Chromium(III) chloride (also called chromic chloride) describes any of several
chemical compounds with the formula CrCl
3, where can be 0, 5, and 6. The
anhydrous
A substance is anhydrous if it contains no water. Many processes in chemistry can be impeded by the presence of water; therefore, it is important that water-free reagents and techniques are used. In practice, however, it is very difficult to achie ...
compound with the formula CrCl
3 is a violet solid. The most common form of the trichloride is the dark green hexa
hydrate
In chemistry, a hydrate is a substance that contains water or its constituent elements. The chemical state of the water varies widely between different classes of hydrates, some of which were so labeled before their chemical structure was understo ...
, CrCl
3. Chromium chlorides find use as
catalysts and as precursors to dyes for wool.
Structure
Anhydrous chromium(III) chloride adopts the
YCl3 structure, with Cr
3+ occupying one third of the
octahedral interstices in alternating layers of a pseudo-
cubic close packed
In geometry, close-packing of equal spheres is a dense arrangement of congruent spheres in an infinite, regular arrangement (or lattice). Carl Friedrich Gauss proved that the highest average density – that is, the greatest fraction of space occu ...
lattice of Cl
− ions. The absence of cations in alternate layers leads to weak bonding between adjacent layers. For this reason, crystals of CrCl
3 cleave
Cleave may refer to:
*Cleave (surname)
*Cleave (fiber), a controlled break in optical fiber
*RAF Cleave, was an airfield in the north of Cornwall, England, May 1939 - Nov 1945
*The process of protein cleaving as a form of post-translational modifi ...
easily along the planes between layers, which results in the flaky (
mica
Micas ( ) are a group of silicate minerals whose outstanding physical characteristic is that individual mica crystals can easily be split into extremely thin elastic plates. This characteristic is described as perfect basal cleavage. Mica is ...
ceous) 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
In chemistry, a space-filling model, also known as a ''calotte model'', is a type of three-dimensional (3D) molecular model where the atoms are represented by spheres whose radii are proportional to the radii of the atoms and whose center-to- ...
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
3−''z''(H2O)''n''">rCl3−''z''(H2O)''n''sup>''z''+. The common hexahydrate can be more precisely described as
2(H2O)4">rCl2(H2O)4l. It consists of the cation ''trans''-
2(H2O)4">rCl2(H2O)4sup>+ and additional molecules of water and a chloride anion in the lattice. Two other hydrates are known, pale green
2O)5">rCl(H2O)5l
2 and violet
2O)6">r(H2O)6l
3. Similar isomerism is seen with other chromium(III) compounds.
Preparation
Anhydrous chromium(III) chloride may be prepared by
chlorination of
chromium
Chromium is a chemical element with the symbol Cr and atomic number 24. It is the first element in group 6. It is a steely-grey, lustrous, hard, and brittle transition metal.
Chromium metal is valued for its high corrosion resistance and hardne ...
metal directly, or indirectly by
carbothermic
Carbothermic reactions involve the reduction of substances, often metal oxides (O^2-), using carbon as the reducing agent. These chemical reactions are usually conducted at temperatures of several hundred degrees Celsius. Such processes are applie ...
chlorination of
chromium(III) oxide at 650–800 °C
:Cr
2O
3 + 3 C + 3 Cl
2 → 2 CrCl
3 + 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 d
3 Cr
3+ ion can be explained using
crystal field theory Crystal field theory (CFT) describes the breaking of degeneracies of electron orbital states, usually ''d'' or ''f'' orbitals, due to a static electric field produced by a surrounding charge distribution (anion neighbors). This theory has been used ...
. One way of opening CrCl
3 up to substitution in solution is to reduce even a trace amount to CrCl
2, for example using
zinc in
hydrochloric acid. This chromium(II) compound undergoes substitution easily, and it can exchange electrons with CrCl
3 via a
chloride bridge, allowing all of the CrCl
3 to react quickly.
With the presence of some chromium(II), however, solid CrCl
3 dissolves rapidly in water. Similarly,
ligand substitution reactions of solutions of
2(H2O)4">rCl2(H2O)4sup>+ are accelerated by chromium(II) catalysts.
