the best way to determine if a molecule is chiral is to see if it contains a mirror plane of symmetry. If it does not contain a mirror plane it is chiral. If it does contain a mirror plane is achiral. Much easier than trying to rotate molecules.
cis [Cr(NH3)4Br2] is not an optical isomer because you can take a mirror image of the molecule and rotate it to match the original. Or, as a commenter said, you can create a mirror plane within the molecule (it would run through the central atom and both Br atoms), thus it's not chiral. The octahedral compounds with bidentate ligands are a different story, because the bridging nature of the bidentate ligands adds an extra complication to symmetry.
That’s the point he’s trying to make right. No matter how u spin them, enantiomers are enantiomers. They can never be superimposed even though you rotate. Super impose means to overlap perfectly or simply, they are congruent or exactly the same object in dimensions. In general (not chemistry) if you mirrored a *symmetrical* object and rotated it, you *can superimpose* it. However if you take an *unsymmetrical* or *chiral object and rotated it about any axis you like however you try, then you can *never ever superimpose it* with In the video, Fe(en)2Cl2. When he rotates the compound, look the mirrored + rotated compound (the third one) and the previous compound (the first one), 😅notice the similarities:- 1) The central atom (Iron) 2) The chlorine ligand situated above the horizontal square plane or simply the top vertex of the octahedron 3) The diethyl amine ligand bonding with the iron in the same way and orientation in both compounds which is situated at the bottom right edge of the octahedron (it does of course tends to come out of the octahedron but to keep it simple, both are same). The above things *can be superimposed with each other* because they are exactly in the same position with respect to the central iron atom Now, notice the differences:- 1) The plane in the first compound consists of diethyl amine on the left side but the the plane of the third compound consists of the diethyl amine more like into the screen/plane/paper/negative Z axis or whatever u like. 2) The chlorine in the plane of both compounds is situated at different locations as well The above things discussed now, cannot be *superimposed* As observed till now, we can notice that the central atom(Iron) and two of the ligands(the ligands *non co-planar* ones or the ones situated on and below the plane) *can be super imposed perfectly* and two of them (the *co-planar* ones) are off and *can’t be super imposed*, we can finally conclude that the *complex on the whole, cannot be superimposed* though it is rotated I tried my best to explain this. I hope you understand.
