What is Valence Bond Theory (VB)?

Valence Bond Theory

Understanding the nature of bonding is vital for studying coordination compounds. The Valence Bond theory explains chemical bonding using quantum mechanics. What is valence bond theory? A description of individual bond formation from the atomic orbitals of the participating atoms during a molecule formation is referred to as VB theory.

The overlapping of atomic orbitals creates molecules, and the valence bond theory continues by explaining the electronic structure of those molecules. Additionally, it highlights how one atom’s nucleus in a molecule is drawn to the electrons of the other atoms. 

What is the Valence Bond Theory?

According to the Valence Bond theory definition,

• Electrons in a molecule stay in atomic orbitals instead of molecular orbitals.
• A covalent bond result from overlapping half-filled atomic orbitals wherein each contains a single electron. These orbitals together yield a pair of shared electrons between the two bonded atoms.
• Orbital overlapping is when a portion of an orbital occupies the same region as a second orbital.
• So, a covalent bond is formed when the following two conditions are present:
  1. An orbital of one atom overlaps the orbital of another atom.
  2. The single electrons of each orbital come together to form an electron pair.

The force of mutual attraction between negatively charged electron pairs and positively charged nuclei of the two atoms links them via a covalent bond. The covalent bond strength depends on the extent of orbital overlap. Therefore, extensively overlapping orbitals form stronger bonds. Metal bonding involves the resonance of electron-pair bonds between every atom and its neighbours.

The VB theory predicts that there are no unpaired electrons in molecular oxygen. The forms of covalent compounds are well qualitatively described by VB theory. In contrast, the Molecular Orbital (MO) hypothesis is useful for comprehending bonds more generally. 

History of Valence Bond Theory

Scientists made several attempts to explain chemical bonding, such as

• The Lewis approach to chemical bonding. It failed to describe the formation of chemical bonds.
• Valence shell electron pair repulsion theory (VSEPR theory) has limited applications. It could not shed light on the geometry of complex molecules.

The German physicists— Fritz Wolfgang and Walter Heinrich Heitler put forth the valence bond theory and addressed all the prevalent issues. Using the Schrodinger wave equation, the formation of a covalent bond was explained in two hydrogen atoms.

The theory concentrates on electronic configuration, overlapping atomic orbitals, and hybridisation concepts. It emphasizes how the nucleus of an atom in a molecule attracts the electrons of the other atoms.

Postulates of Valence Bond Theory 

The key points of the VB theory are as follows:

• Overlapping two half-filled, valence orbitals that belong to two different atoms leads to covalent bond formation.
• The place between the bonding atoms has a high electron density due to overlapping. Thus, the stability of the resulting molecule increases.
• Several valence unpaired electrons of an atom allow it to form multiple covalent bonds with other atoms. The paired electrons in the valence shell don’t participate in chemical bond formation.
• Covalent chemical bonds are directional. They are also parallel to the region corresponding to overlapping atomic orbitals.
• The difference in the overlapping pattern gives rise to sigma and pi bonds. While pi bonds overlap sidewise (parallel overlapping), sigma bonds overlap along the internuclear axis of the two atoms (head-to-head overlapping).

Sigma and pi bonds

Types of Orbital Overlap

The covalent bond can have two types depending upon the

overlapping type, i.e., the sigma bond and the pi bond. The sigma overlap has the following three types:

• s-s overlap:

 

• s-p overlap:

 

• p-p overlap:

 

The types of pi overlap are as follows:

• p-p overlap:

 

• p-d overlap:

 

• d-d overlap:

VBT and Hybridisation

The fusion of atomic orbitals to form new hybridized orbitals is called hybridisation. It influences bonding properties and molecular geometry. The concept of hybridisation originates from the valence bond theory. As per the valence bond theory, a metal atom/ion influenced by a ligand uses its (n-1)d, ns, np, or ns, np, nd orbitals to give rise to certain equivalent orbitals of definite geometry. These hybrid orbitals overlap with ligand orbitals that donate electron pairs for bonding.

