Which one of the following is expected to exhibit optical isomerism ?
(en = ethylenediamine)

To determine which complex is expected to exhibit optical isomerism, we need to analyze the structure of each complex and the conditions required for optical isomerism. So, let's consider each option: option 1 involves cis-[Pt (NH3)2 Cl2], option 2 involves trans-[Co (en)2 Cl2], option 3 involves trans-[Pt (NH3)2 Cl2], and option 4 involves cis-[Co (en)2 Cl2]. Notice that optical isomerism is exhibited by complexes that have a non-superimposable mirror image. Think of it this way: the complex must have a chiral center, which can be a metal atom with a set of different ligands, to exhibit optical isomerism. In other words, the complex must have a non-superimposable mirror image to exhibit optical isomerism. Therefore, the correct answer is the option that involves the complex with a chiral center, which is option 4.