Valence Theory

Covalent Bond Theory (CBT) 

Many theories have been proposed to explain the nature of bonding in coordination compounds. The valence bond (VB) theory is one of them. Valence bond theory was created with the aim of using quantum physics to explain chemical bonds. When building a molecule, much of this theory focuses on forming individual bonds from the atomic orbitals of the contributing atoms. 

What is valence bond theory (VB)? 

The valence bond theory states that in molecular orbitals, the electrons of the molecule occupy the atomic orbitals. In bond formation, the atomic orbitals overlap and the greater the overlap the stronger the bond. 

Metallic bonds are usually covalent in nature and metallic structures carry a resonant bond of electron pairs between each atom and its neighbouring atoms.

History Of Valence Bond Theory 

Lewis’s approach to chemical bonds did not shed any light on chemical bond formation. The valence shell electron-pair repulsion theory (or VSEPR theory) was not applicable to many applications. Also failed to predict the geometry of complex molecules. To address these problems, German physicists Walter Heinrich Heitler and Fritz Wolfgang London proposed the valence bond theory. To explain how the covalent bond formed between two hydrogen atoms, the Schrodinger Wave equation was used.

This theory focuses on the electronic configuration and superposition of atomic orbitals, as well as the hybridization of these orbitals. When atomic orbitals overlap, and electrons are concentrated on the bond region, chemical bonds are formed. Covalent bonds theory can be used to explain the electrical structures of molecules that are formed by superposition of these atomic orbitals. This theory also explains how electrons from other atoms are attracted to the nucleus.

Postulates of valence bond theory 

The main postulates of valence bond theory are listed below. 

• In order to establish a covalent bond, two valence orbitals (half-filled) from two separate atoms need to cross over one another and overlap. Overlapping the two bonding atoms results in an increase of electron density, which in turn increases the stability of the final molecular structure
• An atom’s valence shell contains many unpaired electrons, which allows it to make numerous bonds with other atoms. According to the valence bond theory, the paired electrons in the valence shell do not participate in the creation of chemical bonds
• Chemical covalent bonds have a direction and run parallel to the overlapping atomic orbital region
• The atomic orbitals overlap in a different pattern for pi bonds and sigma bonds, that is pi bonds are produced by sideways overlapping, but in order for two atoms to create sigma bonds, they must overlap along the axis containing the nuclei of the other atom

Covalent bond theory in practice 

• The generation of covalent bonds in many molecules can be explained by the requirement of maximum overlapping as established by the covalent bond theory 
• One of its most essential uses is as follows. For example, the variation in the strength and length of chemical bonds in H2 and F2 molecules changes in their overlapping orbitals can be used to explain variances in their behaviour
• The overlapping 1s orbital of the hydrogen atom and the 2p orbital of the fluorine atom forms a covalent bond in the HF molecule as explained by covalent bond theory.

Valence bond theory – Limitations 

The valence bond theory has several shortcomings:

• It cannot explain the quaternion of carbon
• The electrons’ energies are not specified, and this is a limitation of the experiment
• It is assumed that electrons can be found concentrated in a few distinct areas
• It does not quantify the kinetic or thermodynamic stabilities of coordination compounds
• There is no distinction between ligands that are weak and those that are strong
• The colour of coordination compounds is a mystery, and there is no explanation for it