This section will include:
In 1937 H.A. Jahn and E. Teller put forward a theorem which explained some of the distortions observed in transition metal complexes. This became known as the Jahn-Teller theorem. It states:
"For any non-linear molecule in an electronically degenerate state, distortion must occur to lower the symmetry, remove the degeneracy and lower the energy."
These distortions are called Jahn-Teller distortions. But but how are the molecules distorted, and how does the distortion occur? This section aims to describe what is meant by an electronically degenerate state and explain how Jahn-Teller distortions can lower the energy of molecules in such states.