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?
It is not always as easy to see from a standard octahedral depiction. Optical activity in octahedral complexes can be understood better, if you draw the octahedron as looking onto a triangular face rather than along an edge; this results in a hexagonal arrangement of ligands around the central atom with ligands alternately pointing into and out of the paper plane.

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.