Jahn-Teller effect influence

A dominant influence of certain electronic configurations on the structure type is represented by the Jahn-Teller effect (173). It shall be discussed here only briefly. For further information the reader is referred to a review by Or gel and Dunitz (241) and as for the general base of ligand field theory to the books of Orgel (240) and Figgis (101). 

Considering the influence of electronic configurations on crystal structures it may be asked, whether certain structure t5rpes are restricted to fluorine compounds of the transition elements. Apart from the structure types distorted by the Jahn-Teller effect such a limitation is not obvious at all. On the contrary quite a number of structure prototypes are represented by compounds of the main group elements. Bonding thus must be similar in both, main group and transition element fluorides, at least as for the factors that influence crystal structmes. 

Fig. 7. The influence of the Jahn-Teller effect on bond lengths (A) of copper sulfate trihydrate (top) and pentahydrate (bottom). Note that in the trihydrate one of the Cu(II)-"OSOs bonds is short so that all equatorial Cu(Il)-"0 bonds are 1.94-1.98 A, while the axial bonds are 2.40-2.45 A.

IX. Difficult Metals Jahn-Teller Effects in Cu(II) and the trans Influence in Pt(II)... 

For those systems we have studied so far, many classical ligand field features are successfully captured by LFMM e.g., the double hump variation of structural and thermodynamic properties due to the LFSE (73), o- (36,58,78) and -type (77) Jahn-Teller effects, the trans influence (21), and spin state effects (18,33,59). LFMM is equally at home with small molecules and large proteins and potential future coordination chemistry applications are enormous. 

Because the kinetic energy dissipation of an excess electron by surrounding water molecules plays an essential role during the formation of electron-radical pairs, the influence of the quantum polarization of water molecules and OH radical must be investigated in detail. Further experimental studies on the short-time dependence of vibronic couplings in aqueous environment would permit to understand the contribution of Jahn-Teller effects on the crossing of an elementary redox reaction with OH radical. 

In these molecular type lattices the structure of the CuN402 chromophore is independent of the structure of the host lattice, and the effect is referred to as the noncooperative Jahn-Teller effect.514 It applies to the vast majority of low symmetry copper(II) complexes involving organic type ligands, but even here cooperative effects that influence the ESR properties, such... 

The site preferences shown by cations in the spinel structure demonstrate that transition metal ions prefer coordination sites that bestow on them greatest electronic stability. In addition, certain cations deform their surrounding in order to attain enhanced stability by the Jahn-Teller effect. These two features suggest that similar factors may operate and cause enrichments of cations in specific sites in silicate structures, leading to cation ordering or intersite (intracrystalline ) partitioning within individual minerals which, in turn, may influence distribution coefficients of cations between coexisting phases. 

Even with an assumed or experimentally determined knowledge of the electronic ground state (e. g., by ligand field spectroscopy) modeling of specific electronic effects due to partly filled d-orbitals such as Jahn-Teller effects, trans influences and 7r-backbonding is not trivial. However, if molecular mechanics is used as a technique for the approximate calculation of energy surfaces with a set of functions and corresponding parameters that have been derived from experimental... 

As a consequence of the non-spherical symmetry of the copper(II) ion, d9 configuration, and of the influence of the Jahn-Teller, and pseudo Jahn-Teller effect on six-coordinate geometries, the stereochemistries of the copper(II) ion are characterized by non-rigid geometries (fluxional behaviour), and ranges of distorted geometries (Plasticity Effect). The latter may be connected by a series of Structural Pathways, which may be characterised by an Electronic Criterion of Stereochemistry for a related series of complexes, e.g. the [Cu(bipy)2X] [Y] complexes. 

Bersuker [73-75] has shown the need for a more sophisticated approach to account for such phenomena. We attempt to convey at least the flavor of his ideas here. Jahn-Teller distortions are of a dynamic nature in systems under no external influence. This means that there may be many minimum-energy distorted structures in such systems. Whether an experiment will or will not detect such a dynamic Jahn-Teller effect, depends on the relationship between the time scale of the physical measurement used for the investigation, and the mean lifetime of the distorted configurations. If the time period of the measurement is longer than the mean lifetime of the distorted configurations, only an average structure, corresponding to the undistorted... 

The static Jahn-Teller effect can be observed only in the presence of an external influence. Bersuker [76, 77] stresses this point as the opposite statement is found often in the literature. According to the statement criticized, the effect is not to be expected in systems where low-symmetry perturbations remove electronic degeneracy. However, it is exactly the low-symmetry perturbations that make the Jahn-Teller distortions static and thus observable. Such a low-symmetry perturbation can be the substitution of one ligand by another. In this case one of the previously equivalent minimum-energy structures, or a new one, will become energetically more favored than the others. 

They show also the influence of the Jahn-Teller effect. The emission spectrum shows a vibrational structure that consists of a long progression in V2 (eg), a JT-active mode. This points to a strongly distorted excited state (X). According to Schmidtke et al. (178) the magnitude of the distortion is Ae = -2Ax = -2Ay = 0.2 A, corresponding to a tetragonal Jahn-Teller distortion. 

In this section we discuss the stereochemistry of B-subgroup ions with filled d shells. The most striking feature is that in addition to forming tetra-hedrally and octahedrally coordinated compounds, these ions exhibit a more or less pronounced tendency to occur in linear environments, for example, Ag+ in the linear cation [Ag(NHa)2]+, Au+ in the linearly coordinated infinite chains of Aul, and Hg++ in the discrete molecules of HgCla which exist in both the gas and crystalline phases. We believe that two influences are at work in determining these structures. One is related to the Jahn-Teller effect, and the other to covalent bonding. We shall discuss them in this order and then consider their relative importance. 

Clearly, the extent and influence of the Jahn-Teller effect should be examined in more detail, both theoretically and experimentally. 

Jahn-Teller effect - An interaction of vibrational and electronic motions in a nonlinear molecule which removes the degeneracy of certain electronic energy levels. It can influence the spectrum, crystal structure, and magnetic properties of the substance. 

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