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This theory was proposed by Heitler and London in the year 1972 on the basis of a wave mechanics of electrons and it was further extended by Pauling and Slater in the year 1931. Lewis’ hypothesis described the structure of molecules. It did not, however, explain the production of chemical bonds. VSEPR theory, on the other hand, described the form of small molecules. However, it had a very restricted use. As a result, scientists were forced to develop the idea of valence bonding in order to address and overcome these restrictions.
What is a Valence Bond Theory?
The structure and magnetic characteristics of a wide range of coordination compounds are explained by the valence bond theory. The valence bond theory was used to describe the structure of coordination compounds as well as the bond connections. When impacted by ligands, a metal atom or ion can employ its (n–1)d,ns,np,nd orbitals for hybridisation, yielding a set of equivalent orbitals of definite geometry such as octahedral, tetrahedral, square planar, and so on. The ligand orbitals capable of providing electron pairs for bonding can overlap with the hybridised orbitals.
Valence Bond Theory in Coordination Compounds:
Pauling was primarily responsible for developing the valence bond approach of complicated bonding. It is the simplest of the three theories and adequately describes the structure and magnetic characteristics of a wide range of coordinate compounds.
Postulates of Valence Bond Theory:
. The covalent bond is formed by the overlapping of two half-filled valence orbitals of two distinct atoms. The overlapping increases the electron density between two linked atoms. This provides the molecule the trait of stability.
. More than one bond can be created if the atomic orbitals contain more than one unpaired electron, and electrons coupled in the valence shell cannot participate in such bond formation
. A covalent bond has a directed nature. A bond of this type is also parallel to the area of overlapping atomic orbitals.
. Based on the overlapping pattern, there are two types of covalent bonds: sigma bonds and pi bonds.A pi bond is a covalent link created by overlapping atomic orbitals sideways, whereas a sigma bond is formed by overlapping atomic orbitals along the inter nucleus axis.
Limitations of Valence Bond Theory:
As previously stated, nothing is perfect! Similarly, the Valence Bond idea is not perfect. It has its own set of restrictions.
. It fails to explain carbon’s tetravalency.
. The energy of electrons is not discussed in this theory.
. The electrons are assumed to be confined to certain sites.
. It cannot explain why some complexes are more brittle than others. Labile complexes have one ligand that may easily be substituted by another. In contrast, inert complexes are those with progressive ligand displacement.
Importance of valence bond theory :
. Delocalization of electrons over the two nuclei
. Shielding effect of electrons
.The essential covalent character of bond
. The presence of partial ionic nature in a covalent bond
Solved Example:
Question: How many types of covalent bonds are generated as a result of orbital overlapping, and what are they?
Answer: Two types of covalent bonds are created as a result of orbital overlaps. These are referred to as sigma and pi bonds.
. Sigma bonds are formed by the end-to-end overlap of atomic orbitals along the internuclear axis, known as a head-on or axial overlap. There are three forms of end-on overlapping: s-s overlapping, s-p overlapping, and p-p overlapping.
.When atomic orbitals overlap in such a way that their axes stay parallel to one another and perpendicular to the internuclear axis, a pi bond is produced.
Conclusion:
Therefore we can conclude from this whole article that this theory has no satisfactory explanation of bonding in metals, odd electron molecules and interstitial compounds. This theory is unable to explain the formation of coordinate covalent bonds in which both the electrons forming the bond come from one atom only.
