Chromium(II) chloride describes inorganic compounds with the formula Cr Cl₂(H₂O)_n. The anhydrous solid is white when pure, however commercial samples are often grey or green; it is

hygroscopic Hygroscopy is the phenomenon of attracting and holding water molecules via either absorption or adsorption from the surrounding environment, which is usually at normal or room temperature. If water molecules become suspended among the substan ...

and readily dissolves in water to give bright blue air-sensitive solutions of the tetrahydrate Cr(H₂O)₄Cl₂. Chromium(II) chloride has no commercial uses but is used on a laboratory-scale for the synthesis of other chromium complexes.

Synthesis

CrCl₂ is produced by reducing

chromium(III) chloride Chromium(III) chloride (also called chromic chloride) describes any of several chemical compounds with the formula CrCl3, where can be 0, 5, and 6. The anhydrous compound with the formula CrCl3 is a violet solid. The most common form of the tric ...

either with

hydrogen Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic ...

at 500 °C: :2CrCl₃ + H₂ → 2CrCl₂ + 2HCl or by electrolysis. On the laboratory scale, LiAlH₄,

zinc Zinc is a chemical element with the symbol Zn and atomic number 30. Zinc is a slightly brittle metal at room temperature and has a shiny-greyish appearance when oxidation is removed. It is the first element in group 12 (IIB) of the periodi ...

, and related reductants produce chromous chloride from chromium(III) precursors: :4 CrCl₃ + LiAlH₄ → 4 CrCl₂ + LiCl + AlCl₃ + 2 H₂ :2 CrCl₃ + Zn → 2 CrCl₂ + ZnCl₂ CrCl₂ can also be prepared by treating a solution of

chromium(II) acetate Chromium(II) acetate hydrate, also known as chromous acetate, is the coordination compound with the formula Cr2(CH3CO2)4(H2O)2. This formula is commonly abbreviated Cr2(OAc)4(H2O)2. This red-coloured compound features a quadruple bond. The prepara ...

with

hydrogen chloride The compound hydrogen chloride has the chemical formula and as such is a hydrogen halide. At room temperature, it is a colourless gas, which forms white fumes of hydrochloric acid upon contact with atmospheric water vapor. Hydrogen chloride ga ...

: :Cr₂(OAc)₄ + 4 HCl → 2 CrCl₂ + 4 AcOH Treatment of chromium powder with concentrated

hydrochloric acid Hydrochloric acid, also known as muriatic acid, is an aqueous solution of hydrogen chloride. It is a colorless solution with a distinctive pungent smell. It is classified as a strong acid Acid strength is the tendency of an acid, symbol ...

gives a blue hydrated chromium(II) chloride, which can be converted to a related

acetonitrile complex Transition metal nitrile complexes are coordination compounds containing nitrile ligands. Because nitriles are weakly basic, the nitrile ligands in these complexes are often labile. Scope of nitriles Typical nitrile ligands are acetonitrile, pro ...

. :Cr + nH₂O + 2HCl → CrCl₂(H₂O)_n + H₂

Structure and properties

Anhydrous CrCl₂ is white however commercial samples are often grey or green. It crystallizes in the ''Pnnm''

space group In mathematics, physics and chemistry, a space group is the symmetry group of an object in space, usually in three dimensions. The elements of a space group (its symmetry operations) are the rigid transformations of an object that leave it uncha ...

, which is an orthorhombically distorted variant of the

rutile Rutile is an oxide mineral composed of titanium dioxide (TiO2), the most common natural form of TiO2. Rarer polymorphs of TiO2 are known, including anatase, akaogiite, and brookite. Rutile has one of the highest refractive indices at visib ...

structure; making it isostructural to

calcium chloride Calcium chloride is an inorganic compound, a salt with the chemical formula . It is a white crystalline solid at room temperature, and it is highly soluble in water. It can be created by neutralising hydrochloric acid with calcium hydroxide. Ca ...

. The Cr centres are

octahedral In geometry, an octahedron (plural: octahedra, octahedrons) is a polyhedron with eight faces. The term is most commonly used to refer to the regular octahedron, a Platonic solid composed of eight equilateral triangles, four of which meet a ...

, being distorted by the Jahn-Teller Effect. Chromium(II) chloride tetrahydrate

The hydrated derivative, CrCl₂(H₂O)₄, forms

monoclinic In crystallography, the monoclinic crystal system is one of the seven crystal systems. A crystal system is described by three vectors. In the monoclinic system, the crystal is described by vectors of unequal lengths, as in the orthorhombic s ...

crystals with the ''P2₁/c'' space group. The molecular geometry is approximately octahedral consisting of four short Cr—O bonds (2.078 Å) arranged in a square planar configuration and two longer Cr—Cl bonds (2.758 Å) in a trans configuration.

Reactions

The

reduction potential Redox potential (also known as oxidation / reduction potential, ''ORP'', ''pe'', ''E_'', or E_) is a measure of the tendency of a chemical species to acquire electrons from or lose electrons to an electrode and thereby be reduced or oxidised respe ...

for Cr³⁺ + e⁻ ⇄ Cr²⁺ is −0.41. Since the reduction potential of H⁺ to H₂ in acidic conditions is +0.00, the chromous ion has sufficient potential to reduce acids to hydrogen, although this reaction does not occur without a catalyst.

Organic chemistry

Chromium(II) chloride is used as precursor to other inorganic and organometallic chromium complexes. Alkyl halides and nitroaromatics are reduced by CrCl₂. The moderate electronegativity of chromium and the range of substrates that CrCl₂ can accommodate make organochromium reagents very synthetically versatile. It is a reagent in the Nozaki-Hiyama-Kishi reaction, a useful method for preparing medium-size rings. It is also used in the Takai olefination to form vinyl iodides from aldehydes in the presence of

iodoform Iodoform (also known as triiodomethane and, inaccurately, as carbon triiodide) is the organoiodine compound with the chemical formula C H I3. A pale yellow, crystalline, volatile substance, it has a penetrating and distinctive odor (in older ch ...

.Simple and selective method for aldehydes (RCHO) -> (E)-haloalkenes (RCH:CHX) conversion by means of a haloform-chromous chloride system K. Takai, K. Nitta, K. Utimoto J. Am. Chem. Soc.; 1986; 108(23); 7408-7410. .

References

{{Chlorides Chromium(II) compounds Chlorides Metal halides Reducing agents