Local bond model

The localized bond model is often called valence bond theory. 

We cannot generate a tetrahedron by simple overlap of atomic orbitals, because atomic orbitals do not point toward the comers of a tetrahedron. In this section, we present a modification of the localized bond model that accounts for tetrahedral geometry and several other common molecular shapes. 

The n molecular orbitals described so far involve two atoms, so the orbital pictures look the same for the localized bonding model applied to ethylene and the MO approach applied to molecular oxygen. In the organic molecules described in the introduction to this chapter, however, orbitals spread over three or more atoms. Such delocalized n orbitals can form when more than two p orbitals overlap in the appropriate geometry. In this section, we develop a molecular orbital description for three-atom n systems. In the following sections, we apply the results to larger molecules. 

The concept of aromaticity has been extremely fruitful for both theoretical and experimental organic chemists. Aromatic compounds are cyclic unsaturated molecules characterized by certain magnetic effects and by substantially lower chemical reactivity and greater thermodynamic stability than would be expected from localized bond models. 

Hoffmann and co-workers have used qualitative perturbation theory to analyze systems in which the properties deduced from a standard localized bond model are substantially modified by consideration of the energy changes that occur when the model is improved by allowing interactions among localized orbitals of the same symmetry.11... 

Whether or not the localized bond model will be sufficiently accurate in any given case can of course be determined only by experiment, for existing a priori quantum chemical methods cannot be applied to problems of such complexity. Experiment6 implies that the localized bond model works well for all classical molecules, i.e. molecules for which only single classical structures can be written. Here the localization is probably assisted by correlation effects that tend to concentrate the electrons in pairs in definite two-center bonds. 

The localized bond model fails for odd conjugated systems (ions and radicals such as allyl or benzyl) the mesomeric stabilization of these can be estimated by the perturbation treatment of Section VIII. However, the localized bond model can still4 be used for other types of odd systems, i.e. ones where the... 

Exercise 2.4. Use the localized-bond model to rationalize the dipole moment exhibited by B5H9 (1.7 D). 

To summarize this long section on metal effects, we can state metal clusters can mimic main-group clusters (late-metal clusters with acceptor ligands) metal clusters with four-connect or higher vertices can be described with localized bond models (early-metal clusters with donor ligands) and metal clusters can have reduced (Pt clusters) or no tangential bonding (Au clusters). The characteristically more... 

If we try to apply our semi-empirical LCAO-MO method to metals, we get poor results. Taking lithium metal as an example, and using two Li atoms as our unit (just as for C, Si and Ge), we have only two electrons to fill up the four possible MOs. The cohesive energy is only P instead of 4/ , the same as it is for the diatomic molecule Li2. This gives AEcov — 25 kcal/mol, using Table 5.2. The value of A exp is 77 kcal/mol. A localized bonding model is not useful for the metals. We must exploit the new features that result because a solid is a very large molecule indeed. 

An additional feature of the localized bond model emerges. Small crystallites of metals exist as pseudospherical cubo octahedra as shown in Fig. 4.10. 

These compounds provide some of the closest analogues to the transition metal organometallics. Though the stoichiometries and presumed symmetries of the gas phase molecules vary, their photoelectron spectra may be analysed by a common localized bond model. The range of compounds studied and their ionization energies are given in Table 10, and some representative spectra in Fig. 13. 

The localized bond model is adopted for cr bonds and isolated tt bonds (5, 6). 

A ligogenic process is one in which distinct a or a/n (but not just n) bonding takes place between two unbonded atoms. For canonical structures, one uses localized bonding models. In a ligogenic process, the bond order between two reactive atoms starts at 0 and increases to integral... 

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