Chromium(II) chloride

Chromium(II) chloride

Names
IUPAC name Chromium(II) chloride
Other names Chromous chloride
Identifiers
CAS Number	10049-05-5
ChemSpider	23252
Jmol interactive 3D	Image
PubChem	24871
RTECS number	GB5250000
UNII	CET32HKA21
InChI InChI=1S/2ClH.Cr/h2*1H;/q;;+2/p-2 Key: XBWRJSSJWDOUSJ-UHFFFAOYSA-L InChI=1/2ClH.Cr/h2*1H;/q;;+2/p-2 Key: XBWRJSSJWDOUSJ-NUQVWONBAQ
SMILES Cl[Cr]Cl
Properties
Chemical formula	Cl2Cr
Molar mass	122.90 g·mol^−1
Appearance	White to grey/green powder (anhydrous), very hygroscopic
Odor	Odorless
Density	2.88 g/cm^3 (24 °C)[1]
Melting point	824 °C (1,515 °F; 1,097 K) anhydrous 51 °C (124 °F; 324 K) tetrahydrate, decomposes[1]
Boiling point	1,302 °C (2,376 °F; 1,575 K) anhydrous[1]
Solubility in water	Soluble[1]
Solubility	Insoluble in alcohol, ether
Acidity (pKa)	2
Magnetic susceptibility (χ)	7.23·10^−3 cm^3/mol[1]
Structure
Crystal structure	Orthorhombic (deformed rutile, anhydrous), oP6[2] Monoclinic (tetrahydrate)[3]
Space group	Pnnm, No. 58 (anhydrous)[2] P21/c, No. 14 (tetrahydrate)[3]
Point group	2/m 2/m 2/m (anhydrous)[2] 2/m (tetrahydrate)[3]
Lattice constant	a = 6.64 Å, b = 5.98 Å, c = 3.48 Å (anhydrous)[2] α = 90°, β = 90°, γ = 90°
Coordination geometry	Octahedral (Cr^2+, anhydrous)[2]
Thermochemistry
Specific heat capacity (C)	71.2 J/mol·K[1]
Std molar entropy (S^o298)	115.3 J/mol·K[1]
Std enthalpy of formation (ΔfH^o298)	−395.4 kJ/mol[1]
Gibbs free energy (ΔfG˚)	−356 kJ/mol[1]
Hazards
Safety data sheet	Oxford MSDS
GHS pictograms	[4]
GHS signal word	Warning
GHS hazard statements	H302, H315, H319, H335[4]
GHS precautionary statements	P261, P305+351+338[4]
EU classification (DSD)	Xn
R-phrases	R22, R36/37/38
S-phrases	S26, S36
NFPA 704	[5] 0 2 1
Lethal dose or concentration (LD, LC):
LD50 (Median dose)	1870 mg/kg (rats, oral)[5]
Related compounds
Other anions	Chromium(II) fluoride Chromium(II) bromide Chromium(II) iodide
Other cations	Chromium(III) chloride Chromium(IV) chloride Molybdenum(II) chloride Tungsten(II) chloride
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Infobox references

Chromium(II) chloride is an inorganic compound with the formula CrCl₂. This crystalline 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 [Cr(H₂O)₄]Cl₂. Chromium(II) chloride has no industrial uses but is used on a laboratory-scale for the synthesis of other chromium complexes.

Synthesis

CrCl₂ is produced by reducing chromium(III) chloride with hydrogen at 500 °C:^[6]

2 CrCl₃ + H₂ → 2 CrCl₂ + 2 HCl

Small scale preparations can use LiAlH₄, Zinc or related reagents, to reduce CrCl₃

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 with hydrogen chloride.^[7]

Cr₂(OAc)₄ + 4 HCl → 2 CrCl₂ + 4 AcOH

Structure and Properties

Anhydrous CrCl₂ is white^[7] 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.^[8]

The hydrated derivative, CrCl₂(H₂O)₄, forms monoclinic crystals with the P2₁/c space group. The molecular geometry is approximately square planar with Cr—O distances of 2.078 Å and two Cr—Cl distances of 2.758 Å.^[3]

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 ions 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.^[9] It is a reagent in the Nozaki-Hiyama-Kishi reaction, a useful method for preparing medium-size rings.^[10] It is also used in the Takai olefination to form vinyl iodides from aldehydes in the presence of iodoform.^[11]

References

1. Lide, David R., ed. (2009). CRC Handbook of Chemistry and Physics (90th ed.). Boca Raton, Florida: CRC Press. ISBN 978-1-4200-9084-0.
2. Tracy, Joseph W.; Gregory, N.W.; Lingafelter, E.C.; Dunitz, J.D.; Mez, H.-C.; Rundle, R.E.; Scheringer, Christian; Yakel, H.L.; Wilkinson, M.K. (1961). "The crystal structure of chromium(II) chloride". Acta Crystallographica 4 (9): 927–929. doi:10.1107/S0365110X61002710.
3. Schnering, H.G. von; Brand, B.-H. (1973). "Struktur und Eigenschaften des blauen Chrom(II)-chlorid-tetrahydrats CrCl₂.4H₂O". Wiley Online Library 402 (2): 159–168. doi:10.1002/zaac.19734020206.
4. Sigma-Aldrich Co., Chromium(II) chloride. Retrieved on 2014-07-04.
5. "MSDS of Chromium(II) chloride". https://www.fishersci.ca. Fisher Scientific. Retrieved 2014-07-04. External link in |website= (help)
6. Burg, A. B. (1950). Audrieth, Ludwig F., ed. "Anhydrous Chromium(II) Chloride". Inorg. Synth. 3: 150–153. doi:10.1002/9780470132340.
7. Riley, edited by Georg Brauer ; translated by Scripta Technica, Inc. Translation editor Reed F. (1963). Handbook of preparative inorganic chemistry. Volume 1 (2nd ed.). New York, N.Y.: Academic Press. p. 1337. ISBN 978-0121266011.
8. Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 0-08-037941-9.
9. (a) Kazuhiko Takai, K.; Loh, T.-P. "Chromium(II) Chloride" in Encyclopedia of Reagents for Organic Synthesis John Wiley & Sons: New York; 2005. doi:10.1002/047084289X.rc166. (b) Alois Fürstner, "Carbon−Carbon Bond Formations Involving Organochromium(III) Reagents" Chemical Reviews, 1999, 99 (4), 991–1046 doi:10.1021/cr9703360
10. (a) MacMillan, D. W. C.; Overman, Larry E. "Enantioselective Total Synthesis of (−)-7-Deacetoxyalcyonin Acetate. First Synthesis of a Eunicellin Diterpene" J. Am. Chem. Soc. 1995, 117 (41), 10391–10392. doi:10.1021/ja00146a028. (b) Lotesta, S. D.; Liu, J.; Yates, E. V.; Krieger, I.; Sacchettini, J. C.; Freundlich, J. S.; Sorensen, E. J. "Expanding the pleuromutilin class of antibiotics by de novo chemical synthesis" Chem. Sci. 2011, 2, 1258-1261. doi:10.1039/C1SC00116G.
11. 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. doi:10.1021/ja00283a046.