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Octahedral complex of copper(II) undergo structural distortion (Jahn-Teller). Which one of the given copper(II) complexes will who the maximum structural distortion? (en - ethylenediamine)

(A) $[Cu(H_2O)_6]SO_4$

(B) $[Cu(en)(H_2O)_4]SO_4$

(C) $cis-[Cu(en)_2Cl_2]$

(D) $trans-[Cu(en)_2Cl_2]$

The answer given is A.

How do I compare the magnitude of Jahn-Teller distortion on $e_g^3$ complexes?


I have subjectively understood the JT effect: $Cu^{2+}$ in its $d^9 -t_{2g}^6e_g^3$ configuration has an asymmetric configuration, so the octahedral complex breaks symmetry by distorting, usually by lengthening (z-out) along the $d_{z^2}$ orbital or shortening (z-in)

However in the question, all the complexes have the same configuration. Is it something to do with the strength of the ligands?


This question is from the second shift of the JEE Main exam held on July 29, 2022. There is a possibility the answer is wrong, however I would still like to know how to compare distortion. I have referenced the following material:

  1. JD Lee (Sudarshan Guha): does not compare magnitudes
  2. Libretexts: says "stronger M-L interactions increase chance of observing JT" . That sounds too vague.

I would appreciate some insight.

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  • $\begingroup$ Well until it defines "maximum structural distortion" I have no idea what it means. The en complexes will already be distorted from Oh before J-T is even considered, how are we measuring distortion on top of distortion? $\endgroup$
    – Ian Bush
    Dec 14, 2023 at 20:48
  • $\begingroup$ @IanBush what's "Oh"? Is it a group theory thing? $\endgroup$ Dec 15, 2023 at 15:44
  • $\begingroup$ Sigh. Yes. It is short for Octahedral, and represents the point group for that symmetry. It is where labels such as $t_{2g}$ come from. $\endgroup$
    – Ian Bush
    Dec 15, 2023 at 16:00

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