catalysts. With molten alkali metal chlorides such as potassium chloride, CrCl3 gives salts of the type M3CrCl6 and K3Cr2Cl9, which is also octahedral but where the two chromiums are linked via three chloride bridges. The hexahydrate can also be dehydrated with thionyl chloride: CrCl3 + 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]z, where L is a Lewis base. For example, it reacts with pyridine () to form the pyridine complex: CrCl3 + 3 C5H5N → CrCl3(C5H5N)3 Treatment with trimethylsilylchloride in THF gives the anhydrous THF complex: CrCl3 + 12 Me3SiCl → CrCl3(THF)3 + 6 (Me3Si)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. 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. Dyestuffs A number of chromium-containing dyes are used commercially for wool. Typical dyes are triarylmethanes consisting of ortho-hydroxylbenzoic acid derivatives. Precautions Although trivalent chromium is far less poisonous than hexavalent, chromium salts are generally considered toxic. References Further reading 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. External links International Chemical Safety Card 1316 (anhydr. CrCl3) International Chemical Safety Card 1532 (CrCl3·6H2O) National Pollutant Inventory – Chromium (III) compounds fact sheet NIOSH Pocket Guide to Chemical Hazards IARC Monograph "Chromium and Chromium compounds" Chromium(III) compounds Chlorides Metal