Chromium(III) chloride

Chromium(III) chloride (also called chromic chloride) is an inorganic chemical compound with the chemical formula CrCl₃. This crystalline salt forms several hydrates with the formula CrCl₃·nH₂O, among which are hydrates where n can be 5 (chromium(III) chloride pentahydrate CrCl₃·5H₂O) or 6 (chromium(III) chloride hexahydrate CrCl₃·6H₂O). The anhydrous compound with the formula CrCl₃ are violet crystals, while the most common form of the chromium(III) chloride are the dark green crystals of hexahydrate, CrCl₃·6H₂O. Chromium chlorides find use as catalysts and as precursors to dyes for wool.

Chromium(III) chloride

Anhydrous
Green form of chromium(III) chloride hexahydrate Hexahydrate
Names
IUPAC name Chromium(III) chloride Chromium trichloride
Other names Chromic chloride
Identifiers
CAS Number	10025-73-7 Y 10060-12-5 (hexahydrate) Y
3D model (JSmol)	anhydrous: Interactive image common hexahydrate: Interactive image
ChEBI	CHEBI:53351 Y
ChEMBL	ChEMBL1200528 N
ChemSpider	4954736 Y
DrugBank	DB09129
ECHA InfoCard	100.030.023
Gmelin Reference	1890 130477 532690
PubChem CID	6452300
RTECS number	GB5425000
UNII	Z310X5O5RP Y KB1PCR9DMW (hexahydrate) Y
CompTox Dashboard (EPA)	DTXSID20858722
InChI InChI=1S/3ClH.Cr/h3*1H;/q;;;+3/p-3 Y Key: QSWDMMVNRMROPK-UHFFFAOYSA-K Y InChI=1/3ClH.Cr/h3*1H;/q;;;+2/p-3 Key: HUQISNLCWMVGCG-DFZHHIFOAJ InChI=1/3ClH.Cr/h3*1H;/q;;;+3/p-3 Key: QSWDMMVNRMROPK-DFZHHIFOAF
SMILES anhydrous: Cl[Cr](Cl)Cl common hexahydrate: Cl[Cr-3](Cl)([OH2+])([OH2+])([OH2+])[OH2+].[Cl-].O.O
Properties
Chemical formula	CrCl3
Molar mass	158.36 g/mol (anhydrous) 266.45 g/mol (hexahydrate)[1]
Appearance	Purple (anhydrous), dark green (hexahydrate)
Density	2.87 g/cm3 (anhydrous) 1.760 g/cm3 (hexahydrate)
Melting point	1,152 °C (2,106 °F; 1,425 K) (anhydrous) 81 °C (hexahydrate)[2]
Boiling point	1,300 °C (2,370 °F; 1,570 K) decomposes
Solubility in water	slightly soluble (anhydrous) 585 g/L (hexahydrate)
Solubility	insoluble in ethanol insoluble in ether, acetone
Acidity (pKa)	2.4 (0.2M solution)
Magnetic susceptibility (χ)	+6890.0·10−6 cm3/mol
Structure
Crystal structure	YCl3 structure
Coordination geometry	Octahedral
Hazards
GHS labelling:
Pictograms
Signal word	Danger
Hazard statements	H302, H314, H411
Precautionary statements	P260, P264, P270, P273, P280, P301+P312, P301+P330+P331, P303+P361+P353, P304+P340, P305+P351+P338, P310, P321, P330, P363, P391, P405, P501
NFPA 704 (fire diamond)	[3] 3 0 0
Flash point	Non-flammable
Lethal dose or concentration (LD, LC):
LD50 (median dose)	1870 mg/kg (oral, rat)[4]
NIOSH (US health exposure limits):
PEL (Permissible)	TWA 1 mg/m3[5]
REL (Recommended)	TWA 0.5 mg/m3[5]
IDLH (Immediate danger)	250 mg/m3[5]
Safety data sheet (SDS)	ICSC 1316 (anhydrous) ICSC 1532 (hexahydrate)
Related compounds
Other anions	Chromium(III) fluoride Chromium(III) bromide Chromium(III) iodide
Other cations	Molybdenum(III) chloride Tungsten(III) chloride
Related compounds	Chromium(II) chloride Chromium(IV) chloride
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). N verify (what is YN ?) Infobox references

Structure

Anhydrous chromium(III) chloride adopts the YCl₃ structure,^[6] with Cr³⁺ occupying one third of the octahedral interstices in alternating layers of a pseudo-cubic close packed lattice of Cl⁻ ions. The absence of cations in alternate layers leads to weak bonding between adjacent layers. For this reason, crystals of CrCl₃ cleave easily along the planes between layers, which results in the flaky (micaceous) appearance of samples of chromium(III) chloride.^[7][8] The anhydrous CrCl₃ is exfoliable down to the monolayer limit.^[6] If pressurized to 9.9 GPa it goes under a phase transition.^[9]

