Optical isomerism is not shown by complexes that possess a plane of symmetry. The complex $[\text{Co}(\text{NH}_3)_3\text{Cl}_3]$ is of the type $[Ma_3b_3]$. It exhibits geometrical isomerism, existing as facial (fac) and meridional (mer) isomers. Both of these isomers possess a plane of symmetry and are therefore achiral (optically inactive). On the other hand, octahedral complexes containing bidentate ligands like ethylenediamine (en), such as $[\text{Co}(\text{en})_3]^{3+}$ or the cis-isomer of $[\text{Co}(\text{en})_2\text{Cl}_2]^+$, lack a plane of symmetry and thus exhibit optical isomerism.