Question

The complex(es) that show(s) Jahn–Teller distortion is (are)

• A. [Co(CN)$_5$(H$_2$O)]$^{3-}$
• B. [NiF$_6$]$^{2-}$
• C. [Mn(CNMe)$_6$]$^{2+}$
• D. [Co(en)$_3$]$^{3+}$

Hint:

Jahn–Teller effect is strongest in $d^9$, $d^4$ (high-spin), and $d^7$ (low-spin) configurations with degenerate orbitals.

Answer 1

The Correct Option is A, C

Step 1: Recall Jahn–Teller theorem.
Jahn–Teller distortion occurs in coordination complexes with unevenly occupied degenerate orbitals (especially $e_g$ or $t_{2g}$ levels).
Step 2: Check electronic configurations.
(A) Co(III): d$^6$ low-spin → $t_{2g}^6e_g^0$ (no degeneracy) → no distortion.
(B) Ni(II): d$^8$ → in octahedral field → $t_{2g}^6e_g^2$, unevenly filled $e_g$ orbitals → Jahn–Teller active.
(C) Mn(II): d$^5$ high-spin → symmetric half-filled → no distortion.
(D) Co(III): d$^6$ low-spin → $t_{2g}^6$ → no distortion.
Step 3: Conclusion.
The only complex showing Jahn–Teller distortion is [NiF$_6$]$^{2-$ (Option B)}.