Jahn–teller Distortion

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Jahn–Teller distortion most pronounced in octahedral complexes with electronic configuration:

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1. **Definition**: The Jahn-Teller theorem states that any non-linear degenerate electronic system will undergo distortion to remove the degeneracy and lower its energy.
2. **Octahedral Case**: Distortion is greatest when the **$e_g$ orbitals** (which point directly at ligands) have an **unequal number of electrons**.
3. **Configurations**:
   - **High-spin $d^4$**: Configuration $(t_{2g})^3 (e_g)^1$. The single $e_g$ electron creates strong repulsion along one axis.
   - **High-spin $d^7$** and **Low-spin $d^9$**: Also show strong distortion for similar reasons.
4. **Mechanism**: Typically, the complex undergoes **elongation** or **compression** along the z-axis (Tetragonal distortion).
5. **Non-distorted**: $d^3$ (symmetrical $t_{2g}$), $d^8$ (symmetrical $e_g [2]$), and $d^{10}$ show little to no JT effect.

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Jahn–teller Distortion

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