The study of valence bond theory involves the knowledge of atomic orbitals, hybridisation, types of hybridisation, and principles of superpositions. Many scientists have worked hard to prove how the formation of coordination compounds takes place, and as a result, Valence Bond Theory was proposed to explain the phenomenon.
What are Coordination Compounds?
Coordinate compounds contain a central atom/ion surrounded by oppositely charged ions. These ions are bonded to the atom by the formation of coordinate bonds. Example: Tetracarbonyl Nickel, Tetra Cyanide ferrate, etc.
Central atom: The atoms or ions surrounded by a set of other atoms or molecules are referred to as central atoms.
What is Valence Bond Theory?
Valence bond theory was proposed to explain chemical bonds/molecules’ formation scientifically. It is based on atomic orbitals and the electronic configuration of atoms. It states that “Electrons of any molecule attempt to occupy atomic orbitals instead of molecular orbitals by the process of hybridisation.” The bond strength of the chemical bond formed depends on the overlap. The greater the overlap, the stronger the bond strength of the molecule.
Overlapping orbitals are of two types: Sigma and pi.
Sigma
- Overlapping involves overlap around the internuclear axis called the axial overlapping
- Example: C-H bond or C-X bond
- One head to another overlap
- End to end overlap takes place
Pi
- Pi overlapping involves the overlap of two unhybridized orbitals in sidewise overlapping
- Example: C=O bond
- Lateral overlap
- Overlaps perpendicular to the internuclear axis
History
Valence bond theory came into account when G.N Lewis proposed the geometrical structures of compounds in 1916. He explained how the valence electrons are arranged among the atoms in a molecule. Using Lewis theory, people started drawing Lewis structures that allowed them to predict many properties of molecules such as molecular shape and polarity.
Later, in 1927 Hitler and London attempted to explain the bonding properties by applying quantum mechanics. They took the hydrogen atoms as an example to explain chemical bond formation using Schrodinger’s wave equation.
In 1928 Linus Pauling merged Lewis pair bonding with Heitler-London theory and finally proposed the Valence Bond Theory. He published a paper on valence bond theory in 1931, entitled “On the Nature of Chemical Bond.” He also received a Nobel prize in 1954 for his hard work of introducing the concept of “orbital hybridization.”
In the 1980s, people started applying principles of valence bond theory into application, and since then, it has become recognizable.
Postulates Listed in Valence Bond Theory
- Covalent bonds are formed when two different orbitals from two different atoms overlap with each other
- This increases the stability of the atoms and electron density
- The presence of unpaired electrons determines the formation of multiple bonds with other atoms
- Only Unpaired electrons take part in the formation of bonds
- Pair of electrons in valence shells repel each other as their electron clouds are charged negatively
- These pairs of electrons occupy positions in such a way that they minimise repulsion and maximise separation between them
- As stated earlier, in the case of atoms with more than one unpaired electron, more than one bond can be madeThe number of unpaired electrons determines the number of chemical bonds formed
- The shared electron pair is likely to be found in the space between two nuclei of combining atoms
- Covalent bonds are considered directional
- The resultant overlapping results in a decrease in the energy levels of the atoms forming the bond
- More overlapping results in the formation of strong covalent bonds
- Two types of bond formation that takes place: sigma and pi
- When atoms are far, no intersection occurs; as they move close, intersection takes place
What is Hybridisation?
Hybridisation is the process of merging two atomic orbitals and giving rise to a new hybridised orbital.
The resultant hybridised orbitals formed are of different energy levels and shapes. Hybridisation is an extension of valence bond theory because this theory uses the hybridization concept to explain the geometry of molecules. They are of the following types:
- Sp
- Sp²
- Sp³
- Sp³d
- Sp³d²
Type of Hybridisation | Shape |
sp³ | Tetrahedral |
dsp² | Square Planar |
sp³d | Trigonal Bipyramidal |
sp³d² | Octahedral |
d²sp³ | Octahedral |
Difference between Valence Bond Theory and Molecular Orbital Theory
You might have heard about VSEPR theory, which explains the geometry of compounds geometrically, but Valence bond theory and molecular orbital theory (MO) were proposed to explain them theoretically.
According to molecular orbital theory, atomic orbitals of combining atoms overlap to form new orbitals called molecular orbitals.
- Valence bond theory explains the bonding of atomic orbitals, whereas a molecular orbital theory explains the molecular orbitals
- There is no role of resonance in molecular orbital theory, but it plays a main role in valence bond theory
- Valence bond theory does not explain the paramagnetic character of oxygen, but molecular orbital theory does explain
- Valence bond theory deals with only hybrid orbitals, whereas the molecular orbital theory explains both bonding and antibonding molecular orbitals
- Valence bond theory attempts to use hybridization to explain the geometry of molecules, whereas molecular orbital theory uses the symmetry of atomic orbitals to define the geometry of molecules
- Valence bond theory does not give details about molecular orbitals, whereas molecular orbital theory is purely based on molecular orbitals
- Schrodinger equation is used in valence bond theory, and Linear combination of atomic orbital is used in molecular orbital theory
- Single nucleus affects the electron cloud in valence bond theory, whereas more than two nuclei affect the electron cloud in molecular orbital theory
Applications of Valence Bond Theory
- Valence bond theory is used to explain the formation of covalent bonds between two atoms
- They explained the overlap of atomic orbitals
- This theory explained the structure of molecules based on the hybridisation phenomenon
- It gave the idea that maximum intersection gives rise to the development of strong possible bonds
If we take the example of the F² particle, F-F bonds are made by the intersection of pz orbitals of 2F atoms in which each one of them has unpaired electrons.
Limitations of Valence Bond Theory
- This theory failed to explain the ligand bond formation.
- It did not give the concept of kinetic stabilities and thermodynamics required to form coordinate compounds.
- Valence bond theory does not have the exact value of magnetic movement and the effect of temperature on magnetic movement.
- It didn’t explain inner and outer orbitals.
- The theory failed to explain the colour of coordination compounds.
- It did not categorise between weak and strong ligands.
- This theory did not have any satisfactory explanation of bonding in metals and interstitial compounds.
Conclusion
To put it simply, valence bond theory explains the filling of electrons in the orbitals.
Valence bond theory is very useful for understanding the filling of electrons, electronic spins, and pairing of electrons. Thus, it’s very crucial that you have a good knowledge about valence bond theory.
We have placed all the possible information regarding the theory, and hope that this post was really helpful to you.
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