Chromium(II) chloride

Chromium chloride describes inorganic compounds with the formula CrCl₂_n. The anhydrous solid is white when pure, however commercial samples are often grey or green; it is hygroscopic and readily dissolves in water to give bright blue air-sensitive solutions of the tetrahydrate Cr₄Cl₂. Chromium 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 chloride either with hydrogen at 500 °C:
or by electrolysis.
On the laboratory scale, LiAlH₄, zinc, and related reductants produce chromous chloride from chromium precursors:
CrCl₂ can also be prepared by treating a solution of chromium acetate with hydrogen chloride:
Treatment of chromium powder with concentrated hydrochloric acid gives a blue hydrated chromium chloride, which can be converted to a related acetonitrile complex.

Structure and properties

Anhydrous CrCl₂ is white however commercial samples are often grey or green. It crystallizes in the Pnnm space group, which is an orthorhombically distorted variant of the rutile structure; making it isostructural to calcium chloride. The Cr centres are octahedral, being distorted by the Jahn-Teller Effect.
The hydrated derivative, CrCl₂₄, forms monoclinic crystals with the P2₁/c space group. The molecular geometry is approximately octahedral consisting of four short Cr—O bonds arranged in a square planar configuration and two longer Cr—Cl bonds in a trans configuration.

Reactions

The reduction potential 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 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.