Chromium(III) chloride (also called chromic chloride) is an inorganic chemical compound with the chemical formulaCrCl3. It 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.
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 CrCl3cleave 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
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 [CrCl3−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]
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 d3Cr3+ 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 metalchlorides 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.
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.
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^Meiling Hong (2022). "Pressure-Induced Structural Phase Transition and Metallization of CrCl3 under Different Hydrostatic Environments up to 50.0 GPa". Inorg. Chem. 61 (12): 4852–4864. doi:10.1021/acs.inorgchem.1c03486. PMID35289613. S2CID247452267.
^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. doi:10.1021/ic50033a006
^Brauer, Georg (1965) [1962]. Handbuch Der Präparativen Anorganischen Chemie [Handbook of Preparative Inorganic Chemistry] (in German). Vol. 2. Stuttgart; New York, New York: Ferdinand Enke Verlag; Academic Press, Inc. p. 1340. ISBN978-0-32316129-9. Retrieved 2014-01-10.
^Boudjouk, Philip; So, Jeung-Ho (1992). "Solvated and Unsolvated Anhydrous Metal Chlorides from Metal Chloride Hydrates". Inorganic Syntheses. Inorganic Syntheses. Vol. 29. pp. 108–111. doi:10.1002/9780470132609.ch26. ISBN9780470132609.
^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. doi:10.1016/j.jorganchem.2004.06.008
^Feng Zheng, Akella Sivaramakrishna, John R. Moss "Thermal studies on metallacycloalkanes" Coordination Chemistry Reviews 2007, Volume 251, 2056-2071. doi:10.1016/j.ccr.2007.04.008
^Calvet, G.; Dussaussois, M.; Blanchard, N.; Kouklovsky, C. (2004). "Lewis Acid-Promoted Hetero Diels-Alder Cycloaddition of α-Acetoxynitroso Dienophiles". Organic Letters. 6 (14): 2449–2451. doi:10.1021/ol0491336. PMID15228301.
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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.
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