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

The Jahn-Teller effect occurs in non-linear molecules with electronically degenerate ground states (unevenly filled $t_{2g}$ or $e_g$ orbitals).$d^4$ (high spin): $t_{2g}^3{e}_g^1$ (Uneven $e_g$, shows J-T).$d^7$ (high spin): $t_{2g}^5{e}_g^2$ (Uneven $t_{2g}$, shows J-T).$d^9$: $t_{2g}^6{e}_g^3$ (Uneven $e_g$, shows strong J-T).$d^8$: $t_{2g}^6{e}_g^2$. Here, $t_{2g}$ is completely filled and $e_g$ is half-filled (symmetrical). There is no electronic degeneracy, so no Jahn-Teller effect is observed.