Valence-bond description

Figure 5-20. Reaction coordinate diagram generated from a valence bond description of initial and final state configurations.

This valence bond description leads to an interesting conclusion. Because the transition state occurs at the point where the initial and final state VB configurations cross, the transition state receives equal contributions from each. This is so whether the transition state is early or late. Thus, the nucleophile Y and the leaving group X possess about equal charge densities in the transition state. This conclusion means that an early transition state is not (in this sense) reactantlike , for a reactantlike transition state should have most of the charge on Y. Similarly, a late transition state is not necessarily productlike. This view is at variance with other interpretations. 

Cooper, D. L., Gerratt, J., and Raimondi, M. The Spin-Coupled Valence Bond Description of Benzenoid Aromatic Molecules. 153, 41-56 (1990). 

We are now ready to account for the bonding in methane. In the promoted, hybridized atom each of the electrons in the four sp3 hybrid orbitals can pair with an electron in a hydrogen ls-orbital. Their overlapping orbitals form four o-bonds that point toward the corners of a tetrahedron (Fig. 3.14). The valence-bond description is now consistent with experimental data on molecular geometry. 

According to Lewis s approach and valence-bond theory, we should describe the bonding in 02 as having all the electrons paired. However, oxygen is a paramagnetic gas (Fig. 3.24 and Box 3.2), and paramagnetism is a property of unpaired electrons. The paramagnetism of 02 therefore contradicts both the Lewis structure and the valence-bond description of the molecule. 

A SIMPLE VALENCE BOND DESCRIPTION OF COORDINATE BONDS... 

A Simple Valence Bond Description of Coordinate Bonds 593... 

Polar and apolar covalency NBO and valence-bond descriptions... 

All lone pair orbitals have a node between the two atoms and, hence, have a slightly antibonding character. This destabilizing effect of the lone pair localized molecular orbitals corresponds to the nonbonded repulsions between lone pair atomic orbitals in the valence bond theory. In the MO theory all bonding and antibonding resonance effects can be described as sums of contributions from orthogonal molecular orbitals. Hence, the nonbonded repulsions appear here as intra-orbital antibonding effects in contrast to the valence-bond description. 

Fig. 2.1-17. Valence bond description of the aradino-[B3H6(PMe3)2][B3H8].

Definition A distonic ion is a positive radical ion, which would formally arise by ionization of a zwitterion or a diradical, by isomerization or fragmentation of a classical molecular ion, or by ion-molecule reactions. Consequently, distonic ions have charge and radical at separate atoms in a conventional valence bond description. [42,43]... 

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