One isomer of PtCl2 (NH3)2 reacts with pyridine (py) to give a single isomer of Pt (NH3)2(py)2]Cl2, which in turn decay when heated to yield a single isomer of PtCl2(NH3)(py).
Suppose the other isomer of PtCl2(NH3)2 is given the same treatment, it yields the second isomer of [Pt(NH3)2(py)2]Cl2, as well as finally a mixture of PtCl2(NH3)2 and PtCl2(py)2. Recognize the starting and product isomers as either cis or Trans.