According to John teller any nonlinear molecular system in a degenerate electronic state will be unstable and will undergo some kind of distortion which will lower its symmetry and energy and split the degenerate state.

In case of octahedral $$d^{9}$$ configuration, the last electron may occupy either $$d^{2}$$ or $$d_{x^{2} y^{2}}$$ orbitals of $$e_{g}$$ set.

If it occupies $$\mathrm{dz}^{2}$$ orbital most of the electron density will be concentrated between the metal and the two ligands on the $$\mathrm{z}$$ axis. Thus there will be greater electrostatic repulsion associated with these ligands than with the other four on xy plane.

The Jahn Teller effect is mostly observed in octahedral environments. The considerable distortions are usually observed in high spin $$\mathrm{d}^{4}$$, low spin $$\mathrm{d}^{7}$$ and $$\mathrm{d}^{9}$$ configuration.