Jahn-Teller Distortion in Highspin Transition Metal Ions

The Jahn-Teller effect is a phenomenon observed in high-spin transition metal ions with partially filled d-orbitals in an octahedral field. It occurs due to the interaction between electrons and the degenerate molecular orbitals in the octahedral crystal field.

Explanation:

- Highspin transition metal ions have partially filled d-orbitals and exhibit strong electron-electron repulsion.
- When placed in an octahedral field, the d-orbitals split into two sets of degenerate orbitals, known as t2g and eg.
- In highspin ions, the electrons occupy the t2g orbitals before the eg orbitals due to the Hund's rule.
- However, this arrangement is not energetically favorable as the t2g orbitals are more stable than the eg orbitals.
- The Jahn-Teller effect occurs when the degeneracy of the eg orbitals is lifted by a distortion of the octahedral field, leading to a lowering of the energy of the system.
- The distortion can be either elongation or compression of the octahedral field along one of the axes.
- The distortion causes the electrons to occupy the lower energy eg orbital, resulting in a non-degenerate ground state.
- Only those pairs of electronic configurations of highspin transition metal ions that exhibit the Jahn-Teller effect undergo substantial distortion.

Answer:

Option 'B' is correct as it represents the pair of electronic configurations d4 and d9, which exhibit the Jahn-Teller effect. The d4 ion has two electrons in the eg orbitals and two in the t2g orbitals, while the d9 ion has one electron in the eg orbitals and four in the t2g orbitals. Both these ions have partially filled eg orbitals, which are energetically unstable and undergo Jahn-Teller distortion to lower the energy.