Chromium(II) chloride

Chromium(II) chloride
Names
	IUPAC name Chromium(II) chloride
	Other names Chromous chloride
Identifiers
CAS Number	10049-05-5 (anhydrous) ; 13931-94-7 (tetrahydrate) ;
3D model (JSmol)	Interactive image;
ChemSpider	23252 ;
ECHA InfoCard	100.030.136
EC Number	233-163-3;
PubChem CID	24871;
RTECS number	GB5250000;
UNII	CET32HKA21 (anhydrous) ; Q781GP5E2W (tetrahydrate) ;
UN number	3077
CompTox Dashboard (EPA)	DTXSID20905548 ;
	InChI InChI=1S/2ClH.Cr/h21H;/q;;+2/p-2 Key: XBWRJSSJWDOUSJ-UHFFFAOYSA-L ; 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), 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]
Solubility in water	Soluble[1]
Solubility	Insoluble in alcohol, ether
Acidity (pKa)	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 (So298)	115.3 J/mol·K[1]
Std enthalpy of; formation (ΔfH⦵298)	−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]
NFPA 704 (fire diamond)	[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 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.[6]

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]

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

Lide, David R., ed. (2009). CRC Handbook of Chemistry and Physics (90th ed.). Boca Raton, Florida: CRC Press. ISBN 978-1-4200-9084-0.
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.
Schnering, H.G. von; Brand, B.-H. (1973). "Struktur und Eigenschaften des blauen Chrom(II)-chlorid-tetrahydrats CrCl₂.4H₂O". Zeitschrift fuer Anorganische und Allgemeine Chemie. 402 (2): 159–168. doi:10.1002/zaac.19734020206.
Sigma-Aldrich Co., Chromium(II) chloride. Retrieved on 2014-07-04.
"MSDS of Chromium(II) chloride". fishersci.ca. Fisher Scientific. Retrieved 2014-07-04.
Holah, D. G.; Fackler, Jr. John P. (1967). "Anhydrous Chromium(II) Chloride". Inorg. Synth. 10: 26-35. doi:10.1002/9780470132418.ch4.
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.CS1 maint: extra text: authors list (link)
Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. 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.

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[crc-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.

[iucr-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.

[zeit-3] Schnering, H.G. von; Brand, B.-H. (1973). "Struktur und Eigenschaften des blauen Chrom(II)-chlorid-tetrahydrats CrCl₂.4H₂O". Zeitschrift fuer Anorganische und Allgemeine Chemie. 402 (2): 159–168. doi:10.1002/zaac.19734020206.

[sigma-4] Sigma-Aldrich Co., Chromium(II) chloride. Retrieved on 2014-07-04.

[fca-5] "MSDS of Chromium(II) chloride". fishersci.ca. Fisher Scientific. Retrieved 2014-07-04.

[6] Holah, D. G.; Fackler, Jr. John P. (1967). "Anhydrous Chromium(II) Chloride". Inorg. Synth. 10: 26-35. doi:10.1002/9780470132418.ch4.

[brauer-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.CS1 maint: extra text: authors list (link)

[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.