Chromium(II) chloride

Chromium(II) chloride
Names


IUPAC name Chromium(II) chloride
Other names Chromous chloride
Identifiers
CAS Number	10049-05-5 (anhydrous)^Y 13931-94-7 (tetrahydrate)^Y
3D model (JSmol)	Interactive image
ChemSpider	23252^Y
ECHA InfoCard	100.030.136
EC Number	233-163-3
PubChem CID	24871
RTECS number	GB5250000
UNII	CET32HKA21 (anhydrous)^Y Q781GP5E2W (tetrahydrate)^Y
UN number	3077
CompTox Dashboard (EPA)	DTXSID20905548
InChI InChI=1S/2ClH.Cr/h21H;/q;;+2/p-2^Y Key: XBWRJSSJWDOUSJ-UHFFFAOYSA-L^Y InChI=1/2ClH.Cr/h21H;/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) blue solid (tetrahydrate)
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]
Solubility in water	Soluble^[1]
Solubility	Insoluble in alcohol, ether
Acidity (pK_a)	2
Magnetic susceptibility (χ)	+7230·10⁻⁶ cm³/mol
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 (Cr²⁺, anhydrous)^[2]
Thermochemistry
Heat capacity (C)	71.2 J/mol·K^[1]
Std molar entropy (S^⦵₂₉₈)	115.3 J/mol·K^[1]
Std enthalpy of formation (Δ_fH^⦵₂₉₈)	−395.4 kJ/mol^[1]
Gibbs free energy (Δ_fG^⦵)	−356 kJ/mol^[1]
Hazards
GHS labelling:
Pictograms	^[4]
Signal word	Warning
Hazard statements	H302, H315, H319, H335^[4]
Precautionary statements	P261, P305+P351+P338^[4]
NFPA 704 (fire diamond)	^[5] 2 0 1
Lethal dose or concentration (LD, LC):
LD₅₀ (median dose)	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). Y verify (what is ^YN ?) Infobox references

Chemical compound

Chromium(II) chloride describes inorganic compounds with the formula Cr Cl₂(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.

Synthesis

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 properties

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]

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(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

^ ^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.
^ ^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.
^ ^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.
^ ^a ^b ^c Sigma-Aldrich Co., Chromium(II) chloride. Retrieved on 2014-07-04.
^ ^a ^b "MSDS of Chromium(II) chloride". fishersci.ca. Fisher Scientific. Retrieved 2014-07-04.
^ ^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)CS1 maint: multiple names: authors list (link)
^ Holah, David G.; Fackler, John P. (1967). "Chromium(II) Salts and Complexes". Inorganic Syntheses. Inorganic Syntheses. Vol. 10. pp. 26–35. doi:10.1002/9780470132418.ch4. ISBN 9780470132418.
^ Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. pp. 1021–1022. ISBN 978-0-08-037941-8.
^ (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
^ (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.
^ 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.