FeSO4 does not form any complex with (NH4)2SO4. Instead, it forms a double salt FeSO4.(NH4)2SO4.6H2O which dissociates completely into ions. CuSO4 when mixed with NH3 forms a complex [CU(NH3)4]SO4 in which the complex ion [CU(NH3)4]2+ does not dissociate to give Cu2+ ion.