Chromium(II) chloride

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
CAS Number	10049-05-5 (anhydrous) Y 13931-94-7 (tetrahydrate) Y
3D model (JSmol)	anhydrous: Interactive image tetrahydrate: 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/h2*1H;/q;;+2/p-2 Y Key: XBWRJSSJWDOUSJ-UHFFFAOYSA-L Y InChI=1/2ClH.Cr/h2*1H;/q;;+2/p-2 Key: XBWRJSSJWDOUSJ-NUQVWONBAQ
SMILES anhydrous: [Cr+2].[Cl-].[Cl-] tetrahydrate: Cl[Cr-4](Cl)([OH2+])([OH2+])([OH2+])[OH2+]
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 (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 (S⦵298)	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
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

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]