Chromium ammines, decompositions

The color and solubilities of these chromium(III) ammines are very similar to those of the corresponding cobalt(III) complexes.5 The chlorides, bromides, nitrates, and perchlorates in the acidopentamminechromium(III) series are not very soluble in water, whereas the analogous aquopentamminechromium(III) salts are soluble. In aqueous solutions, these chromium(III) ammines are much more readily decomposed than the corresponding cobalt(III) ammines. Decomposition may be perceptible within a few minutes. The absorption spectra of aqueous solutions of some acidopentamminechromium(III) salts have been studied.6,7... 

Decomposition of the metal ammines have probably been most extensively investigated. Some qualitative features of the thermal decomposition of metal ammine compounds are conveniently illustrated [1116— 1118] by the somewhat contrasting behaviour of the compounds [Cr(NH3)6]X3 and [Co(NH3)6]X3 where X is Cl- or Br . During decomposition of the chromium compound, the oxidation number of the metal remains unchanged, viz. 

As examples of series of related reactions, compensation effects have been described [53] for the thermal decompositions of [CoXj (aromatic amine)2] type complexes (7 reactions) and also for a series of cobalt (III) and chromium (III) complexes (22 compounds studied in which two compensation trends were identified). Later work [54] examined the dehydrations and deamminations of dioximine complexes (two compensation trends identified), and [Co(NCS)2(ammine)2]-type complexes (three compensation trends identified). The systems involving larger entropy changes required less energy for activation [53]. Separate compensation plots for the dehydrations and the decompositions of eleven alkali and alkaline-earth metal dithionates were described by Zsako et al. [55]. 

Jorgensen, E. and B)errum, J. (1958) Kinetics of complex formation. IV. Rates of decomposition and transformation mechanisms of chromium(III) ammines in aqueous solution. Acta Chem, Scand, 12, 1047-1061. 

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