text: It's Jahn-Teller effect, not John Teller.\n\nJahn-Teller distortions occur for transition metal complexes where by distortion of the molecule can result in a lower energy state for the molecule.\n\nIn transition metal chemistry, bonds from ligands to the central metal atom are coordinate covalent, i.e. both electrons come from one atom.  The orbitals which contain those electrons bond with usually the dz2 and dx2-y2 orbitals from the metal, but are lower in energy, so the orbitals based on the metal are essentially anti-bonding orbitals.  The dxz, dyz, and dxy orbitals are mostly non-bonding orbitals, but their energies do go up somewhat upon bonding interaction.  In a typical octahedral bonding arrangement, the dz2 and dx2-y2 are degenate, as are the dxz, dyz, and dxy orbitals (they have the same energy).  In certain electron configurations (the classic example is d9), if the system can make those orbitals non-degenerate by lengthening or shortening bonding interactions along a certain axis, it can lower the overall energy by putting electrons in lower energy orbitals, which is why you will often see Cu2+ complexes where some bond distances are longer than others.