# Optical activity of cis coordination compounds

I came across a statement saying,

"Consider a Octahedral coordination compound of type [MA2X2]. Then, if the compound is in a cis form, it shows optical activity as there is no plane of symmetry and hence is non-superimposable on its mirror image. On the other hand, a trans form is symmetric and superimposable on its mirror image and so is optically inactive."

I do not see why the cis form is not superimposable on its mirror image. If all the ligands lie on the same plane, then isn't there a line of symmetry passing diagonally? Does that not make it superimposable on its mirror image?

• $\ce{[CoCl2(en)2]+}$ isn't square planar. Please add a reference for the quoted part. – andselisk Oct 28 '19 at 5:17
• Point 1. The compound is not planar(it is octahedral). Point 2. Line/Axis of symmetry doesn't affect chirality. Only POS,COS,alternating Axis of symmetry does. But for any planar molecule, molecular plane is itself a plane of symmetry so planar compounds are always achiral – user600016 Oct 28 '19 at 5:17
• ah duly noted. Turns out I got the structures messed up. I will edit my question for future viewers. – The Jade Reaper Oct 29 '19 at 1:40

If we do that with $$\ce{[Co(en)2Cl2]}$$, we get the following two enantiomers which you cannot convert into each other:
The complex is chiral because the $$\ce{en}$$ ligands necessarily cross the paper plane and can do so either in a right-turn screw motion or in a left-turn screw motion.