Chromium(II) chloride

Chromium(II) chloride
Names
IUPAC name
Chromium(II) chloride
Other names
Chromous chloride
Identifiers
10049-05-5 
ChemSpider 23252 Yes
Jmol-3D images Image
PubChem 24871
RTECS number GB5250000
UNII CET32HKA21 Yes
Properties
Molecular formula
 Cl2Cr
Molar mass 122.90 g·mol−1
Appearance White to grey/green powder (anhydrous), very hygroscopic
Odor Odorless
Density 2.88 g/cm3 (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]
Soluble[1]
Solubility Insoluble in alcohol, ether
Acidity (pKa) 2
7.23·10−3 cm3/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]
Lattice constant α = 90°, β = 90°, γ = 90°
Octahedral (Cr2+, anhydrous)[2]
Thermochemistry
Specific
heat capacity (C)
 71.2 J/mol·K[1]
115.3 J/mol·K[1]
Std enthalpy of
formation (ΔfHo298)
 −395.4 kJ/mol[1]
−356 kJ/mol[1]
Hazards
MSDS Oxford MSDS
GHS pictograms [4]
GHS signal word Warning
H302, H315, H319, H335[4]
P261, P305+351+338[4]
EU classification Xn
R-phrases R22, R36/37/38
S-phrases S26, S36
NFPA 704
Flammability code 0: Will not burn. E.g., water Health code 2: Intense or continued but not chronic exposure could cause temporary incapacitation or possible residual injury. E.g., chloroform Reactivity code 1: Normally stable, but can become unstable at elevated temperatures and pressures. E.g., calcium Special hazards (white): no codeNFPA 704 four-colored diamond
0
2
1
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 noted otherwise, data is 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. ↑ 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 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. ↑ 2.0 2.1 2.2 2.3 2.4 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. ↑ 3.0 3.1 3.2 3.3 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. ↑ 4.0 4.1 4.2 Sigma-Aldrich Co., Chromium(II) chloride. Retrieved on 2014-07-04.
  5. ↑
  6. ↑ Burg, A. B. (1950). Audrieth, Ludwig F., ed. "Anhydrous Chromium(II) Chloride". Inorg. Synth. 3: 150–153. doi:10.1002/9780470132340.
  7. ↑ 7.0 7.1 Riley, edited by Georg Brauer ; translated by Scripta Technica, Inc. Translation editor Reed F. (1963). Handbook of preparative inorganic chemistry. Volume 1 (2nd ed. 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 0080379419.
  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.