$[\mathrm{Co}(\mathrm{NH}{}_3){}_5\mathrm{Cl}]\mathrm{SO}{}_4$ and $[\mathrm{Co}(\mathrm{NH}{}_3){}_5\mathrm{SO}{}_4]\mathrm{Cl}$ are related as

The two compounds have the same formula but exchange an anion between the coordination sphere and the counter-ion position ($\mathrm{Cl}{}^{-}$ and $\mathrm{SO}{}_4{}^{2-}$). This is ionisation isomerism.