$$
\left[\mathrm{Co}\left(\mathrm{NH}_3ight)_5 \mathrm{SO}_4ight] \mathrm{Br} \text { and }\left[\mathrm{Co}\left(\mathrm{NH}_3ight)_5 \mathrm{Br}ight] \mathrm{SO}_4
$$
are a pair of isomers.
ionisation
ligand
coordination
hydrate
Solution
$$
\begin{aligned}
& {\left[\mathrm{Co}\left(\mathrm{NH}_3ight)_5 \mathrm{SO}_4ight] \mathrm{Br} ightleftharpoons\left[\mathrm{Co}\left(\mathrm{NH}_3ight)_5 \mathrm{SO}_4ight]^{+}+\mathrm{Br}^{-}} \\
& {\left[\mathrm{Co}\left(\mathrm{NH}_3ight)_5 \mathrm{Br}ight] \mathrm{SO}_4 ightleftharpoons\left[\mathrm{Co}\left(\mathrm{NH}_3ight)_5 \mathrm{Br}ight]^{2+}+\mathrm{SO}_4^{2-}}
\end{aligned}
$$
the molecular formula of both of the above compounds is same but on ionisation they give different ions in solution, so they are called ionisation isomers.