Bonding considerations valence bond theory

Another approach is spin-coupled valence bond theory, which divides the electrons into two sets core electrons, which are described by doubly occupied orthogonal orbitals, and active electrons, which occupy singly occupied non-orthogonal orbitals. Both types of orbital are expressed in the usual way as a linear combination of basis functions. The overall wavefunction is completed by two spin fimctions one that describes the coupling of the spins of the core electrons and one that deals with the active electrons. The choice of spin function for these active electrons is a key component of the theory [Gerratt ef al. 1997]. One of the distinctive features of this theory is that a considerable amount of chemically significant electronic correlation is incorporated into the wavefunction, giving an accuracy comparable to CASSCF. An additional benefit is that the orbitals tend to be... 

Delocalization occurs in molecules wherever it is possible by symmetry considerations and wherever an energetic advantage can be gained from its operation. MO theory deals very satisfactorily with delocalization, but with valence bond theory the concept is somewhat clumsily incorporated as an addition to the conventional two-electron, two-centre bonding. In general, the stability conferred upon a molecule by delocalization is because the orbitals are more extensive, so that interelectronic repulsion is minimized. 

In the case of tt complexes of substituted cyclopentadienones, such as the iron tricarbonyl derivatives prepared by Weiss and H libel (30), qualitative molecular-orbital theory (20) predicted a considerable reduction of the ketonic carbonyl bond order. It was observed that the ketonic carbonyl frequency dropped by as much as 65 cm-1, in agreement with theory. A similar explanation can also be provided in terms of valence bond theory (Fig. 14). It has been suggested that n complexing of arenes such as benzene results in loss of aromaticity of the ring in contrast to the dicyclopentadienyl... 

As a tribute to Pauling s contributions, I shall restate and summarize some of the implications for bonding theory that arise when the three-electron bond is incorporated as a mainstream component for VB descriptions of the electronic structures of electron-rich molecules. Attention will be focussed on increased-valence structures for molecular systems that involve four-electron three-centre and six-electron four-centre bonding units. However initially, consideration will be given to the one-electron bond, for which Pauling also provided some attention to both the theory and examples of systems that involve this type of bond in their VB structures. As indicated in ref. [8(a)], experimentally one-electron bonds and three-electron bonds are abundant and well-characterized for odd-electron systems. 

Valence-bond theory is over 90% successful in explaining much of the descriptive chemistry of ground states. VB theory is therefore particularly popular among chemists, since it makes use of familiar concepts such as chemical bonds between atoms, resonance hybrids and the like. It can perhaps be characterized as a theory which explains but does not predict. Valence-bond theory fails to account for the triplet ground state of O2 or for the bonding in electron-deficient molecules such as diborane, B2H6. It is not very useful in consideration of excited states, hence for spectroscopy. Many of these deficiencies are remedied by molecular orbital theory, which we take up in the next two chapters. 

The valence-bond theory of covalent binding involves the reinterpretation of the Lewis picture of the chemical bond in terms of the more detailed description of the electronic structure of the elements discussed in chapter 2, and immediately new considerations arise. The bond is still conceived as due to a sharing of two electrons, or more precisely to an overlapping of their orbitals, but now sharing is possible only between electrons of opposite spin. It follows at once that electrons already paired in orbitals can play no part in covalent binding, so that for any given element it becomes important to consider which particular orbitals will be available for bond formation. 

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