Chromium(III) chloride
1510928
197674042
2008-03-12T08:02:25Z
Mehdi ish
6649004
{{Chembox new
| Name = Chromium(III) chloride
| ImageFile = Chromium(III)_chloride_green_hexahydrate.jpg
<!-- | ImageSize = 200px -->
| ImageName = Green form of chromium(III) chloride hexahydrate
| IUPACName = Chromium(III) chloride<br />Chromium trichloride
| OtherNames = Chromic chloride
| Section1 = {{Chembox Identifiers
| CASNo = 10025-73-7
| RTECS = GB5425000
}}
| Section2 = {{Chembox Properties
| Formula = CrCl<sub>3</sub>
| MolarMass = 158.35 g/mol (anhydrous)<br />
| MolarMass_notes = ''Cr=32.84%, Cl=67.16%''
| Appearance = ''see text''
| Density = 2.76 g/cm<sup>3</sup>, solid
| Solubility =
| MeltingPt = 1150 °C (anhydrous)
| BoilingPt = 1300 °C ''decomp.''
}}
| Section3 = {{Chembox Structure
| Coordination = [[Octahedron|Octahedral]]
| CrystalStruct = [[Yttrium trichloride#Structure|YCl<sub>3</sub> structure]]
}}
| Section7 = {{Chembox Hazards
| ExternalMSDS =
| EUClass = not listed
| FlashPt = non flammable
}}
| Section8 = {{Chembox Related
| OtherAnions = [[Chromium(III) fluoride]]<br />[[Chromium(III) bromide]]<br />[[Chromium(III) iodide]]
| OtherCations = [[Molybdenum(III) chloride]]<br />[[Tungsten(III) chloride]]
| OtherCpds = [[Chromium(II) chloride]]<br />[[Chromium(IV) chloride]]
}}
}}
'''Chromium(III) chloride''' (also called '''chromic chloride''') is a violet coloured solid with the formula CrCl<sub>3</sub>.
Although it is [[ion]]ic, the solid state structure is kinetically inert so that anhydrous CrCl<sub>3</sub> is surprisingly reluctant to dissolve in water. However, in the presence of a trace of a [[Redox|reducing agent]] capable of reducing Cr<sup>3+</sup> to Cr<sup>2+</sup>, the CrCl<sub>3</sub> dissolves rapidly to form soluble complexes containing hydrated Cr<sup>3+</sup> ions. The common commercial form of the hydrate is the dark green complex shown in the picture, [CrCl<sub>2</sub>(H<sub>2</sub>O)<sub>4</sub>]Cl.2H<sub>2</sub>O, but two other forms are known, ''viz.'', pale green [CrCl(H<sub>2</sub>O)<sub>5</sub>]Cl<sub>2</sub>.H<sub>2</sub>O and violet [Cr(H<sub>2</sub>O)<sub>6</sub>]Cl<sub>3</sub>.
This inertness means that CrCl<sub>3</sub> is generally sluggish to react without the presence of a reducing agent. When it does react it undergoes [[ligand]] substitution reactions to form other complexes of chromium(III). It reacts as a [[Lewis acid]], forming stable chloro complexes such as [CrCl<sub>6</sub>]<sup>3-</sup>.
==Chemical properties==
'''Chromium(III) chloride''' is a [[Lewis acid]], classified as "hard" according to the [[HSAB concept|Hard-Soft Acid-Base theory]]. However it is also a chloro [[complex (chemistry)|complex]] which is quite inert to substitution, so in fact it is ordinarily quite unreactive. The low reactivity of the d<sup>3</sup> Cr<sup>3+</sup> ion can be explained using [[crystal field theory]]. One way of opening CrCl<sub>3</sub> up to substitution in solution is to reduce even a trace amount to CrCl<sub>2</sub>, for example using [[zinc]] in [[hydrochloric acid]]. This chromium(II) compound undergoes substitution easily, and it can exchange electrons with CrCl<sub>3</sub> via a [[chloride]] bridge, allowing all of the CrCl<sub>3</sub> to react quickly.
The most common form of CrCl<sub>3</sub> sold commercially is a dark green hexahydrate with the structure [CrCl<sub>2</sub>(H<sub>2</sub>O)<sub>4</sub>]Cl.2H<sub>2</sub>O, and like the anhydrous form it is also very inert towards substitution.
If substitution reactions are performed in the presence of a trace of Cr<sup>2+</sup>, then CrCl<sub>3</sub> can undergo substitution with ligands such as water (giving violet [Cr(H<sub>2</sub>O)<sub>6</sub>]<sup>3+</sup>) or [[pyridine]]:
CrCl<sub>3</sub> + 3 [[pyridine|C<sub>5</sub>H<sub>5</sub>N]] → [CrCl<sub>3</sub>(C<sub>5</sub>H<sub>5</sub>N)<sub>3</sub>]
Such complexes are usually octahedral.
