Chromic chloride, preparation anhydrous

Potassium Tetrathiocyanato-dipyridino-chromium, [Cr py2 (SCN)4]K.2H,0, is prepared by heating potassium ehromithiocyanate with anhydrous pyridine. The melt is reerystallised from a little wTater, and on cooling a stable red powder separates. The thiocyanato-groups are all within the complex, and the salt in solution therefore gives no coloration with ferric chloride. It is decomposed by chlorine in presence of cold water with formation of tetraquo-dipyridino-chromic chloride. If crystallised in the dry state from pyridine it forms transparent red crystals of the addition compound, [Cr py2(SCN)JK.4py, which quickly effloresce in air. 

Pfeiffer prepared trichloro(tripyridine)chromium(III) from anhydrous chromium(III) chloride and from dichloro-tetraaquochromium(III) chloride 2-hydrate (the common hydrated chromic chloride of commerce), but neither details of the preparations nor yields were reported. The reaction of [Cr(H20)4Cl2]Cl-2H20 with pyridine gives [Cr(C6H6N)3-CI3] with a yield of about 4%. In the procedure described here, the complex can be prepared in about 90% yield when anhydrous chromium(III) chloride is used as the starting material. 

The chromium may be used to prepare anhydrous chromic chloride, from which certain complexes of chromium can be made the manganese may be used in the electrolytic preparation of potassium permanganate (see Experiment 52). 

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