With molten
alkali metal
The alkali metals consist of the chemical elements lithium (Li), sodium (Na), potassium (K),The symbols Na and K for sodium and potassium are derived from their Latin names, ''natrium'' and ''kalium''; these are still the origins of the names ...
chlorides such as
potassium chloride
Potassium chloride (KCl, or potassium salt) is a metal halide salt composed of potassium and chlorine. It is odorless and has a white or colorless vitreous crystal appearance. The solid dissolves readily in water, and its solutions have a salt ...
, CrCl
3 gives salts of the type M
3CrCl
6 and K
3Cr
2Cl
9, which is also octahedral but where the two chromiums are linked via three chloride bridges.
The hexahydrate can also be dehydrated with
thionyl chloride:
:CrCl
3 + 6 SOCl
2 → CrCl
3 + 6 SO
2 + 12
HCl HCL may refer to:
Science and medicine
* Hairy cell leukemia, an uncommon and slowly progressing B cell leukemia
* Harvard Cyclotron Laboratory, from 1961 to 2002, a proton accelerator used for research and development
* Hollow-cathode lamp, a spe ...
Complexes with organic ligands
CrCl
3 is a
Lewis acid
A Lewis acid (named for the American physical chemist Gilbert N. Lewis) is a chemical species that contains an empty orbital which is capable of accepting an electron pair from a Lewis base to form a Lewis adduct. A Lewis base, then, is any sp ...
, classified as "hard" according to the
Hard-Soft Acid-Base theory. It forms a variety of adducts of the type
3L3">rCl3L3sup>''z'', where L is a
Lewis base
A Lewis acid (named for the American physical chemist Gilbert N. Lewis) is a chemical species that contains an empty orbital which is capable of accepting an electron pair from a Lewis base to form a Lewis adduct. A Lewis base, then, is any sp ...
. For example, it reacts with
pyridine () to form the
pyridine complex:
:CrCl
3 + 3 C
5H
5N → CrCl
3(C
5H
5N)
3
Treatment with
trimethylsilylchloride
Trimethylsilyl chloride, also known as chlorotrimethylsilane is an organosilicon compound ( silyl halide), with the formula (CH3)3SiCl, often abbreviated Me3SiCl or TMSCl. It is a colourless volatile liquid that is stable in the absence of water. ...
in THF gives the anhydrous THF complex:
:CrCl
3 + 12 Me
3SiCl → CrCl
3(THF)
3 + 6 (Me
3Si)
2O + 12 HCl
Precursor to organochromium complexes
Chromium(III) chloride is used as the precursor to many
organochromium compound Organochromium chemistry is a branch of organometallic chemistry that deals with organic compounds containing a chromium to carbon bond and their reactions. The field is of some relevance to organic synthesis. The relevant oxidation states for organ ...
s, for example
bis(benzene)chromium
Bis(benzene)chromium is the organometallic compound with the formula Cr( η6-C6H6)2. It is sometimes called dibenzenechromium. The compound played an important role in the development of sandwich compounds in organometallic chemistry and is th ...
, an analogue of
ferrocene
Ferrocene is an organometallic compound with the formula . The molecule is a complex consisting of two cyclopentadienyl rings bound to a central iron atom. It is an orange solid with a camphor-like odor, that sublimes above room temperature, a ...
:
:

Phosphine complexes derived from CrCl
3 catalyse the trimerization of
ethylene
Ethylene (IUPAC name: ethene) is a hydrocarbon which has the formula or . It is a colourless, flammable gas with a faint "sweet and musky" odour when pure. It is the simplest alkene (a hydrocarbon with carbon-carbon double bonds).
Ethylene i ...
to
1-hexene.
Use in organic synthesis
One niche use of CrCl
3 in
organic synthesis
Organic synthesis is a special branch of chemical synthesis and is concerned with the intentional construction of organic compounds. Organic molecules are often more complex than inorganic compounds, and their synthesis has developed into one o ...
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 CrCl
3 per mole of
lithium aluminium hydride
Lithium aluminium hydride, commonly abbreviated to LAH, is an inorganic compound with the chemical formula Li Al H4. It is a white solid, discovered by Finholt, Bond and Schlesinger in 1947. This compound is used as a reducing agent in organic ...
, 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
In chemistry, the valence (US spelling) or valency (British spelling) of an element is the measure of its combining capacity with other atoms when it forms chemical compounds or molecules.
Description
The combining capacity, or affinity of an ...
, 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. CrCl
3)
(CrCl
3·6H
2O)
*
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