Explanation

(a)
$\lbrack Co\left( {NH}_{3} \right)_{5}{SO}_{4}\rbrack Br\ \rightleftharpoons \left\lbrack Co\left( {NH}_{3} \right)_{5}{SO}_{4}\ \right\rbrack^{+} + {Br}^{-}$
$\left\lbrack Co\left( {NH}_{3} \right)_{5}Br \right\rbrack{SO}_{4} \rightleftharpoons \left\lbrack Co\left( {NH}_{3} \right)_{5}Br \right\rbrack^{2 +} + \ \ \ {SO}_{4}^{2 -}\$
The molecular formula of both of the above compounds is same but on ionisation they give different ions in solution, so they are called ionization isomers.