Air Oxidation of Cobalt II Ammine Complexes

It has been known for some time (60-63) that aqueous ammonia solutions containing cobalt(II) salts turn brown on exposure to air. Thermodynamic (64) and kinetic (65, 66) studies on cobalt(II) ammines have therefore been carried out in inert atmosphere. 

Both are active towards molecular oxygen with which they react rapidly through second order 67, 41) rate processes (75)  

There is tentative evidence for the lower reactivity of the hexaammine species in high ammonia concentration. The tetraammine (diaquated) and lower ammine Co(II) species do not pick up O2 over relatively long periods (75). 

Complex B) can be reduced in acid aqueous medium by Fe to complex A), the process results in the evolution of molecular oxygen 74), this being ascribed to the decomposition of A). In analogous reductions using Cr , and Eu , a common intermediate was detected (75), which has been assigned the stmcture 

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