my-server
← Wiki Redirected from Chromous chloride

Chromium(II) chloride

Chromium(II) chloride describes inorganic compounds with the formula CrCl<sub>2</sub>(H<sub>2</sub>O)<sub>n</sub>. 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 Cr(H<sub>2</sub>O)<sub>4</sub>Cl<sub>2</sub>. Chromium(II) chloride has no commercial uses but is used on a laboratory-scale for the synthesis of other chromium complexes.

Structure and properties

Anhydrous CrCl<sub>2</sub> 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, CrCl<sub>2</sub>(H<sub>2</sub>O)<sub>4</sub>, forms monoclinic crystals with the P2<sub>1</sub>/c space group. The molecular geometry is approximately octahedral consisting of four short Cr—O bonds (2.078&nbsp;Å) arranged in a square planar configuration and two longer Cr—Cl bonds (2.758&nbsp;Å) in a trans configuration.

Synthesis

CrCl<sub>2</sub> is produced by reducing chromium(III) chloride either with hydrogen at 500&nbsp;°C:

2CrCl<sub>3</sub> + H<sub>2</sub> → 2CrCl<sub>2</sub> + 2HCl

or by electrolysis.

On the laboratory scale, LiAlH<sub>4</sub>, zinc, and related reductants produce chromous chloride from chromium(III) precursors:

4 CrCl<sub>3</sub> + LiAlH<sub>4</sub> → 4 CrCl<sub>2</sub> + LiCl + AlCl<sub>3</sub> + 2 H<sub>2</sub>
2 CrCl<sub>3</sub> + Zn → 2 CrCl<sub>2</sub> + ZnCl<sub>2</sub>

CrCl<sub>2</sub> can also be prepared by treating a solution of chromium(II) acetate with hydrogen chloride:

Cr<sub>2</sub>(OAc)<sub>4</sub> + 4 HCl → 2 CrCl<sub>2</sub> + 4 AcOH

Treatment of chromium powder with concentrated hydrochloric acid gives a blue hydrated chromium(II) chloride, which can be converted to a related acetonitrile complex.

Cr + nH<sub>2</sub>O + 2HCl → CrCl<sub>2</sub>(H<sub>2</sub>O)<sub>n</sub> + H<sub>2</sub>

Reactions

The reduction potential for Cr<sup>3+</sup> + e<sup>−</sup> ⇄ Cr<sup>2+</sup> is −0.41. Since the reduction potential of H<sup>+</sup> to H<sub>2</sub> 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 CrCl<sub>2</sub>. The moderate electronegativity of chromium and the range of substrates that CrCl<sub>2</sub> 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.

References