Answer:
The Jahn-Teller effect is a phenomenon that occurs in molecules and ions with certain electron configurations. Specifically, it describes the geometrical distortion of species with a spatially degenerate electronic ground state, which results in the removal of the degeneracy and a lowering of the overall energy of the species.
For a complex having d^4 electrons, the Jahn-Teller effect is most pronounced when the complex is in a high-spin state and has an odd number of electrons occupying the eg orbitals. This situation arises in complexes with the configurations d^9, low-spin d^7, or high-spin d^4, all of which have doubly degenerate ground states. In such compounds, the eg orbitals involved in the degeneracy point directly at the ligands, so distortion can result in a large energetic stabilization. The elongation Jahn-Teller is more distinctive when the degeneracy occurs in the eg level.
In summary, the Jahn-Teller effect is a geometrical distortion that occurs in molecules and ions with certain electron configurations. For a complex having d^4 electrons, the effect is most pronounced when the complex is in a high-spin state and has an odd number of electrons occupying the eg orbitals. The elongation Jahn-Teller is more distinctive when the degeneracy occurs in the eg level.