Transitions bonding characteristics

The interaction between two adjacent bulky groups can depend on steric factors which are not necessarily related to the stability of the radicals produced on homolysis. It is estimated from linear free energy relationships that only 65-70% of the ground-state strain energy is relieved in the transition state for homolysis of a bond between two quaternary centres (Ruchardt and Beckhaus, 1980, 1986). Thus steric constraints to delocalization in the radicals produced may persist. A pertinent example is 2,3-di(l-adamantyl)-2,3-dimethylbutane [123] which has four such centres, linked by the long C-C bonds characteristic of this sort of structure. The strongest... 

This chapter summarizes recent developments in the expanding field of electron-deficient compounds having from three up to 13 skeletal boron and carbon atoms. In particular, the focus will be on the transition of classical organoboranes into non-classical compounds. Therefore, we first want to briefly review electron counting rules and bonding characteristics of these classes. For a more thorough discussion see Chapter 1 by King and Schleyer. 

An alternative and more likely interpretation would be that the electron pairs of the double bond are equivalent and both participate in the transition. This would correspond to a charge migration around a two-membered ring system, the transition being forbidden , as in the case of the B-bands, only by the comparatively small transition moment. These qualitatively similar interpretations account for the small intensities and low transition energies characteristic of the... 

Similar results have been obtained for a variety of other analogs with poor hydrogen bonding characteristics. Apparently shape is most important.666 674 676 6763 Binding strengths in transition state structures must also be considered.1676b... 

Aluminum—tetradentate ligand catalyst system, in epoxide homopolymerization, 11, 601 Aluminum(I) tetrahedra, synthesis, 9, 262 Aluminum(III)-tin exchange, process, 9, 265 Aluminum-transition metal bonds, characteristics, 9, 264 Amavadine, for alkane carboxylations, 10, 234—235 Ambruticin S, via ring-closing diene metathesis, 11, 218 Amide-allenes, cyclizations, 10, 718 Amide ether complexes, with Zr(IV) and Hf(IV), 4, 783 Amide hybrid ligands, in organometallic synthesis, 1, 64 Amides... 

Ever since this breakthrough, C60 has been intensively studied and has shown its rich bonding characteristics in various environments, from van der Waals cohesion to ionic and metallic cohesions, and even to the extent of covalent bonding, together also with its rich electronic properties including high transition-temperature (Tc) superconductivity in alkali-doped C60 fullerides. This article reviews the first-principles quantum-mechanical studies of these electronic and bonding properties of C60. 

Before the molybdenum catalysts can be used in hydrotreating reactions, they must be sulfided, and Raman spectroscopy sensitively reveals this transition. The characteristic Mo-S frequencies of M0S2 are at 389 and 411 cm-1 [51], much lower than the Mo-O bands. This occurs because, first, sulfur is twice as heavy as oxygen and, second, the Mo-S bond is weaker than the Mo-O bond [see Eq. (8-3)]. The Raman spectra of a sulfided M0O3/AI2O3 also show a band at 529 cm-1, due to the (S-S)2 disulfide species, which is probably located at the edges of the... 

In earlier chapters, allusions were made to die effects of covalent bonding. For example, covalent interactions were invoked to account for the intensification of absorption bands in crystal field spectra when transition metal ions occupy tetrahedral sites ( 3.7.1) patterns of cation ordering for some transition metal ions in silicate crystal structures imply that covalency influences the intracrystalline (or intersite) partitioning of these cations ( 6.8.4) and, the apparent failure of the Goldschmidt Rules to accurately predict the fractionation of transition elements during magmatic crystallization was attributed to covalent bonding characteristics of these cations ( 8.3.2). 

Trifluorophosphine and carbon monoxide readily undergo ligand-exchange reactions in their transition metal complexes. The close similarity in bonding characteristics of the two ligands toward transition metals has been discussed extensively in several review articles (72,174,272) and the evidence will not be repeated here. Extensive vibrational spectroscopic studies have been made on mixed carbonyl-PF3 metal complexes (72,174) and force constant calculations have been carried out in some cases. 

Each compound will have particular bonding characteristics, which will generate different values of kmax- As noted earlier, bonding depends on the behaviour of electrons and electronic transitions. These electronic transitions will be observed at different wavelengths on the absorption spectrum and reveal important features of the compounds and chromophores being studied. Values of kmax and absorbance can be taken from the absorbance spectmm, and from this the maximum extinction coefficient (smax) can be determined using the Beer-Lambert law equation. Table 5.1 gives information on electronic absorption characteristics of typical chromophores in non-aromatic compounds. 

The lanthanides, elements 58 through 71, constitute a so-called inner transition series, as do the actinides, elements 90 through 103. Scandium (21) and yttrium (39), together with the lanthanides, are traditionally referred to as the rare earth elements. The lanthanides, with 3+ ions and decreasing radii, show strong ionic bonding and weaker covalent bonding characteristics. As discussed below, the lanthanides tend to exhibit hard sphere or A-type behavior in their coordination compounds. 

The [la,3s] sigmatropic shift of the fluorine atom in the 3-fluorpropene system has been also previously discussed in detail. The transition state has been thoroughly characterized in terms of a ion-pair structure with a charge separation of 0.6e, and the changes in the bonding characteristics along the intrinsic reaction coordinate reaction (IRC) path (see Schemes 5a and 5b) were described in terms of the ELF basin properties, i.e. electron populations, variance and delocalization indexes.84... 

Soon after the development of the quantum mechanical model of the atom, physicists such as John H. van Vleck (1928) began to investigate a wave-mechanical concept of the chemical bond. The electronic theories of valency, polarity, quantum numbers, and electron distributions in atoms were described, and the valence bond approximation, which depicts covalent bonding in molecules, was built upon these principles. In 1939, Linus Pauling s Nature of the Chemical Bond offered valence bond theory (VBT) as a plausible explanation for bonding in transition metal complexes. His application of VBT to transition metal complexes was supported by Bjerrum s work on stability that suggested electrostatics alone could not account for all bonding characteristics. 

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