JEE Main 2020 · 5 September · evening · Single correct · +4 / −1

Consider the complex ions,
trans-[Co(en)₂Cl₂]⁺ (A) and
cis-[Co(en)₂Cl₂]⁺ (B).
The correct statement regarding them is :

  1. (a) both (A) and (B) can be optically active.
  2. (b) both (A) and (B) can not be optically active.
  3. (c) (A) can not be optically active, but (B) can be optically active.
  4. (d) (A) can be optically active, but (B) can not be optically active.
Answer and solution

Answer (c) (A) can not be optically active, but (B) can be optically active.

Solution

Chemistry ma’am · 4 steps

Look for a mirror plane: the trans form has one, the cis form does not.

  1. Test for chirality. A complex is optically active only if it cannot be superimposed on its mirror image. If it has a plane of symmetry, it is achiral and cannot be optically active.
  2. Trans isomer (A). In trans-[Co(en)₂Cl₂]⁺ the two Cl atoms sit opposite each other, and both en rings lie flat in the plane between them. That plane is a mirror plane, so (A) is identical to its mirror image and cannot be optically active.
  3. Cis isomer (B). In cis-[Co(en)₂Cl₂]⁺ the two Cl atoms are next to each other, and the two en rings twist around cobalt like the blades of a propeller. There is no mirror plane, so the left-handed and right-handed forms are non-superimposable mirror images (d and l), and (B) can be optically active.
  4. So. (A) cannot be optically active, but (B) can. The answer is (c).

The idea to keep. For [M(AA)₂B₂] complexes, trans is always optically inactive because of its mirror plane. The cis form is chiral and exists as a d and l pair.

Answer from the NTA final answer key. Solution by PrepWiser.

Study Coordination Compounds

Asked about 250 times in JEE Main since 2020. This question is from 2020.