Jahn–Teller effect

The Jahn -Teller effect, discovered in 1937, going to the scientists Edward Teller ( 1908-2003 ) and Hermann Arthur Jahn ( 1907-1979 ) back. The Jahn- Teller theorem explains the distortion in the geometry of the ligand field of some octahedral complexes along a spatial axis.

The theorem states:

The Jahn -Teller effect can also be observed in complexes with tetrahedral structure, but leads to no structural changes. In particular, complexes with weak ligand fields appear susceptible to Jahn -Teller distortion, so you in the strong ligand field only in octahedral d7 low-spin complexes ( t62ge1g ) can observe a strong Jahn -Teller distortion.

Two of the ligands take you through a distortion ( Jahn -Teller distortion ) of greater or lesser distance from the central atom as a ligand in the equatorial plane of the complex compound. The square planar coordination can be seen as an extreme case of a Jahn -Teller distortion when the ligands are removed along the z- axis infinitely far from the central atom.

The distortion proves on the affected complexes as energetically favorable, since the occupied molecular orbitals are lowered in energy. Unoccupied molecular orbitals are raised in energy. Due to the energy splitting of the molecular orbitals, the degeneracy is lost.

A Jahn -Teller distortion ( stretching or compression ) occurs when there are groups of d states due to the ligand field splitting, so that there is more than one way of electron arrangement and this electron arrangement energetically equivalent, ie it is energetically degenerate. Since degenerate states are unstable, the symmetry of the structure is removed by the distortion; are thereby orbitals of different energy from the partially occupied orbitals of the same energy. This means an energy gain, because only the lower energy orbitals are occupied. A distortion is, for example, for the electron configurations d4 high-spin, and at d7 low-spin and d9 to be expected.

Is now a high-spin complex before, for example, d4 ( t32ge1g ), then the fourth d- electron either the dx ² -y ² - or the same energy dz ² orbital occupy. If the dx ² -y ² orbital occupied, the four equatorial ligands are arranged repelled, resulting in a compression of the octahedron result. If, however, occupied the dz ² orbital, so only the two axially arranged ligands are repelled, resulting in a stretching of the octahedron in the z- direction. In both cases, the occupation of the orbitals leads to a lowered though not very large energy gain, which is called the Jahn -Teller stabilization energy. Whether it comes to stretching or compression, depends among other things also the counterion. For example, one finds a [Cu (NO ) 6] 4 - complex, depending on the counterion partly as octahedral - compressed or as an octahedral complex - crocheted stretch again. Others, such as [Cu ( py ') 6 ] 2 py ' = pyridine oxide even have a fluctuating between the two forms construction.

A very similar mechanism also occurs in one-dimensional chains of atoms and in solids, it is then referred to as the Peierls distortion.

As an example of such a distortion in organic chemistry can be the Antiaromat Cyclobutadiene use. (See Frost Musulin diagram)

Follow

Between the split by the Jahn -Teller effect energy levels may be caused by light to electron transitions, the light of the appropriate wavelength is absorbed. This mechanism is mainly responsible for the color of various minerals, such as the green coloring of malachite and azurite blue color.