Chromium(III) chloride
Chromium(III) chloride (also called chromic chloride) is an inorganic chemical compound with the chemical formula Template:Chem2. This crystalline salt forms several hydrates with the formula Template:Chem2, among which are hydrates where n can be 5 (chromium(III) chloride pentahydrate Template:Chem2) or 6 (chromium(III) chloride hexahydrate Template:Chem2). The anhydrous compound with the formula Template:Chem2 are violet crystals, while the most common form of the chromium(III) chloride are the dark green crystals of hexahydrate, Template:Chem2. Chromium chlorides find use as catalysts and as precursors to dyes for wool.
StructureEdit
Anhydrous chromium(III) chloride adopts the [[Yttrium(III) chloride|Template:Chem2]] structure,<ref name=":0">Template:Cite journal</ref> with Template:Chem2 occupying one third of the octahedral interstices in alternating layers of a pseudo-cubic close packed lattice of Template:Chem2 ions. The absence of cations in alternate layers leads to weak bonding between adjacent layers. For this reason, crystals of Template:Chem2 cleave easily along the planes between layers, which results in the flaky (micaceous) appearance of samples of chromium(III) chloride.<ref>Template:Greenwood&Earnshaw2nd</ref><ref>A. F. Wells, Structural Inorganic Chemistry, 5th ed., Oxford University Press, Oxford, UK, 1984.</ref> The anhydrous Template:Chem2 is exfoliable down to the monolayer limit.<ref name=":0" /> If pressurized to 9.9 GPa it goes under a phase transition.<ref name="pres">Template:Cite journal</ref>
- Chromium(III)-chloride-sheet-from-monoclinic-xtal-3D-balls-SF-overlay.png
Space-filling model of cubic close packing of chloride ions in the crystal structure of Template:Chem2
- Chromium(III)-chloride-sheet-from-monoclinic-xtal-3D-balls.png
Ball-and-stick model of part of a layer
- Chromium(III)-chloride-layers-stacking-from-monoclinic-xtal-3D-balls.png
Stacking of layers
Chromium(III) chloride hydratesEdit
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 Template:Chem2. The common hexahydrate can be more precisely described as Template:Chem2. It consists of the cation trans-Template:Chem2 and additional molecules of water and a chloride anion in the lattice.<ref>Ian G. Dance, Hans C. Freeman "The Crystal Structure of Dichlorotetraaquochromium(III) Chloride Dihydrate: Primary and Secondary Metal Ion Hydration" Inorganic Chemistry 1965, volume 4, 1555–1561. {{#invoke:doi|main}}</ref> Two other hydrates are known, pale green Template:Chem2 and violet Template:Chem2. Similar hydration isomerism is seen with other chromium(III) compounds.Template:Citation needed
PreparationEdit
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<ref>D. Nicholls, Complexes and First-Row Transition Elements, Macmillan Press, London, 1973.</ref><ref name="German">Template:Cite book</ref>
The hydrated chlorides are prepared by treatment of chromate with hydrochloric acid and aqueous methanol.
ReactionsEdit
Slow reaction rates are common with chromium(III) complexes. The low reactivity of the d3 Template:Chem2 ion can be explained using crystal field theory. One way of opening Template:Chem2 up to substitution in solution is to reduce even a trace amount to Template:Chem2, for example using zinc in hydrochloric acid. This chromium(II) compound undergoes substitution easily, and it can exchange electrons with Template:Chem2 via a chloride bridge, allowing all of the Template:Chem2 to react quickly. With the presence of some chromium(II), solid Template:Chem2 dissolves rapidly in water. Similarly, ligand substitution reactions of solutions of Template:Chem2 are accelerated by chromium(II) catalysts.
With molten alkali metal chlorides such as potassium chloride, Template:Chem2 gives salts of the type Template:Chem2 and Template:Chem2, which is also octahedral but where the two chromiums are linked via three chloride bridges.
The hexahydrate can also be dehydrated with thionyl chloride:<ref>Template:Cite book</ref>
Complexes with organic ligandsEdit
Template:Chem2 is a Lewis acid, classified as "hard" according to the Hard-Soft Acid-Base theory. It forms a variety of adducts of the type Template:Chem2, where L is a Lewis base. For example, it reacts with pyridine (Template:Chem2) to form the pyridine complex:
Treatment with trimethylsilylchloride in THF gives the anhydrous THF complex:<ref>Template:Cite book</ref>
Precursor to organochromium complexesEdit
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 Template:Chem2 catalyse the trimerization of ethylene to 1-hexene.<ref>John T. Dixon, Mike J. Green, Fiona M. Hess, David H. Morgan "Advances in selective ethylene trimerisation – a critical overview" Journal of Organometallic Chemistry 2004, Volume 689, pp 3641-3668. {{#invoke:doi|main}}</ref><ref>Feng Zheng, Akella Sivaramakrishna, John R. Moss "Thermal studies on metallacycloalkanes" Coordination Chemistry Reviews 2007, Volume 251, 2056-2071. {{#invoke:doi|main}}</ref>
Use in organic synthesisEdit
One niche use of Template:Chem2 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 Template:Chem2 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.<ref>Template:Cite journal</ref>
DyestuffsEdit
A number of chromium-containing dyes are used commercially for wool. Typical dyes are triarylmethanes consisting of ortho-hydroxylbenzoic acid derivatives.<ref>Thomas Gessner and Udo Mayer "Triarylmethane and Diarylmethane Dyes" in Ullmann's Encyclopedia of Industrial Chemistry 2002, Wiley-VCH, Weinheim. {{#invoke:doi|main}}</ref>
PrecautionsEdit
Although trivalent chromium is far less poisonous than hexavalent, chromium salts are generally considered toxic.
ReferencesEdit
Further readingEdit
- 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.
- 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.