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Jahn-Teller effect not observed in high spin complexes of:
A
$d^4$
B
$d^9$
C
$d^7$
D
$d^8$
Explanation
Symmetric $e_g^2$ occupancy in $d^8$ gives no distortion.
Detailed Solution
The Jahn–Teller effect explains axial distortion in octahedral geometry. It is present in $d^4$ high spin, $d^7$ low spin and $d^9$ configurations, which have an odd number of electrons in the $e_g$ set.
A weak Jahn–Teller effect is also present in $d^7$ high spin complexes, which have an odd number of electrons in the $t_{2g}$ set.
High spin $d^8$ ($t_{2g}^6e_g^2$) has symmetric filling, so no Jahn–Teller effect.
A weak Jahn–Teller effect is also present in $d^7$ high spin complexes, which have an odd number of electrons in the $t_{2g}$ set.
High spin $d^8$ ($t_{2g}^6e_g^2$) has symmetric filling, so no Jahn–Teller effect.