What is the main cause of the Jahn-Teller effect?

The Jahn-Teller effect is primarily related to the presence of degenerate electronic states in a system. When a molecule or coordination complex has a degenerate electronic ground state, it means that multiple electron configurations have the same energy. This degeneracy can lead to instabilities in the geometry of the complex, as the system tends to lower its energy by distorting away from a high-symmetry configuration.

By breaking the degeneracy through geometric distortion, the system can achieve a lower energy state, resulting in a more stable configuration. Such distortion often results in the elongation or compression of certain bond lengths, thereby reducing the symmetry of the complex and removing the degeneracy of the electronic states.

The presence of high symmetry coordination may initially suggest that the complex is stable; however, the Jahn-Teller effect is inherently about the instability caused by degenerate electronic states, prompting the need for distortion to achieve a more favorable energy configuration. Similarly, while stable geometries and stable electronic configurations can be important concepts in coordination chemistry, they do not directly cause the Jahn-Teller effect itself. Thus, the main driving force behind this phenomenon is indeed the presence of degenerate electronic states, which necessitates a distortion to lower the overall energy of the system.