Question

Why only cis isomers show optical activity

Answer 1

A compound will be optically active,if there exists no center or plane of symmetry.This follows one more thing,that the compound exhibiting optically isomerism need to form non-superimposable mirror images.As in case of cis isomers of co-ordination compounds,the mirror images are non-superimpossable,hence they are optically active.For example in a cordination entity of the type (PtCl2(en)2)^2+,only the cis-isomer shows optical activity due to lack of plane of symmetry.In trans isomer,the molecule will be symmetric,because of which its mirror images will be superimpossable.Thus the compound will be optically inactive.