Which of the following complex has a possibility to exist as meridional isomer?

Solution

Chemistry ma’am · 3 steps

Mer isomers need three identical ligands on an octahedron, so look for the MA₃B₃ pattern.

1. What mer means. In an octahedral MA₃B₃ complex, the three A ligands can sit on one face of the octahedron. That is the facial (fac) isomer. They can also lie in one plane that passes through the metal, with two of them trans to each other. That is the meridional (mer) isomer.
2. Check each complex. So a mer isomer needs two things: an octahedral complex, and a set of three identical donor groups.

   Complex	Geometry and type	Isomerism shown
   [Pt(NH₃)₂Cl₂]	square planar, MA₂B₂	cis–trans only
   [Co(en)₂Cl₂]	octahedral, M(AA)₂B₂	cis–trans, and the cis form is optically active
   [Co(en)₃]	octahedral, M(AA)₃	optical isomers only
   [Co(NH₃)₃(NO₂)₃]	octahedral, MA₃B₃	fac and mer

3. So. Only [Co(NH₃)₃(NO₂)₃] has three NH₃ and three NO₂⁻ around an octahedral cobalt, so only (d) can exist as a meridional isomer.

The idea to keep. Fac–mer isomerism belongs to octahedral MA₃B₃ complexes. MA₂B₂ and M(AA)₂B₂ give cis–trans, and M(AA)₃ gives only optical isomers.

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