page_title: Jahn–Teller effect
section_title: Transition metal chemistry
hierarchical_section_title: Jahn–Teller effect / Simplified overview / Transition metal chemistry
caption_attribution_description: Ball-and-stick model of the hexaaquacopper(II) complex, [Cu(H2O)6]2+. Image generated in Accelrys DS Visualizer.
context_page_description: The Jahn–Teller effect is an important mechanism of spontaneous symmetry breaking in molecular and solid-state systems which has far-reaching consequences in different fields, and is responsible for a variety of phenomena in spectroscopy, stereochemistry, crystal chemistry, molecular and solid-state physics, and materials science. The effect is named for Hermann Arthur Jahn and Edward Teller, who first reported studies about it in 1937.
context_section_description: The Jahn–Teller effect is most often encountered in octahedral complexes of the transition metals.  The phenomenon is very common in six-coordinate copper(II) complexes. The d⁹ electronic configuration of this ion gives three electrons in the two degenerate eg orbitals, leading to a doubly degenerate electronic ground state. Such complexes distort along one of the molecular fourfold axes (always labelled the z axis), which has the effect of removing the orbital and electronic degeneracies and lowering the overall energy. The distortion normally takes the form of elongating the bonds to the ligands lying along the z axis, but occasionally occurs as a shortening of these bonds instead (the Jahn–Teller theorem does not predict the direction of the distortion, only the presence of an unstable geometry). When such an elongation occurs, the effect is to lower the electrostatic repulsion between the electron-pair on the Lewis basic ligand and any electrons in orbitals with a z component, thus lowering the energy of the complex. The inversion centre is preserved after the distortion.
  In octahedral complexes, the Jahn–Teller effect is most pronounced when an odd number of electrons occupy the eg orbitals. This situation arises in complexes with the configurations d⁹, low-spin d⁷ or high-spin d⁴ complexes, all of which have doubly degenerate ground states. In such compounds the eg orbitals involved in the degeneracy point directly at the ligands, so distortion can result in a large energetic stabilisation. Strictly speaking, the effect also occurs when there is a degeneracy due to the electrons in the t2g orbitals (i.e. configurations such as d¹ or d², both of which are triply degenerate). In such cases, however, the effect is much less noticeable, because there is a much smaller lowering of repulsion on taking ligands further away from the t2g orbitals, which do not point directly at the ligands (see the table below).  The same is true in tetrahedral complexes (e.g. manganate: distortion is very subtle because there is less stabilisation to be gained because the ligands are not pointing directly at the orbitals.
  The expected effects for octahedral coordination are given in the following table:
  w: weak Jahn–Teller effect (t2g orbitals unevenly occupied)
  s: strong Jahn–Teller effect expected (eg orbitals unevenly occupied)
  blank: no Jahn–Teller effect expected.
  The Jahn–Teller effect is manifested in the UV-VIS absorbance spectra of some compounds, where it often causes splitting of bands. It is readily apparent in the structures of many copper(II) complexes. Additional, detailed information about the anisotropy of such complexes and the nature of the ligand binding can be however obtained from the fine structure of the low-temperature electron spin resonance spectra.
caption: The Jahn–Teller effect is responsible for the tetragonal distortion of the hexaaquacopper(II) complex ion, [Cu(OH2)6]2+, which might otherwise possess regular octahedral geometry.  The two axial Cu−O distances are 238 pm, whereas the four equatorial Cu−O distances are ~195 pm. This geometry is common in crystal structures; the geometry in solution is uncertain.[1]
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