Valence Bond Theory of Coordination Compound

Valence bond theory (VBT) of coordination compound, is based on the assumption that electron pairs are donated to form a metal-ligand bond. Linus Pauling developed the theory

• This donating of electrons forms the coordinate bond between the ligand and the metal. 
• This theory explains the filling of the atomic orbitals with the electrons.
• The earlier system by Lewis although describe the configuration of the molecule but fails to explain the chemical bond.
• On the other hand, the VSEPR theory was restricted to only certain molecules, therefor there was a need for a new theory. 

Key Terms: Valence Bond Theory, electrons, atomic orbitals, VSEPR theory, coordinate bond. 

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Valence Bond Theory 

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According to the Valence Bond Theory:

• The electrons are present in the atomic orbitals and not in the molecular orbitals.
• When these atomic orbitals overlap, they result in the formation of chemical bonds. There is a localization of electrons in the bond region, due to the overlap. 
• The origin of the bond is covalent when it comes to metallic bonding. The electron pair bond between the atoms shows resonance. 

Also Read: Quantum Mechanical Model of Atom

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Postulates of Valence Bond Theory 

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The postulates of valence bond theory are as follows: 

• A formation of a covalent bond is observed when there is an overlap between the valence orbitals of two different atoms. 
• There is an increase in the electron density, which results in the stability of the molecules. 
• The nature of the covalent bond is directional and parallel to the atomic orbitals that are overlapping. 
• There are many unpaired electrons, which results in the formation of multiple bonds. 
• On the other hand, the paired electrons, do not take part in the formation of chemical bonds.
• There is a difference in the way, the atomic orbitals overlap in sigma and pi bonds. 
• The pi bonds are formed on a sideways overlapping and along the axis overlapping results in a sigma bond. 

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Features of Valence Bond Theory 

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The features of the Valence Bond Theory of Coordination Compounds are as follows:

• The vacant orbitals like s,p, and d orbitals are made available by the central atom. This is equal to the coordination number and forms the covalent coordination bond with the ligand.
• The vacant orbitals form hybrid orbitals, by hybridization and forming equivalent bonding orbitals. All these hybrid orbitals have the same energy, definite directional properties, and also the same geometry. 
• The bond in the metal complexes is formed when the lone pairs of the filled ligand orbitals overlap with the vacant hybrid orbitals of the central atoms. This forms a covalent bond. 
• Ligands have at least one orbital with a lone pair. The ligands are classified as weak ligands (F⁻) and strong ligands (CN⁻). 
• The tendency of a strong ligand is to form a pair with the d orbitals of the central atom, which cannot be done by a weak ligand. 
• If the complex has an unpaired electron, it is paramagnetic. If the complex has all the electrons paired then it is diamagnetic. 

Also Read:

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Hybridization and Geometry 

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The geometry and hybridization depend on the coordination number. It is as given below: 

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Limitation of Valence Bond Theory 

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Even though some limitations of Lewis dot structure and VSEPR theory were out-ruled by VBT. It has some limitations as mentioned follows: 

• The nature of the ligand cannot be explained.
• The color and electronic spectra of the complexes were not explained by VBT.
• It cannot give an explanation as to why the pairing of electrons takes place in the presence of strong ligands. 
• The reaction rate and the mechanism of the reaction are left untouched by VBT. 

Also Read: Energy level Diagram

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Things to Remember 

• Valence Bond theory suggests that the electrons are present in the atomic orbitals. 
• There is an overlapping between the atomic orbitals that result in the formation of the chemical bond. 
• The density of electrons increases at the site of overlap. 
• Sigma bonds show along the axis overlap and a pi bond shows sideways overlap. 
• There are two types of ligands, strong and weak. Only strong ligands can form bonds with d orbitals. 
• VBT cannot explain the nature of the ligand. 
• Complexes with unpaired electrons are paramagnetic and electrons with paired electrons are diamagnetic. 

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Previous Year Questions

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