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Chromium(III) chloride
Encyclopedia
Chromium chloride (also called chromic chloride) is a violet coloured solid with the formula CrCl3. The most common form of CrCl3 sold commercially is a dark green hexahydrate with the formula [CrCl2(H2O)4]Cl.2H2O. Two other hydrates are known, pale green [CrCl(H2O)5]Cl2.H2O and violet [Cr(H2O)6]Cl3. This unusual feature of chromium(III) chlorides, having a series of [CrCl3−n(H2O)n]z+, each of which is isolable, is also found with other chromium(III) compounds.
structure, with Cr3+ occupying two thirds of the octahedral
interstices in every other layer 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 appearance of samples of chromium(III) chloride.
of chromium
metal directly, or indirectly by chlorination of chromium(III) oxide
in the presence of carbon
at 800 °C, with carbon monoxide
as a side-product:
It may also be prepared by treating the hexahydrate with thionyl chloride
.
In laboratory it's usually prepared by dissolving the elemental chromium
or chromium(III) oxide
in hydrochloric acid
.
However, it is also a chloro complex
which is quite inert to substitution, so in fact it is ordinarily quite unreactive. Slow reaction rates are common with chromium(III) complexes in general. 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.
, 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
to form an adduct:
Like most chromium(III) compounds, CrCl3 is chemically inert but its neighboring redox state chromium(II) is reactive. Thus, CrCl3 and its complexes generally react only sluggishly in the absence of a reducing agent
capable of reducing Cr3+ to Cr2+. 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
chloride
s 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. Chromium(III) chloride is used as the precursor to many inorganic compound
s of chromium, for example dibenzenechromium(0)
, an analogue of ferrocene
:
One 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 mole
s 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 Louis acid in organic reactions, for example to catalyse the nitroso Diels-Alder reaction
.
Structure
Anhydrous chromium(III) chloride adopts the YCl3Yttrium(III) chloride
Yttrium chloride is an ionic compound of yttrium and chlorine. It is a salt that is solid at room temperature, highly soluble in water, and deliquescent...
structure, with Cr3+ occupying two thirds of the octahedral
Octahedral molecular geometry
In chemistry, octahedral molecular geometry describes the shape of compounds where in six atoms or groups of atoms or ligands are symmetrically arranged around a central atom, defining the vertices of an octahedron...
interstices in every other layer 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
Cleavage (crystal)
Cleavage, in mineralogy, is the tendency of crystalline materials to split along definite crystallographic structural planes. These planes of relative weakness are a result of the regular locations of atoms and ions in the crystal, which create smooth repeating surfaces that are visible both in the...
easily along the planes between layers, which results in the flaky appearance of samples of chromium(III) chloride.
Preparation
Commercially anhydrous chromium(III) chloride may be prepared by chlorinationHalogenation
Halogenation is a chemical reaction that incorporates a halogen atom into a molecule in substitution of hydrogen atom. Halogenation takes place in the gas phase. There are four types of halogenation: fluorination, chlorination, bromination, and iodination...
of chromium
Chromium
Chromium is a chemical element which has the symbol Cr and atomic number 24. It is the first element in Group 6. It is a steely-gray, lustrous, hard metal that takes a high polish and has a high melting point. It is also odorless, tasteless, and malleable...
metal directly, or indirectly by chlorination of chromium(III) oxide
Chromium(III) oxide
Chromium oxide is the inorganic compound of the formula Cr2O3. It is one of principal oxides of chromium and is used as a pigment. In nature, it occurs as the rare mineral eskolaite.-Structure and properties:...
in the presence of carbon
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...
at 800 °C, with carbon monoxide
Carbon monoxide
Carbon monoxide , also called carbonous oxide, is a colorless, odorless, and tasteless gas that is slightly lighter than air. It is highly toxic to humans and animals in higher quantities, although it is also produced in normal animal metabolism in low quantities, and is thought to have some normal...
as a side-product:
- Cr2O3 + 3 C + 3 Cl2 → 2 CrCl3 + 3 CO
It may also be prepared by treating the hexahydrate with thionyl chloride
Thionyl chloride
Thionyl chloride is an inorganic compound with the formula SOCl2. It is a reactive chemical reagent used in chlorination reactions. It is a colorless, distillable liquid at room temperature and pressure that decomposes above 140 °C. Thionyl chloride is sometimes confused with sulfuryl...
