Octahedral complexes of copper (II) undergo structural distortion (Jahn-Teller). Which one of the given copper (II) complexes will show the maximum structural distortion ?
(en–ethylenediamine; $\left.{\mathrm{H}}_2\mathrm{N}-{\mathrm{CH}}_2-{\mathrm{CH}}_2-{\mathrm{NH}}_2\right)$

The complexes in which the metal ion have a unsymmetrical eg or t_2​g orbital shows Jahn-taller distortions. In the complex [Cu(H_2​O)_6​]²⁺ the configuration is  t_2​g6_eg3 i.e eg orbital is unsymmetrical and hence will show Jahn-Taller distortions.