It is readily apparent in the structures of many copper II complexes. This situation is not unique to coordination complexes and can be encountered in other areas of chemistry. In organic chemistry the phenomenon of antiaromaticity has the same cause and also often sees molecules distorting; as in the case of cyclobutadiene  and cyclooctatetraene COT. The argument of Jahn and Teller assumes no details about the electronic structure of the system. Jahn and Teller made no statement about the strength of the effect, which may be so small that it is immeasurable. Indeed, for electrons in non-bonding or weakly bonding molecular orbitals , the effect is expected to be weak.
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Contributors The Jahn-Teller effect is a geometric distortion of a non-linear molecular system that reduces its symmetry and energy. This distortion is typically observed among octahedral complexes where the two axial bonds can be shorter or longer than those of the equatorial bonds.
This effect can also be observed in tetrahedral compounds. This effect is dependent on the electronic state of the system. Introduction In , Hermann Jahn and Edward Teller postulated a theorem stating that "stability and degeneracy are not possible simultaneously unless the molecule is a linear one," in regards to its electronic state. Since , the theorem has been revised which Housecroft and Sharpe have eloquently phrased as "any non-linear molecular system in a degenerate electronic state will be unstable and will undergo distortion to form a system of lower symmetry and lower energy, thereby removing the degeneracy.
For a given octahedral complex, the five d atomic orbitals are split into two degenerate sets when constructing a molecular orbital diagram. When a molecule possesses a degenerate electronic ground state, it will distort to remove the degeneracy and form a lower energy and by consequence, lower symmetry system.
When an octahedral complex exhibits elongation, the axial bonds are longer than the equatorial bonds. For a compression, it is the reverse; the equatorial bonds are longer than the axial bonds. Elongation and compression effects are dictated by the amount of overlap between the metal and ligand orbitals. Thus, this distortion varies greatly depending on the type of metal and ligands.
In general, the stronger the metal-ligand orbital interactions are, the greater the chance for a Jahn-Teller effect to be observed. Jahn-Teller elongations are well-documented for copper II octahedral compounds. This is due to the z-component d orbitals having greater overlap with the ligand orbitals, resulting in the orbitals being higher in energy.
Since the dz2 orbital is antibonding, it is expected to increase in energy due to compression. The dxz and dyz orbitals are still nonbonding, but are destabilized due to the interactions. Electronic Configurations For Jahn-Teller effects to occur in transition metals there must be degeneracy in either the t2g or eg orbitals. Low spin and high spin configurations exist only for the electron counts d4, d5, d6, and d7. These electronic configurations correspond to a variety of transition metals.
The electron configurations highlighted in red d3, high spin d5, d8, and d10 do not exhibit Jahn-Teller distortions. This is primarily caused by the occupation of these high energy orbitals. Since the system is more stable with a lower energy configuration, the degeneracy of the eg set is broken, the symmetry is reduced, and occupations at lower energy orbitals occur.
Spectroscopic Observation Jahn-Teller distortions can be observed using a variety of spectroscopic techniques. Consider a hypothetical molecule with octahedral symmetry showing a single absorption band. The red arrows indicate electronic transitions.
A similar phenomenon can be seen with IR and Raman vibrational spectroscopy. The number of vibrational modes for a molecule can be calculated using the 3n - 6 rule or 3n - 5 for linear geometry rule.
If a molecule exhibits an Oh symmetry point group, it will have fewer bands than that of a Jahn-Teller distorted molecule with D4h symmetry. This effect can also be observed in EPR experiments as long as there is at least one unpaired electron.
References Jahn, H. London A, , , DOI: Inorganic Chemistry. Prentice Hall, 3rd Ed. Metal-ligand bonding. The Open University, , p. ISBN P. Miller, P. Lenhert and M.
Joesten, Inorg. Wood, C. Keijzers and R. Day, Acta Crystallogr. Joesten, M. Hussain and P. Lenhert, Inorg. Practice Questions Why do d3 complexes not show Jahn-Teller distortions? Does the spin system high spin v. What spectroscopic method would one utilize in order to observe Jahn-Teller distortions in a diamagnetic molecule?
What spectroscopic method s would one utilize in order to observe Jahn-Teller distortions in a paramagnetic molecule? Why are Jahn-Teller effects most prevalent in inorganic transition metal compounds? Answers Complexes with d3 electron configurations do not show Jahn-Teller distortions because there is no ground state degeneracy.
Examining the d5 electron configuration, one finds that the high spin scenario contains all singly occupied d orbitals no degeneracy. However, the low spin d5 electron configuration shows degeneracy, which then leads to possible Jahn-Teller effects. UV-VIS absorption spectroscopy is one of the most common techniques for observing these effects. In general, it is independent of magnetism diamagnetic v.
Inorganic, specifically transition metal, complexes are most prevalent in showing Jahn-Teller distortions due to the availability of d orbitals. The most common geometry that the Jahn-Teller effect is observed is in octahedral complexes see Figures 2, 4, 5 and 6 above due to the splitting of d orbitals into two degenerate sets. Due to stabilization, the degeneracies are removed, making a lower symmetry and lower energy molecule.