The Jahn–Teller distortion is an important concept in transition-metal chemistry that explains why some metal complexes are not perfectly symmetrical.
What is Jahn–Teller Distortion?
- It is a geometric distortion of a molecule or complex that happens when the electronic arrangement in its d-orbitals is uneven.
- In simple words: if electrons are not evenly distributed among orbitals of the same energy, the complex will distort to remove that imbalance and lower its overall energy.
When does it occur?
- Most common in octahedral complexes with certain electron configurations.
- Typical cases:
- d⁹ (e.g., Cu²⁺)
- high-spin d⁴ (e.g., Mn³⁺)
- These have unevenly filled orbitals, which makes the complex unstable in perfect symmetry.
How does it distort?
- In an octahedral complex, the distortion usually happens by:
- Lengthening some bonds (usually along the z-axis).
- Shortening others (in the xy-plane).
- The result: the octahedron becomes slightly elongated or sometimes compressed.
Effects on Metal Complexes
- Geometry changes
- Complexes lose perfect octahedral symmetry.
- Example: [Cu(H₂O)₆]²⁺ often has four short Cu–O bonds and two longer ones.
- Spectroscopic properties
- Distortion changes d-orbital splitting, affecting the color of the complex.
- Magnetic properties
- Uneven orbital occupancy influences magnetic behavior.
- Reactivity
- Distorted complexes may have weaker or stronger bonds in certain directions, affecting substitution and redox chemistry.
In summary:
The Jahn–Teller distortion is nature’s way of “fixing” instability caused by uneven electron distribution in degenerate orbitals. It makes complexes more stable, but also changes their shape, color, magnetism, and reactivity.