Stability of Cobalt (III) and Chromium (III) Ammine Complexes in a Strongly Alkaline Solution

Abstract

The rate of decomposition of the cobalt (III) and chromium (III) hexammine and monoaquopentammine complexes in an alkaline solution was measured. The activation energies of the decomposition were found to be 36, 32, 31 and 31 Kcal/ mole for [Co(NH3)6]3+, [Co(NH3)5H2O]3+, [Cr(NH3)6]3+ and [Cr(NH3)5H2O]3+ respectively. At the same time the infrared absorption spectra of these complexes were also measured. The frequencies of the rocking vibration of the coordinated ammonia were found to be 815, 853, 744 and 747 cm−1 for [Co(NH3)6]3+, [Co(NH3)5H2O]3+, [Cr(NH3)6]3+ and [Cr(NH3)5H2O]3+ respectively. From these results it was concluded that the decomposition of the cobalt (III) ammine complex was not a mere replacement of the coordinated ammonia by hydroxyl ion, but a reaction through the proton dissociation step as exemplified in the following scheme[Co(NH_3)_6]^3++OH^-→[Co(NH_3)_5NH_2]^2++H_2O,while, in the chromium (III) ammine complex, the decomposition was inferred to proceed through a substitution of hydroxyl ion for the coordnated ammonia.