‌[Co(NH3)5‌SO4]‌Br⇌[Co(NH3)5‌SO4]++Br− ‌[Co(NH3)5‌Br]SO4⇌[Co(NH3)5‌Br]2++SO42− 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.