.
In laboratory it's usually prepared by dissolving the elemental chromium
Chromium
Chromium is a chemical element which has the symbol Cr and atomic number 24. It is the first element in Group 6. It is a steely-gray, lustrous, hard metal that takes a high polish and has a high melting point. It is also odorless, tasteless, and malleable...
or chromium(III) oxide
Chromium(III) oxide
Chromium oxide is the inorganic compound of the formula Cr2O3. It is one of principal oxides of chromium and is used as a pigment. In nature, it occurs as the rare mineral eskolaite.-Structure and properties:...
in hydrochloric acid
Hydrochloric acid
Hydrochloric acid is a solution of hydrogen chloride in water, that is a highly corrosive, strong mineral acid with many industrial uses. It is found naturally in gastric acid....
.
However, it is also a chloro complex
Complex (chemistry)
In chemistry, a coordination complex or metal complex, is an atom or ion , bonded to a surrounding array of molecules or anions, that are in turn known as ligands or complexing agents...
which is quite inert to substitution, so in fact it is ordinarily quite unreactive. Slow reaction rates are common with chromium(III) complexes in general. The low reactivity of the d3 Cr3+ ion can be explained using crystal field theory
Crystal field theory
Crystal field theory is a model that describes the electronic structure of transition metal compounds, all of which can be considered coordination complexes. CFT successfully accounts for some magnetic properties, colours, hydration enthalpies, and spinel structures of transition metal complexes,...
. One way of opening CrCl3 up to substitution in solution is to reduce even a trace amount to CrCl2, for example using zinc
Zinc
Zinc , or spelter , is a metallic chemical element; it has the symbol Zn and atomic number 30. It is the first element in group 12 of the periodic table. Zinc is, in some respects, chemically similar to magnesium, because its ion is of similar size and its only common oxidation state is +2...
in hydrochloric acid
Hydrochloric acid
Hydrochloric acid is a solution of hydrogen chloride in water, that is a highly corrosive, strong mineral acid with many industrial uses. It is found naturally in gastric acid....
. This chromium(II) compound undergoes substitution easily, and it can exchange electrons with CrCl3 via a chloride
Chloride
The chloride ion is formed when the element chlorine, a halogen, picks up one electron to form an anion Cl−. The salts of hydrochloric acid HCl contain chloride ions and can also be called chlorides. The chloride ion, and its salts such as sodium chloride, are very soluble in water...
bridge, allowing all of the CrCl3 to react quickly.
Reactions and uses
CrCl3 is a Lewis acidLewis acid
]The term Lewis acid refers to a definition of acid published by Gilbert N. Lewis in 1923, specifically: An acid substance is one which can employ a lone pair from another molecule in completing the stable group of one of its own atoms. Thus, H+ is a Lewis acid, since it can accept a lone pair,...
, 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
Pyridine
Pyridine is a basic heterocyclic organic compound with the chemical formula C5H5N. It is structurally related to benzene, with one C-H group replaced by a nitrogen atom...
to form an adduct:
- CrCl3 + 3 C5H5N → CrCl3(C5H5N)3
Like most chromium(III) compounds, CrCl3 is chemically inert but its neighboring redox state chromium(II) is reactive. Thus, CrCl3 and its complexes generally react only sluggishly in the absence of a reducing agent
Redox
Redox reactions describe all chemical reactions in which atoms have their oxidation state changed....
capable of reducing Cr3+ to Cr2+. With the presence of some chromium(II), however, solid CrCl3 dissolves rapidly in water. Similarly, ligand
Ligand
In coordination chemistry, a ligand is an ion or molecule that binds to a central metal atom to form a coordination complex. The bonding between metal and ligand generally involves formal donation of one or more of the ligand's electron pairs. The nature of metal-ligand bonding can range from...
substitution reactions of solutions of [CrCl2(H2O)4]+ are accelerated by chromium(II) catalysts.
