Jahn-Teller effect is not observed in high spin complexes of:

Solution & Explanation

\textbf{Answer:} (B) High-spin $d^8$ \textbf{Solution:} The Jahn-Teller effect arises when the $e_g$ orbitals (which point directly at the ligands) are unevenly occupied, causing a distortion that lifts the degeneracy. For a high-spin octahedral configuration the $e_g$ set is $d_{z^2}$ and $d_{x^2-y^2}$: $d^4$ (high-spin): $t_{2g}^3{e}_g^1$ — $e_g$ unevenly filled $\Rightarrow$ distortion. $d^7$ (high-spin): $t_{2g}^5{e}_g^2$ — wait, $t_{2g}$ uneven; $d^7$ HS is $t_{2g}^5{e}_g^2$, $e_g$ evenly filled but $t_{2g}$ uneven (weak distortion still occurs). $d^9$: $t_{2g}^6{e}_g^3$ — $e_g$ unevenly filled $\Rightarrow$ strong distortion. $d^8$ (high-spin): $t_{2g}^6{e}_g^2$ — both $e_g$ orbitals are singly/evenly occupied, so there is no Jahn-Teller distortion. Hence $d^8$ is the answer.