Jahn–Teller-effect-Jahn-Teller效应.doc

本文转自wiki, wiki网络有时不通，仅供网络不便时参考！如何使用版权，请与版权商联系！JahnTeller effectFrom Wikipedia, the free encyclopediaTheJahnTeller effect, sometimes also known asJahnTeller distortion, describes the geometrical distortion ofmoleculesandionsthat is associated with certain electron configurations. Thiselectronic effectis named afterHermann Arthur JahnandEdward Teller, who proved, usinggroup theory, that orbital nonlinear spatially degenerate moleculescannotbe stable.1TheJahnTeller theoremessentially states that any nonlinear molecule with a spatiallydegenerateelectronic ground state will undergo a geometrical distortion that removes that degeneracy, because the distortion lowers the overall energy of the species. For a description of another type of geometrical distortion that occurs in crystals with substitutional impurities see articleoff-center ions.Contentshide 1Transition metal chemistry 2Related effects 3See also 4References 5External linksTransition metal chemistryeditThe JahnTeller effect is responsible for the tetragonal distortion of the hexaaquacopper(II) complex ion, Cu(OH2)62+, which might otherwise possessoctahedralgeometry. The two axial CuO distances are 238pm, whereas the four equatorial CuO distances are 195 pm.A recent study shows that the copper(II) ion coordinates five water molecules in an elongated square pyramid with four Cu-Oeq bonds (2x1.98and 2x1.95 ) and a long Cu-Oax bond (2.35 ). The four equatorial ligands were distorted from the mean equatorial plane by 17 degrees.2The JahnTeller effect is most often encountered inoctahedral complexesof the transition metals.3The phenomenon is very common in six-coordinatecopper(II) complexes.4Thed9electronic configuration of thisiongives three electrons in the two degenerateegorbitals, leading to a doubly degenerate electronic ground state. Such complexes distort along one of the molecular fourfold axes (always labelled thezaxis), 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 thezaxis, but occasionally occurs as a shortening of these bonds instead (the JahnTeller 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 theLewis basicligand and any electrons in orbitals with azcomponent, thus lowering the energy of the complex. If the undistorted complex would be expected to have an inversion centre, this is preserved after the distortion.In octahedral complexes, the JahnTeller effect is most pronounced when an odd number of electrons occupy theegorbitals. This situation arises in complexes with the configurationsd9,low-spind7orhigh-spind4complexes, all of which have doubly degenerate ground states. In such compounds theegorbitals 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 thet2gorbitals (i.e.configurations such asd1ord2, 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 thet2gorbitals, which do not pointdirectlyat 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:JahnTeller effectNumber of d electrons12345678910High/Low SpinHSLSHSLSHSLSHSLSStrength of J-T Effectwwswwwwssw: weak JahnTeller effect (t2gorbitals unevenly occupied)s: strong JahnTeller effect expected (egorbitals unevenly occupied)blank: no JahnTeller effect expected.The JahnTeller effect is manifested in theUV-VISabsorbance spectra of some compounds, where it often causes splitting of bands. It is readily apparent in the structures of many copper(II) complexes.2Additional, 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-temperatureelectron spin resonancespectra.The JahnTeller effect forces the radical anion of cyclooctatetraene (1) to be non-symmetric (see text)Related effectseditThe underlying cause of the JahnTeller effect is the presence ofmolecular orbitalswhich are bothdegenerateandopen shell. This situation is not unique to coordination complexes and can be encountered in other areas of chemistry. In organic chemistry the phenomenon ofantiaromaticityhas the same cause and also often sees molecules distorting; as in the case ofcyclobutadiene5andcyclooctatetraene(COT).6See alsoedit Crystal field theory Potential energy surface Diabatic Conical intersection Avoided crossing Vibronic coupling Renner-Teller effectReferencesedit1. Jump upJahn, H.;Teller, E.(1937). Stability of Polyatomic Molecules in Degenerate Electronic States. I. Orbital Degeneracy.Proceedings of the Royal Society A161(905): 220235.Bibcode:1937RSPSA.161.220J.doi:10.1098/rspa.1937.0142.2. Jump up to:abFrank, Patrick; Benfatto, Maurizio; Szilagyi, Robert K.; DAngelo, Paola; Della Longa, Stefano; Hodgson, Keith O. (2005). The Solution Structure of Cu(aq)2+and Its Implications for Rack-Induced Bonding in Blue Copper Protein Active Sites.Inorganic Chemistry44: 19221933.doi:10.1021/ic0400639.3. Jump