oxford University Press, Oxford, UK, 1984.
  6. ↑ Ian G. Dance, Hans C. Freeman "The Crystal Structure of Dichlorotetraaquochromium(III) Chloride Dihydrate: Primary and Secondary Metal Ion Hydration" Inorganic Chemistry 1965, volume 4, 1555–1561. doi:10.1021/ic50033a006
  7. ↑ D. Nicholls, Complexes and First-Row Transition Elements, Macmillan Press, London, 1973.
  8. ↑ Brauer, Georg (1965) [1962]. Handbuch Der Präparativen Anorganischen Chemie [Handbook of Preparative Inorganic Chemistry] (in German). 2. Stuttgart; New York, New York: Ferdinand Enke Verlag; Academic Press, Inc. p. 1340. ISBN 978-0-32316129-9. Retrieved 2014-01-10.
  9. ↑ Pray, A. P. "Anhydrous Metal Chlorides" Inorganic Syntheses, 1990, vol 28, 321–2. doi:10.1002/9780470132401.ch36
  10. ↑ 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
  11. ↑ 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, pp 3641-3668. doi:10.1016/j.jorganchem.2004.06.008
  12. ↑ 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
  13. ↑ 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.
  14. ↑ 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

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