Question

Jahn Teller effect is observed in octahedral transition metal complexes with the electron configuration

• A. d$^4$ in high spin state
• B. d$^5$ in low spin state
• C. d$^5$ in high spin state
• D. d$^7$ in low spin state

Answer 1

Answer: (A), (B), (D)

d$^4$ in high spin state, d$^5$ in low spin state, d$^7$ in low spin state

Answer 2

The Jahn-Teller effect occurs when an electronic ground state is orbitally degenerate. This degeneracy leads to a distortion of the complex, lowering its symmetry and energy. In octahedral complexes, the d-orbitals split into $t_{2g}$ and $e_g$ sets.

1. d$^4$ in high spin state: $(t_{2g})^3 (e_g)^1$. The $e_g$ orbitals ($d_{z^2}$, $d_{x^2-y^2}$) are degenerate and contain only one electron, leading to orbital degeneracy. This results in a strong Jahn-Teller effect.
2. d$^5$ in low spin state: $(t_{2g})^5 (e_g)^0$. The $t_{2g}$ orbitals ($d_{xy}$, $d_{xz}$, $d_{yz}$) are degenerate and contain five electrons (e.g., two paired and one unpaired electron), leading to orbital degeneracy. This results in a weak Jahn-Teller effect.
3. d$^5$ in high spin state: $(t_{2g})^3 (e_g)^2$. Both $t_{2g}$ and $e_g$ orbitals are symmetrically occupied (one electron in each orbital). There is no orbital degeneracy in the ground state, hence no Jahn-Teller effect.
4. d$^7$ in low spin state: $(t_{2g})^6 (e_g)^1$. The $e_g$ orbitals are degenerate and contain only one electron, leading to orbital degeneracy. This results in a strong Jahn-Teller effect.

Thus, d$^4$ in high spin state, d$^5$ in low spin state, and d$^7$ in low spin state all exhibit the Jahn-Teller effect.