Chemistry - Coordination Chemistry Question with Solution | TestHub

Strong Jahn-Teller distortion requires unsymmetrical occupation of the $e_g$ orbitals in an octahedral field.

${\left[\mathrm{Cu}{\left({\mathrm{H}}_2\mathrm{O}\right)}_6\right]}^{2+}$: ${\mathrm{Cu}}^{2+}$ is ${\mathrm{d}}^9\left(t_{2g}^6{e}_g^3\right)$ - the $e_g$ set is unsymmetrically filled, so a strong (tetragonal) Jahn-Teller distortion is observed.

The others show no or only weak distortion: ${\left[\mathrm{Ni}{\left({\mathrm{NH}}_3\right)}_6\right]}^{2+}$ (${\mathrm{d}}^8,e_g^2$ symmetrical), ${\left[\mathrm{Mn}(\mathrm{CN}{)}_6\right]}^{2-}$ (${\mathrm{Mn}}^{4+},{\mathrm{d}}^3,e_g^0$) and ${\left[{\mathrm{FeF}}_6\right]}^{3-}$ (${\mathrm{Fe}}^{3+},{\mathrm{d}}^5$ high spin, $t_{2g}^3{e}_g^2$ symmetrical).

Answer: (C)