|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||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)|
|Crystal structure||YCl3 structure|
|MSDS||ICSC 1316 (anhydrous)
ICSC 1532 (hexahydrate)
|EU classification||Not listed|
LD50 (Lethal dose)
Except where noted otherwise, data is given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
|what is: / ?)(|
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.
- 1 Structure
- 2 Preparation
- 3 Reactions and uses
- 4 Precautions
- 5 References
- 6 Further reading
- 7 External links
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.
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
Different Chemical Forms of 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 water 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. 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.
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:
- Cr2O3 + 3 C + 3 Cl2 → 2 CrCl3 + 3 CO
- CrCl3•6H2O + 6 SOCl2 → CrCl3 + 6 SO2 + 12 HCl
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
5N) to form an adduct:
- CrCl3 + 3 C5H5N → CrCl3(C5H5N)3
- CrCl3.(H2O)6 + 12 (CH3)3SiCl + 3 THF → CrCl3(THF)3 + 6 ((CH3)3Si)2O + 12 HCl
Precursor to organochromium complexes
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.
Although trivalent chromium is far less poisonous than hexavalent, chromium salts are generally considered toxic.
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- 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.
|Wikimedia Commons has media related to Chromium(III) chloride.|
- 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"