Chromium(II) chloride

Summary

Chromium(II) chloride describes inorganic compounds with the formula CrCl2(H2O)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 Cr(H2O)4Cl2. Chromium(II) chloride has no commercial uses but is used on a laboratory-scale for the synthesis of other chromium complexes.

Chromium(II) chloride
3D model of chromium(II) chloride, green atom is chloride
Sample of chromium(II) chloride
Names
IUPAC name
Chromium(II) chloride
Other names
Chromous chloride
Identifiers
  • 10049-05-5 (anhydrous) checkY
  • 13931-94-7 (tetrahydrate) checkY
3D model (JSmol)
  • Interactive image
ChemSpider
  • 23252 checkY
ECHA InfoCard 100.030.136 Edit this at Wikidata
EC Number
  • 233-163-3
  • 24871
RTECS number
  • GB5250000
UNII
  • CET32HKA21 (anhydrous) checkY
  • Q781GP5E2W (tetrahydrate) checkY
UN number 3077
  • DTXSID20905548 Edit this at Wikidata
  • InChI=1S/2ClH.Cr/h2*1H;/q;;+2/p-2 checkY
    Key: XBWRJSSJWDOUSJ-UHFFFAOYSA-L checkY
  • InChI=1/2ClH.Cr/h2*1H;/q;;+2/p-2
    Key: XBWRJSSJWDOUSJ-NUQVWONBAQ
  • Cl[Cr]Cl
Properties
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
+7230·10−6 cm3/mol
Structure
Orthorhombic (deformed rutile, anhydrous), oP6[2]
Monoclinic (tetrahydrate)[3]
Pnnm, No. 58 (anhydrous)[2]
P21/c, No. 14 (tetrahydrate)[3]
2/m 2/m 2/m (anhydrous)[2]
2/m (tetrahydrate)[3]
a = 6.64 Å, b = 5.98 Å, c = 3.48 Å (anhydrous)[2]
α = 90°, β = 90°, γ = 90°
Octahedral (Cr2+, anhydrous)[2]
Thermochemistry
71.2 J/mol·K[1]
115.3 J/mol·K[1]
−395.4 kJ/mol[1]
−356 kJ/mol[1]
Hazards
GHS labelling:
GHS07: Exclamation mark[4]
Warning
H302, H315, H319, H335[4]
P261, P305+P351+P338[4]
NFPA 704 (fire diamond)
Lethal dose or concentration (LD, LC):
1870 mg/kg (rats, oral)[5]
Safety data sheet (SDS) Oxford MSDS
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

SynthesisEdit

CrCl2 is produced by reducing chromium(III) chloride either with hydrogen at 500 °C:

2 CrCl3 + H2 → 2 CrCl2 + 2 HCl

or by electrolysis.

On the laboratory scale, LiAlH4, zinc, 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 with hydrogen chloride:[6]

Cr2(OAc)4 + 4 HCl → 2 CrCl2 + 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.[7]

Cr + n H2O + 2 HCl → CrCl2(H2O)n + H2

Structure and propertiesEdit

Anhydrous CrCl2 is white[6] 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]

 
Ball-and-stick model of chromium(II) chloride tetrahydrate.

The hydrated derivative, CrCl2(H2O)4, forms monoclinic 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.[3]

ReactionsEdit

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 chemistryEdit

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]

ReferencesEdit

  1. ^ a b c d e f g h 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. ^ a b c d e 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. ^ a b c d Schnering, H.G. von; Brand, B.-H. (1973). "Struktur und Eigenschaften des blauen Chrom(II)-chlorid-tetrahydrats CrCl2.4H2O". Zeitschrift für anorganische und allgemeine Chemie. 402 (2): 159–168. doi:10.1002/zaac.19734020206.
  4. ^ a b c Sigma-Aldrich Co., Chromium(II) chloride. Retrieved on 2014-07-04.
  5. ^ a b "MSDS of Chromium(II) chloride". fishersci.ca. Fisher Scientific. Retrieved 2014-07-04.
  6. ^ a b 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. {{cite book}}: |first= has generic name (help)
  7. ^ Holah, David G.; Fackler, John P. (1967). "Chromium(II) Salts and Complexes". Inorganic Syntheses. 10: 26–35. doi:10.1002/9780470132418.ch4.
  8. ^ Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 978-0-08-037941-8.
  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.