Chromium(III) Chloride
Properties
| State | Solid (crystalline) |
| Color | Violet (anhydrous); dark green (hexahydrate) |
| Solubility | Anhydrous nearly insoluble in water; hexahydrate freely soluble |
| Melting Point | 1152°C (anhydrous) |
| Boiling Point | 1300°C (sublimes) |
About Chromium(III) Chloride
Chromium(III) chloride is the textbook hydrate-isomerism compound. Anhydrous CrCl3 is a violet, layered solid with chromium in octahedral holes between close-packed chloride sheets, and it is famously slow to dissolve in water — Cr(III) is kinetically inert (d³, low-spin, with the half-filled t2g configuration giving large crystal-field stabilization) and the activation barrier for ligand exchange is enormous, on the order of 10⁻⁶ s⁻¹ at 25 °C. Add a pinch of zinc dust or Cr(II) and dissolution becomes instantaneous through electron-self-exchange catalysis. The hexahydrate CrCl3·6H2O exists as three distinct, isolable hydrate isomers: violet [Cr(H2O)6]Cl3, pale green [Cr(H2O)5Cl]Cl2·H2O, and dark green trans-[Cr(H2O)4Cl2]Cl·2H2O. They have the same empirical formula and molecular weight but different ions in the coordination sphere, different conductivities, and different AgNO3 titration end-points — Werner used exactly this kind of evidence in the 1890s to nail down coordination theory. Industrially, anhydrous CrCl3 is the precursor to most homogeneous chromium catalysts, including the Phillips and Union Carbide ethylene-trimerization systems that make 1-hexene for LLDPE comonomer feed. Mixed with LiAlH4 in ether it generates 'CrCl2-LAH', a selective reductant that takes vinyl halides through to alkenes with retention of geometry.
Where you'll encounter it
If you've ever tried to dissolve anhydrous CrCl3 in water for a synthesis and given up after an hour of stirring, you've met chromium's kinetic inertness in person. The standard fix is to add a few mg of Cr(II) (made by zinc-amalgam reduction in situ) — the Cr(II)/Cr(III) self-exchange is fast, and the chromium dissolves in seconds. In the textile industry you'll find CrCl3 in mordant dye baths for wool, where the Cr³⁺ binds covalently to both fiber and dye to give wash-fast colors that simple anionic dyes can't achieve.
Common Uses
- Precursor for ethylene oligomerization catalysts producing 1-hexene comonomer for LLDPE
- Lewis-acid catalyst for Diels-Alder reactions and Friedel-Crafts acylations
- Mordant in wool dyeing for binding anionic dyes covalently to keratin fibers
- Starting material for Cr(II) reagents like Cr(II)Cl2 and the Takai-Utimoto olefination
- Teaching demonstration for Werner-style hydrate isomerism in undergraduate inorganic labs
- Source of Cr³⁺ ions for chromium-electrodeposition baths in trivalent-chromium plating
- Precursor to organochromium reagents like NHC-CrCl3 complexes for cross-coupling research
- Component of CrCl3-LiAlH4 reductant systems for stereoselective alkyne reductions
Safety Information
Cr(III) is roughly 1000-fold less toxic than Cr(VI) — the hydrated Cr³⁺ ion is too large and charge-dense to cross cell membranes through the sulfate/phosphate transporters that Cr(VI) exploits. GHS H302 (harmful if swallowed), H315 (skin irritation), H317 (skin sensitization, can cause allergic contact dermatitis), H319 (eye irritation). OSHA does not list a specific PEL for Cr(III), but the metal-and-insoluble-compounds limit of 0.5 mg Cr/m³ applies. Skin sensitization is the realistic occupational hazard — once you're sensitized to chromium you'll cross-react to Cr(VI) at much lower exposure thresholds, so glove discipline matters. Standard nitrile gloves and a dust mask for powder handling are sufficient.
This safety summary is for educational reference only and may not be complete. It is not a substitute for Safety Data Sheets (SDS), medical advice, or professional chemical safety guidance. Always consult appropriate SDS and qualified professionals before handling chemicals.