With molten alkali metal
Alkali metal
The alkali metals are a series of chemical elements in the periodic table. In the modern IUPAC nomenclature, the alkali metals comprise the group 1 elements, along with hydrogen. The alkali metals are lithium , sodium , potassium , rubidium , caesium , and francium...
chloride
Chloride
The chloride ion is formed when the element chlorine, a halogen, picks up one electron to form an anion Cl−. The salts of hydrochloric acid HCl contain chloride ions and can also be called chlorides. The chloride ion, and its salts such as sodium chloride, are very soluble in water...
s such as potassium chloride
Potassium chloride
The chemical compound potassium chloride is a metal halide salt composed of potassium and chlorine. In its pure state, it is odorless and has a white or colorless vitreous crystal appearance, with a crystal structure that cleaves easily in three directions. Potassium chloride crystals are...
, 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. Chromium(III) chloride is used as the precursor to many inorganic compound
Inorganic compound
Inorganic compounds have traditionally been considered to be of inanimate, non-biological origin. In contrast, organic compounds have an explicit biological origin. However, over the past century, the classification of inorganic vs organic compounds has become less important to scientists,...
s of chromium, for example dibenzenechromium(0)
Bis(benzene)chromium
Bischromium is the organometallic compound with the formula Cr2. It is sometimes called dibenzenechromium. The compound played an important role in the development of sandwich compounds in organometallic chemistry and is the prototypical complex containg two arene ligands.-Preparation:The...
, an analogue of ferrocene
Ferrocene
Ferrocene is an organometallic compound with the formula Fe2. It is the prototypical metallocene, a type of organometallic chemical compound consisting of two cyclopentadienyl rings bound on opposite sides of a central metal atom. Such organometallic compounds are also known as sandwich compounds...
:
One use of CrCl3 in organic synthesis
Organic synthesis
Organic synthesis is a special branch of chemical synthesis and is concerned with the construction of organic compounds via organic reactions. Organic molecules can often contain a higher level of complexity compared to purely inorganic compounds, so the synthesis of organic compounds has...
is for the in situ preparation of chromium(II) chloride
Chromium(II) chloride
Chromium chloride is the chemical compound with the formula CrCl2. This white, crystalline solid is used for the synthesis of other chromium complexes. CrCl2 is hygroscopic. It dissolves in water to give bright blue solutions that are easily oxidized by air to give Cr-containing products...
, 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 mole
Mole (unit)
The mole is a unit of measurement used in chemistry to express amounts of a chemical substance, defined as an amount of a substance that contains as many elementary entities as there are atoms in 12 grams of pure carbon-12 , the isotope of carbon with atomic weight 12. This corresponds to a value...
s of CrCl3 per mole of lithium aluminium hydride
Lithium aluminium hydride
Lithium aluminium hydride, commonly abbreviated to LAH or known as LithAl, is an inorganic compound with the chemical formula LiAlH4. It was discovered by Finholt, Bond and Schlesinger in 1947. This compound is used as a reducing agent in organic synthesis, especially for the reduction of esters,...
, although if aqueous acidic conditions are appropriate zinc
Zinc
Zinc , or spelter , is a metallic chemical element; it has the symbol Zn and atomic number 30. It is the first element in group 12 of the periodic table. Zinc is, in some respects, chemically similar to magnesium, because its ion is of similar size and its only common oxidation state is +2...
and hydrochloric acid
Hydrochloric acid
Hydrochloric acid is a solution of hydrogen chloride in water, that is a highly corrosive, strong mineral acid with many industrial uses. It is found naturally in gastric acid....
may be sufficient.
Chromium(III) chloride has also been used as a Louis acid in organic reactions, for example to catalyse the nitroso Diels-Alder reaction
Diels-Alder reaction
The Diels–Alder reaction is an organic chemical reaction between a conjugated diene and a substituted alkene, commonly termed the dienophile, to form a substituted cyclohexene system. The reaction can proceed even if some of the atoms in the newly formed ring are not carbon...
.
Precautions
Although trivalent chromium is far less poisonous than hexavalent, chromium salts are generally considered toxic.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.