• 
  Space-filling model of cubic close packing of chloride ions in the crystal structure of CrCl₃
• 
  Ball-and-stick model of part of a layer
• 
  Stacking of layers

Chromium(III) chloride hydrates

The hydrated chromium(III) chlorides display the somewhat unusual property of existing in a number of distinct chemical forms (isomers), which differ in terms of the number of chloride anions that are coordinated to Cr(III) and the water of crystallization. The different forms exist both as solids and in aqueous solutions. Several members are known of the series of [CrCl_3−q(H₂O)_n]^q+. The common hexahydrate can be more precisely described as [CrCl₂(H₂O)₄]Cl·2H₂O. It consists of the cation trans-[CrCl₂(H₂O)₄]⁺ and additional molecules of water and a chloride anion in the lattice.^[10] Two other hydrates are known, pale green [CrCl(H₂O)₅]Cl₂·H₂O and violet [Cr(H₂O)₆]Cl₃. Similar hydration isomerism is seen with other chromium(III) compounds.^{[citation needed]}

Preparation

Anhydrous chromium(III) chloride may be prepared by chlorination of chromium metal directly, or indirectly by carbothermic chlorination of chromium(III) oxide at 650–800 °C^[11][12]

Cr₂O₃ + 3 C + 3 Cl₂ → 2 CrCl₃ + 3 CO

The hydrated chlorides are prepared by treatment of chromate with hydrochloric acid and aqueous methanol.

Reactions

Slow reaction rates are common with chromium(III) complexes. The low reactivity of the d³ Cr³⁺ ion can be explained using crystal field theory. One way of opening CrCl₃ up to substitution in solution is to reduce even a trace amount to CrCl₂, for example using zinc in hydrochloric acid. This chromium(II) compound undergoes substitution easily, and it can exchange electrons with CrCl₃ via a chloride bridge, allowing all of the CrCl₃ to react quickly. With the presence of some chromium(II), solid CrCl₃ dissolves rapidly in water. Similarly, ligand substitution reactions of solutions of [CrCl₂(H₂O)₄]⁺ are accelerated by chromium(II) catalysts.

With molten alkali metal chlorides such as potassium chloride, CrCl₃ gives salts of the type M₃[CrCl₆] and K₃[Cr₂Cl₉], which is also octahedral but where the two chromiums are linked via three chloride bridges.

The hexahydrate can also be dehydrated with thionyl chloride:^[13]

CrCl₃·6H₂O + 6 SOCl₂ → CrCl₃ + 6 SO₂ + 12 HCl

Complexes with organic ligands

CrCl₃ is a Lewis acid, classified as "hard" according to the Hard-Soft Acid-Base theory. It forms a variety of adducts of the type [CrCl₃L₃]^q+, where L is a Lewis base. For example, it reacts with pyridine (C₅H₅N) to form the pyridine complex:

CrCl₃ + 3 C₅H₅N → CrCl₃(C₅H₅N)₃

Treatment with trimethylsilylchloride in THF gives the anhydrous THF complex:^[14]

CrCl₃·6H₂O + 12 (CH₃)₃SiCl → CrCl₃(THF)₃ + 6 ((CH₃)₃Si)₂O + 12 HCl

Precursor to organochromium complexes

Chromium(III) chloride is used as the precursor to many organochromium compounds, for example bis(benzene)chromium, an analogue of ferrocene:

Phosphine complexes derived from CrCl₃ catalyse the trimerization of ethylene to 1-hexene.^[15][16]

Use in organic synthesis

One niche use of CrCl₃ in organic synthesis is for the in situ preparation of chromium(II) chloride, a reagent for the reduction of alkyl halides and for the synthesis of (E)-alkenyl halides. The reaction is usually performed using two moles of CrCl₃ per mole of lithium aluminium hydride, although if aqueous acidic conditions are appropriate zinc and hydrochloric acid may be sufficient.

Chromium(III) chloride has also been used as a Lewis acid in organic reactions, for example to catalyse the nitroso Diels-Alder reaction.^[17]

Dyestuffs

A number of chromium-containing dyes are used commercially for wool. Typical dyes are triarylmethanes consisting of ortho-hydroxylbenzoic acid derivatives.^[18]

Precautions

Although trivalent chromium is far less poisonous than hexavalent, chromium salts are generally considered toxic.