Jahn-Teller unstable

Octahedral complexes of Cu11 are Jahn—Teller unstable, so that the formal Eg and terms are split. The spectra do not usually correspond to the simple 2T2g(t2g) -2Eg(eg) excitation,38,67,100 but rather to one based upon Figure 23. The Cu(H20)62+ ion is almost an exception the band at 12000 cm-1 (e as 10) is broadened to —6000 cm-1 but structure is not resolved. Often the splitting is considerable Cu(bipy)32+, for example, shows bands at 15000 and at 6000 cm-1, which must be interpreted with the aid of a diagram of the type of Figure 23, but for trigonal symmetry, as 2AX -2E and 2E<-2E. 

This idea is a failure of simple resonance theory, not of VB theory. Taking into account the sign of the matrix element (overlap) between the five VB structures shows that singlet CsH5+ is Jahn—Teller unstable, and the ground state is in fact the triplet state. As shown later in Chapter 5, this is generally the case for all of the antiaromatic ionic species having 4n electrons over 4n + 1 or 4n - 1 centers (61). 

Investigation of 1 upon removal of an electron has intrigued both theoreticians and experimentalists because the properties of the formed cyclopropyl radical cation provide a basis to test MO models of three-membered rings (the Walsh MO model cannot explain the properties of the cyclopropyl radical cation ). Ejection of an electron from the 3e MOs of 1 (MOs 11,12 in Figure 6) leads to the Jahn-Teller unstable electron configuration (see Section X), which can be stabilized by distortion to the C2v-symmet-rical states and 82 (first-order Jahn-Teller effect, see Figure 23). 

However, most impurities and defects are Jahn-Teller unstable at high-symmetry sites or/and react covalently with the host crystal much more strongly than interstitial copper. The latter is obviously the case for substitutional impurities, but also for interstitials such as 0 (which sits at a relaxed, puckered bond-centred site in Si), H (which bridges a host atom-host atom bond in many semiconductors) or the self-interstitial (which often forms more exotic structures such as the split-(llO) configuration). Such point defects migrate by breaking and re-forming bonds with their host, and phonons play an important role in such processes. 

Furthermore, the unusual valence state of Co" or on fully charged LiCoO or LiNiOj and Jahn-Teller-unstable Mn high-spin state on the dis-... 

An epikernel is an intermediate subgroup in the decomposition scheme of a given point group. The epikernel principle states that the preferred distortions of Jahn-Teller unstable molecules are directed towards the maximal allowed epikernels of the undistorted parent group. The group theoretical foundations of this principle are explained, and a wide variety of applications in different areas of chemistry is discussed. 

Thermal interconversion of these d -MLs systems will be quite complicated. It is clear that going from the far ri t in 17.25 to the Dsf, isomer will be energetically difficult. Not only do the two electrons in the hj level rise to high energy, but the Diif geometry is Jahn-Teller unstable. A more favorable way for the thermal interconversion is illustrated in 17.26>17.28. One of the four CO ligands is bent toward... 

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