The most iconic and prominent of the JT systems in coordination chemistry is probably the case of Cu(II) octahedral complexes. While in perfectly equivalent coordination, like a CuF 6 complex associated to a Cu(II) impurity in a cubic crystal like KMgF 3, perfect octahedral (O h) symmetry is expected. In fact a lower tetragonal symmetry is usually found experimentally. The origin of this JTE distortion it revealed by examining the electronic configuration of the undistorted complex. For an octahedral geometry, the five 3d orbitals partition into t 2g and e g orbitals (see diagram). These orbitals are occupied by nine electrons corresponding to the d 9 electronic configuration of Cu(II). Thus, the t 2g shell is filled, and the e g shell contains 3 electrons. Overall the unpaired electron produces a E g state, which is Jahn–Teller active. The third electron can occupy either of the orbitals comprising the e g shell: the mainly 3 z 2 − − r 2 orbital or the mainly x 2 − − y 2 orbital. If the electron occupies the mainly 3 z 2 − − r 2 level, which antibonding orbital the final geometry of the complex would be elongated as the axial ligands will be pushed away to reduce the global energy of the system. On the other hand, if the electron went into the mainly x 2 − − y 2 antibonding orbital the complex would distort into a compressed geometry. Experimentally elongated geometries are overwhelmingly observed and this fact has been attributed both to metal-ligand anharmonic interactions and 3d-4s hybridisations. Given that all the directions containing a fourfold axis are equivalent the distortion is equally likely to happen in any of these orientations. From the electronic point of view this means that the 3 z 2 − − r 2 and x 2 − − y 2 orbitals, that are degenerate and free to hybridise in the octahedral geometry, will mix to produce appropriate equivalent orbitals in each direction like 3 x 2 − − r 2 or 3 y 2 − − r 2 .