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?