With molten [[alkali metal]] [[chloride]]s such as [[potassium chloride]], CrCl<sub>3</sub> gives octahedral complexes of the type K<sub>3</sub>CrCl<sub>6</sub>, as well as K<sub>3</sub>Cr<sub>2</sub>Cl<sub>9</sub> which is also octahedral but where the two chromiums are linked via three chloride bridges.
==Preparation==
Anhydrous '''chromium(III) chloride''' may be prepared from [[chromium]] metal and [[chlorine]] directly, or indirectly using [[chromium(III) oxide]] with [[carbon]] and [[chlorine]] at 800 °C<sup>[4]</sup>:
[[chromium(III) oxide|Cr<sub>2</sub>O<sub>3</sub>]]([[solid|s]]) + 3 [[carbon|C]]([[solid|s]]) + 3 [[chlorine|Cl<sub>2</sub>]]([[gas|g]]) → 2 CrCl<sub>3</sub>([[solid|s]]) + 3 [[carbon monoxide|CO]]([[gas|g]])
It may also be prepared from the hexahydrate, by heating with [[thionyl chloride]] which reacts with the water of hydration.
The hydrated [[chloride]] may be made by dissolving the metal in [[hydrochloric acid]].
==Uses==
'''Chromium(III) chloride''' is used as the source of chromium for many [[inorganic compound]]s of chromium, for example [[Bis(benzene)chromium|dibenzenechromium(0)]], an analogue of [[ferrocene]]:
[[Image:CrCl3 dibenzenechromium.gif|400px]]
A significant use of CrCl<sub>3</sub> in [[organic synthesis]] is for the ''in situ'' preparation of [[chromium(II) chloride]], a popular reagent for (A) reduction of [[alkyl halide]]s and for (B) the synthesis of (''E'')-alkenyl halides. The reaction is usually performed using two [[mole (unit)|mole]]s of CrCl<sub>3</sub> per mole of [[lithium aluminium hydride]], although if aqueous acidic conditions are appropriate [[zinc]] and [[hydrochloric acid]] may be sufficient.
[[Image:CrCl3 CrCl2.gif|500px]]
Chromium(III) chloride has also been used as a Lewis acid in organic reactions, for example to catalyse the nitroso [[Diels-Alder reaction]].<sup>8</sup>
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==Precautions==
Although [[Valency (chemistry)|trivalent]] chromium is far less poisonous than hexavalent, chromium salts are generally considered highly toxic. Avoid ingestion and inhalation of dust. Wear gloves and goggles.
==References==
{{Citationstyle|date=September 2007}}
<references/>
# N. N. Greenwood, A. Earnshaw, ''Chemistry of the Elements'', 2nd ed., Butterworth-Heinemann, Oxford, UK, 1997.
# ''Handbook of Chemistry and Physics'', 71st edition, CRC Press, Ann Arbor, Michigan, 1990.
# ''The Merck Index'', 7th edition, Merck & Co, Rahway, New Jersey, USA, 1960.
# D. Nicholls, ''Complexes and First-Row Transition Elements'', Macmillan Press, London, 1973.
# A. F. Wells, '''Structural Inorganic Chemistry'', 5th ed., Oxford University Press, Oxford, UK, 1984.
# J. March, ''Advanced Organic Chemistry'', 4th ed., p. 723, Wiley, New York, 1992.
# K. Takai, in ''Handbook of Reagents for Organic Synthesis, Volume 1: Reagents, Auxiliaries and Catalysts for C-C Bond Formation'', (R. M. Coates, S. E. Denmark, eds.), pp. 206-211, Wiley, New York, 1999.
# Calvet, G.; Dussaussois, M.; Blanchard, N.; Kouklovsky, C. ''[[Organic Letters]]'' '''2004''', ''6'', 2449-2451.
==External links==
*[http://www.ilo.org/public/english/protection/safework/cis/products/icsc/dtasht/_icsc13/icsc1316.htm International Chemical Safety Card 1316] (anhydr. CrCl<sub>3</sub>)
*[http://www.ilo.org/public/english/protection/safework/cis/products/icsc/dtasht/_icsc15/icsc1532.htm International Chemical Safety Card 1532] (CrCl<sub>3</sub>·6H<sub>2</sub>O)
* [http://www.npi.gov.au/database/substance-info/profiles/24.html National Pollutant Inventory - Chromium (III) compounds fact sheet]
*[http://www.cdc.gov/niosh/npg/npgd0140.html NIOSH Pocket Guide to Chemical Hazards]
*[http://www-cie.iarc.fr/htdocs/monographs/vol49/chromium.html IARC Monograph "Chromium and Chromium compounds"]
[[Category:Chromium compounds]]
[[Category:Chlorides]]
[[Category:Metal halides]]
[[Category:Coordination compounds]]
[[ar:كلوريد كروم ثلاثي]]
[[de:Chrom(III)-chlorid]]
[[pl:Chlorek chromu(III)]]
[[ru:Хлорид хрома(III)]]