An exchange reaction is one in which ligands in the coordination sphere of a metal ion are interchanged for other ligands. Exchange reactions are observed at relatively slow rates between Co²⁺ and Co³⁺ ions. For hexaamine cobalt (III), the exchange reaction is given by Equation-1.
$\text{ Equation-1: }\mathrm{Co}{\left({\mathrm{NH}}_3\right)}_6^{3+}+\mathrm{Co}{\left({\mathrm{NH}}_3\right)}_6^{2+}\rightleftharpoons \mathrm{Co}{\left({\mathrm{NH}}_3\right)}_6^{2+}+\mathrm{Co}{\left({\mathrm{NH}}_3\right)}_6^{3+}$
This reaction is believed to proceed via the following mechanism
$\mathrm{Co}{\left({\mathrm{NH}}_3\right)}_6^{3+}+{\mathrm{H}}_2\mathrm{O}\rightleftharpoons \mathrm{Co}{\left({\mathrm{NH}}_3\right)}_6^{3+}+{\mathrm{OH}}^{-}+{\mathrm{H}}^{+}\left(\mathrm{Fast}\right)$
${\mathrm{OH}}^{-}+{\left[\mathrm{Co}{\left({\mathrm{NH}}_3\right)}_6\right]}^{+3}⟶{[\mathrm{Co}\left(\mathrm{NH}\right)}^{}$
${\mathrm{H}}^{+}+{\left[\mathrm{Co}{\left({\mathrm{NH}}_3\right)}_6\mathrm{OH}\right]}^{2+}+{\left[\mathrm{Co}{\left({\mathrm{NH}}_3\right)}_6\right]}^{+2}⟶{\left[\mathrm{Co}{\left({\mathrm{NH}}_3\right)}_6\right]}^{+2}+{\mathrm{H}}_2\mathrm{O}+{\left[\mathrm{Co}{\left({\mathrm{NH}}_3\right)}_6\right]}^{+3}\left(\text{ fast }\right)$
The rates of reaction for this type of process are often elucidated using isotopic labeling.
Figure-1 shows how changes in pH affect the relative rate of Equaiton - 1
 

A chemist prepares a new solution for observation by placing 6.7 g of [Co(NH₃)₆]Cl; and 5.5 g of [Co(NH₃)₆]Cl₂ in a beaker containing 1L of water. The pH of the solution is adjusted to 9.
$$\begin{gathered} \mathrm{Mol}. \mathrm{wt}.\left[\mathrm{Co}{\left({\mathrm{NH}}_3\right)}_6\right]{\mathrm{Cl}}_3=268\mathrm{g} \\ \text{ Mol. wt. [}\mathrm{Co}({\mathrm{NH}}_3{)}_6]{\mathrm{Cl}}_2=232\mathrm{g} \end{gathered}$$