Jahn-Teller distorted cations

The best known transition-metal cation distortions are those found around 

Cu and Mn, systems that have partially filled d shells with a degenerate ground state. According to the Jahn-Teller theorem, any degenerate electronic system will spontaneously distort in such a way as to remove the degeneracy (Dunitz and Orgel 1960). The distortions around Cu have been extensively studied and may play a role in the superconductivity found in some copper oxides (Section 13.3.2), while the distortions around Mn + play an important role in the potentially useful magnetoresistive properties of LaMnOs (Section 13.3.3). 

Since the d orbitals have quadrupolar character, this distortion must be modelled using a point electric quadrupole with the shape of the d i orbital oriented with a negative pole along the z axis and the positive pole in the x-y equatorial plane (Fig. 8.7(a)). No fluxes have been calculated for Jahn-Teller distorted cations, either with or without the quadrupolar contribution but, as in the other cases, the experimental bond valences are expected to provide a good approximation to the bond fluxes. 

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Lufaso MW, Woodward PM. Jahn-Teller distortions, cation ordering and octahedral tilting in perovskites. Acta Crystallographica Section B Structural Science 2004 60 10-20. 

Instead the allyl cation 97 was obtained. The reported experimental 13C NMR data (<513C(exp) 8 — 250/243, 153, 60 ppm) are in agreement with the cyclopentenyl cation structure 97, but differ significantly from the calculated 13C NMR chemical shift for both Jahn-Teller distorted valence isomeric cyclopentadienyl cation structures 95 and 96. 

A number of empirical studies have attempted to define the boundaries that indicate which metal ions can form LDHs. For example, the roles of the absolute values of ionic radii and the difference in ionic radii of m and [3,17-21] as well as the values of the solubiUty products of m (OH)2 and M COs [22] have been speculated on in the literature, hi earlier publications [1,23], it was often stated that ions such as Cu " that are subject to a Jahn-Teller distortion do not form LDHs unless diluted by other cations [24], but more recently examples of Cu -containing LDHs such as [Cuo.69Cro.3i(OH)2]Clo,3i O.6IH2O [25,26] have been well characterized (see Sect. 3.2.2), although the material does show a corrugation of the sheets associated with the Jahn-Teller distortion around individual Cu centers as discussed in Sect. 3.4. 

From published photoelectron data for these compounds we can infer that cations formed under these conditions will carry some excess internal energy. The photoelectron spectrum of borazine indicates no significant change in the structure of the cation relative to the precursor although a slight Jahn-Teller distortion is possible. The alternate cation structure... 

It is well known that crystal and electronic structures are interdependent and define the reactivity of chemical substances. In Section 1.4.2, it was noted that copper-porphyrin complex gives cation-radicals with significant reactivity at the molecular periphery. This reactivity appears to be that of nucleophilic attack on this cation-radical, which belongs to n-type. The literature sources note, however, some differences in the reactivity of individual positions. A frequently observed feature in these n-cation derivatives is the appearance of an alternating bond distance pattern in the inner ring of porphyrin consistent with a localized structure rather than the delocalized structure usually ascribed to cation-radical. A pseudo Jahn-Teller distortion has been named as a possible cause of this alternation, and it was revealed by X-ray diffraction method (Scheidt 2001). 

Wade expanded the 1971 hypothesis to incorporate metal hydrocarbon 7T complexes, electron-rich aromatic ring systems, and aspects of transition metal cluster compounds [a parallel that had previously been noted by Corbett 19) for cationic bismuth clusters]. Rudolph and Pretzer chose to emphasize the redox nature of the closo, nido, and arachno interconversions within a given size framework, and based the attendant opening of the deltahedron after reduction (diagonally downward from left to right in Fig. 1) on first- and second-order Jahn-Teller distortions 115, 123). Rudolph and Pretzer have also successfully utilized the author s approach to predict the most stable configuration of SB9H9 (1-25) 115) and other thiaboranes. 

The cyclopropane radical cation can be prepared by y-radiolysis in rigid matrices. At temperatures as low as 4.2 K, its ESR spectrum shows evidence for static Jahn-Teller distortion, resulting in a structure of the (ring-closed) trimethylene ( Ai) type, r " ... 

Figure 6.9. Top to bottom Two possible Jahn-Teller distorted geometries for the methane radical cation and calculated geometry for its energy minimum schematic representation of the SOMO for ethane radical cation SOMOs of three different propane radical cations observed by ESR the SOMO of butane radical cation and SOMOs for two conformers of pentane radical cation.

FIGURE 23. First-order Jahn-Teller distortion of Dn symmetrical cyclopropyl radical cation. Geometries [UMP2/6-31G(d) calculations] and relative energy [CISD/6-31G(d)] from Reference 231... 

Krogh-Jespersen and Roth231 also investigated Jahn-Teller distortions in mono-, di- and tetramethyl substituted cyclopropyl radical cations. In all cases, the ground state of the corresponding radical cation can be considered to be derived from a 2 4, -type state and (the) transition state(s) of pseudorotation from (a) 2B2-type state(s). Several minima and transition states were found along the pseudorotation path where the most stable form corresponds to an asymmetrical2A, -type state. 

The structure of diphosphallenic radical cations, generated from the allene ArP=C=PAr by electrochemical oxidation, has been examined using EPR spectroscopy. Ab initio calculations including correlation effects at the MP2 and MCSCF levels have determined that two rotamers exist compatible with Jahn-Teller distortion of the allene.146 Anodically generated radical cations of alkyl phosphites [(RO P] and silylphosphites [(RO)2POSiMe3] reacted with alkenes by initial attack at the C=C bond followed by electron transfer, deprotonation, and elimination of an alkyl or trimethylsilyl cation to form identical alkyl phosphate adducts.147 The electron ionization-induced McLafferty rearrangement of n-hexylphosphine afford the a-distonic radical cation CTEPH, the distinct reactivity of which suggests there is no... 

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