Diaquabis ethylenediamine chromium III Bromide

The pure salt is obtained by the following method from pure cis- [diaquabis-(ethylenediamine)chromium(III)] bromide dihydrate. [Diaquabis(ethylenedi-amine)chromium(III)] bromide dihydrate (2.00 g, 0.00413 mole) (see Sec. G) is dissolved at 0° in 5.00 mL of 0.100 jW hydrochloric acid. To this solution is added a solution of 1.10 g (0.00454 mole) of sodium dithionate dihydrate in 10 mL of water at room temperature. To the filtered solution is added, with stirring and cooling in an ice bath, 23.0 mL of 0.200 M sodium hydroxide. Precipitation of red crystals of cis- [aquabis(ethylenediamine)hydroxochromium(III)] dithionate commences almost instantaneously. After a few minutes the sample is filtered and washed thoroughly with water, 96% ethanol, and then diethyl ether. Drying in air yields 0.9 g (59%) of pure ds-[aquabis(ethylenediamine)hydroxochrom-ium(III)] dithionate. Found Cr, 14.04 N, 15.16 C, 13.17 H, 5.23. 

The c/s-[aquabis(ethylenediamine)hydroxochromium(III)] dithionate salt is stable for years. It is insoluble in water but very soluble in strong acid and strong base, giving the corresponding m-diaqua and m-dihydroxo species. The crude product is used in the preparation of m-[diaquabis(ethylenediamine)chromium-(III)] bromide (Sec. G) and of di-/a-hydroxobis[bis(ethylenediamine)chromium-(III)] dithionate (Sec. J). 

Starting material. The preparation given below is based on an improved method. Here the bromide salt is obtained from crude c/s-[aquabis(ethylene-diamine)hydroxochromium(III)] dithionate (Sec. F) by treatment with hydro-bromic acid. The pure c/s-[diaquabis(ethylenediamine)chromium(IIl)] bromide is used to prepare pure cfs-[aquabis(ethylenediamine)hydroxochromium(III)] dithionate (Sec. F). 

The c/s-[diaquabis(ethylenediamine)chromium(IlI)] bromide salt is sensitive to light. If stored in the dark at -20°, it is stable for years. In acidified solution it is converted slowly to an equilibrium mixture containing the trans-isomer. The equilibrium constant is = [cis]/[trans] = 12.8 (25°, 1 MNaNOs). The acid dissociation constants are pK = 4.8 and pA 2 = 7.17 (25°, 1 MNaNOa).  

The sulfate salt of di- i-hydroxo-bis[tetraamminechromium(III)] has been prepared traditionally by heating of cK-[tetraammineaquahydroxochromium(III)] sulfate in acetic anhydride. In the method given here, the impure dithionate salt is obtained by heating ds-[tetraammineaquahydroxochromium(III)] dithionate at 100°. The pure bromide salt is then obtained from the crude dithionate, and the pure perchlorate is obtained from the bromide. 

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Methods for preparation of ds-[tetraammineaquachlorochromium(III)] sulfate, ds-[tetraammineaquahydroxochromium(III)] dithionate monohydrate, and ds-[tetraammineaquahydroxocobalt(III)] dithionate monohydrate as nearly pure intermediates are given. The preparation of crude dx-[aquabis(ethylenediamine)-hydroxochromium(lll)] dithionate and a method for obtaining the pure salt are given. These dx-aquahydroxo salts have been used to prepare the following pure dx-diaqua salts dx-[tetraamminediaquachromium(Ill)] perchlorate, dx-[tetra-amminediaquacobalt(lll)] perchlorate, and dx-[diaquabis(ethylenediamine)-chromium(lll)] bromide dihydrate. 

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