Valence bond theory
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The valence bond theory is a concept in chemistry that explains the nature of a chemical bond in a molecule <ref>Murrel, JN, Kettle, SF Tedder, JM "The Chemical Bond", John Wiley & Sons (1985) ISBN 0471907596</ref> . It has its origins with G.N. Lewis who in 1916 first proposed that a chemical bond forms by the interaction of two shared bonding electrons, with the representation of molecules as Lewis structures, and with the Heitler-London theory (1927) which for the first time enabled the calculation of properties of hydrogen based on quantum mechanical considerations. Two other key concepts in VB theory are resonance (1928) and orbital hybridization (1930) both developed by Linus Pauling.
[edit] Theory
A valence bond structure has a close relationship to a Lewis structure. Where no single Lewis structure can be written, several valence bond structures are used, each arising from a specific Lewis structure. This combination of valence bond structures is the key idea of resonance theory.
Valence bond theory considers that the overlapping atomic orbitals of the participating atoms form a chemical bond. Due to the overlapping, it is most probable that electrons should be in the bond region. Valence bond theory views bonds as weakly coupled orbitals (small overlap). Valence bond theory is typically easier to employ in ground state molecules.
The overlapping atomic orbitals can be of different types. There are two different types of overlapping orbitals: sigma and pi. Sigma bonds occur when the orbitals of two shared electrons overlap co-axially(i.e head-on overlap). Pi bonds occur when two orbitals overlap laterally(i.e. the side-to-side overlap of p-orbitals). For example, a bond between two s-orbital electrons is a sigma bond, because two spheres are always coaxial. In terms of bond order, single bonds consist of one sigma bond, double bonds consist of one sigma bond and one pi bond, and triple bonds consist of one sigma bond and two pi bonds.
However, the atomic orbitals for bonding may not be "pure" atomic orbitals. Often, the bonding atomic orbitals have a character of several possible types of orbitals. The methods to get an atomic orbital with the proper character for the bonding is called hybridization (also spelled hybridisation). Hybridization can only occur when electrons need to be promoted to the next energy level.
[edit] VB theory today
Valence bond theory now complements Molecular Orbital Theory (MO theory), which does not adhere to the VB idea that electron pairs are localized between two specific atoms in a molecule but that they are distributed in sets of molecular orbitals which can extend over the entire molecule. MO theory can predict magnetic properties in a straight forward manner, but valence bond theory is more complicated although giving the similar results. Valence bond theory views aromatic properties of molecules as due to resonance between Kekule, Dewar and possibly ionic structures, while molecular orbital theory views it as delocalisation of the π-electrons. The underlying mathematics are also more complicated limiting VB treatment to relatively small molecules. On the other hand, VB theory provides a much more accurate picture of the reorganization of electronic charge that takes place when bonds are broken and formed during the course of a chemical reaction. In particular, valence bond theory correctly predicts the dissociation of homonuclear diatomic molecules into separate atoms, while simple molecular orbital theory predicts disociation into a mixture of atoms and ions.
More recently, several groups have developed what is often called modern valence bond theory. This replaces the overlapping atomic orbitals by overlapping valence bond orbitals that are expanded over all basis functions in the molecule. The resulting energies are more competitive with energies where electron correlation is introduced based on a Hartree-Fock reference wavefunction.
