Chromium chloride, anhydrous hydrates

Twenty grams of chromium nitrate 9-hydrate, or 0.05 moles of the hydrated sulfate or chloride, are fused with 30g of potassium thiocyanate as in (I), and the melt allowed to cool in a desiccator and then triturated with 25ml portions of ethyl acetate until no more material dissolves. The violet liquid is filtered a small green residue remains behind together with the potassium nitrate, sulfate, or chloride. The ethyl acetate extract is evaporated to dryness on the steam bath and the violet residue is further dried in vacuo. The anhydrous salt is violet. 

When the apparatus has been flushed, a boiling solution of 16 g. (0.20 mol) of anhydrous sodium acetate dissolved in 35 ml. of water is added through E A slow nitrogen flow is maintained, and a solution of 9 g. (0.034 mol) of chromium(III) chloride 6-hydrate in 15 ml. of 0.4 N sulfuric acid is poured into the top of the redactor. The rate of flow of the chromium solution can be controlled by stopcock B. If too fast a rate is used, there is a possibility of incomplete reduction. Distilled water is poured after the chromium chloride until the effluent is only slightly colored by chromium. This requires approximately 125 ml. 

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. 

Sulphur dichloride oxide (thionyl chloride) on the hydrated chloride can also be used to produce the anhydrous chloride in certain cases, for example copper(II) chloride and chromium(III) chloride ... 

This is prepared by passing dry hydrogen chloride over chromium, or hydrogen over anhydrous chromium(III) chloride. It is a white solid. If pure chromium is dissolved in dilute hydrochloric acid in the absence of air, a blue solution of the hydrated chloride, containing the hexaaquo-ion [Cr(H20)6]2+. is obtained. The same solution is also obtained by reduction of the + 6 oxidation state (through the + 3) using a solution of a dichromate(VI) and reducing with zinc and hydrochloric acid ... 

The anhydrous chromium (III) chloride may be obtained by heating the hydrated salt CrCls 6H2O with SOCI2 and subliming the product in a stream of chlorine at 600°C. Alternatively, the red-violet anhydrous chloride can be obtained by passing chlorine gas over a mixture of chromic oxide and carbon ... 

If tri8(ethylenediamine)chromium(III) chloride is prepared by the method of Rollinson and Bailar,7 it is worthy of note that if the dehydration of the hydrated chromium (III) sulfate is performed in a vacuum oven (2-3 mm. Hg, 70-80°C.) the anhydrous sulfate is obtained in a much more reactive form so that the reaction with ethylenediamine, when started by local heating, proceeds by itself and is finished within an hour. Editor s note For an alternative preparation, see Inorganic Syntheses, 10, 33 (1967) also ref. 26. 

Some salts could not be dehydrated in THF. Iron(III) chloride for example, polymerizes THF and must be prepared using neat trimethylchlorosilane. Cobalt(II) chloride dihydrate gave a similar result requiring neat trimethylchlorosilane. In both cases, nearly quantitative yields of the anhydrous salt were obtained. Hydrated zinc chloride, which we prepared by adding 10 wt % water to the anhydrous chloride because well-defined hydrates are not commercially available, was very efficiently dehydrated in neat trimethylchlorosilane to give a 96% yield of zinc chloride. In contrast, the THF/trimethylchlorosilane mixture afforded a comparatively modest 71% yield of the tetrahydrofuranate. Chromium(III) chloride hexahydrate, on the other hand, could not be completely dehydrated in neat trimethylchlorosilane and required THF for an efficient reaction giving 89% yield of chromium(III) chloride tris(tetrahydrofuran). Our results are summarized in Table I. 

Pure anhydrous chromium(II) chloride forms white hygroscopic needles that are stable in dry air but oxidize rapidly if moist. The compound dissolves in water, forming the blue hydrated chromium(II) ion which has strong reducing properties. Several hydrates and ammines have been described. 

In the preparation of some complexes, particularly organometallic complexes, the presence of water must be avoided. An important example from classical coordination chemistry is that the action of ammonia (either as a gas or in solution) on hydrated chromium(III) salts—those commercially available—leads to the precipitation of insoluble hydroxy complexes and not to the formation of [Cr(NH3)6]. This complex is prepared by reaction between liquid ammonia and anhydrous chromium(III) chloride. 

Apparently chromic chloride was a violet pigment discovered by Wohler (1800-82), Professor of Chemistry at the Chemical University of Gottingen (cf. Colour Index, 1971 Cl 77295). In practice this compound generally occurs either as an anhydrous chromium(III) chloride (CrCl3) or as a hexahydrate (chromium(III) chloride hexahydrate, Cl3CrH]20g, probably as [Cr(OH2)6]Cl3). The anhydrous compound is red-violet, while the hydrated form has greenish black to violet crystals it is unclear which is the basis of die pigment. Chromium(II) and chromium(IV) chlorides also exist. 

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