Coordination Number	Hybridisation type	Hybrid Orbitals’ Distribution in Space
4	dsp2	Square planar
4	sp3	Tetrahedral
5	sp3 d	Trigonal bipyramidal
6	d2sp3	Octahedral
6	sp3d2	Octahedral

Example: Formation of the octahedral complex

The valence-bond theory can be applied to coordination complexes like Co(NH₃ )₆³⁺ ions. Beginning with the electron configuration of Co³⁺ (the transition-metal ion)

EC = [Ar]d⁶

 

The valence-shell orbitals 4s and 4p are empty. Thus, the 3d electrons are in the d_xy, d_xz, and d_yz orbitals. The mixing of 4s, 4px, 4py, 3dx²-y², and 3dz² orbitals form empty d²sp³ orbitals that point towards the octahedron corners. Each orbital can accept a non-bonding pair of electrons from an NH₃ molecule for a complex formation.

Valence Bond Theory Examples

The VB theory’s maximum overlap explains the formation of covalent bonds in various molecules. For instance, H₂ and F₂ chemical bonds differ in length and strength due to the difference in the extent of overlapping orbitals in these molecules. It also explains the structure of the covalent bond in an HF molecule. It comprises the overlap of the 1s orbital of the H atom and a 2p orbital of the fluorine atom.

Advantages of Valence Bond Theory

Valence bond theory explains how covalent bonds are formed by maximum overlapping. Some of the advantages of valence bond theory are as follows:

• It allows you to understand the covalent bonds of various molecules.
• It also provides insights into the ionic character of chemical bonds.
• Explains the geometrical shape of complexes.
• Describes the magnetic properties of complexes.
• Illustrates the formation of inner complexes when strong ligands are present.

Limitations of Valence Bond Theory

Like every other theory, the valence bond theory also has shortcomings, such as

• It fails to explain the tetravalency of carbon atoms.
• It doesn’t provide insights into the energies of the electrons.
• It assumes the localisation of electrons in specific areas.
• It doesn’t provide a quantitative interpretation of kinetic stability or thermodynamics.
• It doesn’t explain the phenomenon of the colour exhibition by coordination compounds.
• It fails to explain the paramagnetic nature of oxygen.
• It does not determine the shapes of the polyatomic molecules, like methane, water ammonia and water, which are tetrahedral, bent, and pyramidal, respectively.

How is Valence Bond Theory different from VSEPR?  

The main difference between valence shell electron pair repulsion theory and valence bond theory is that VSEPR specifies the molecule’s shape while the valence band theory describes the molecule’s chemical bonding. The following table enumerates the significant difference between VSEPR and VBT:

VSEPR	VBT
Predicts the geometry of a molecule.	Explains chemical bonding between atoms.
Based on the repulsive force between bonded and lone electron pairs.	Based on orbital overlapping for bond formation.
Does not provide details about the atomic orbitals.	Provides details about orbitals present in atoms.
Does not indicate the types of bonds.	Explains the various types of bonds in atoms.

How is the Valence Bond Theory Different from the Molecular Orbital Theory?

Five years after the VB theory, the molecular orbital theory came forward to explain the bonding in molecules that the VBT could not. The following table states the main differences between VBT and MOT:

Valence Bond Theory	Molecular Orbital Theory
Explains the bonding of atoms in a molecule.	Explains the chemical bonding of molecules.
It defines the hybridisation of molecular orbitals.	It does not define hybridisation.
Only suitable for diatomic molecules.	It can be applied to polyatomic molecules.
Atoms retain their characteristics.	They do not retain their characteristics.
Bonds remain localized to two atoms and not molecules.	Bonds are localized to both atoms and molecules.
Resonance plays an important role in VBT.	Resonance does not play a significant role in MOT.
It fails to explain the paramagnetic nature of oxygen.	It extensively explains the paramagnetic nature of oxygen.

Conclusion

Valence bond theory explains covalent bonding by stating that atoms attract one another, leading to an overlap of two separate atomic orbitals of different bits. A region of high electron density is created with a pair of electrons shared by the two participating atoms. Overlapping along the internuclear axis gives rise to a σ bond, and overlapping creates a node along this axis, resulting in a π bond.

Frequently Asked Questions

Q1. How are pi and sigma bonds formed?

Atomic orbitals overlapping creates sigma and pi bonds. Sigma bonds are created via end-to-end overlap, whereas talking about how are Pi bonds formed then Pi bonds are created when the lobes of two different atomic orbitals come together. When viewed from the bond axis, both names derive from the Greek letters and the bond.

Q2. What assumptions are a part of the valence bond theory?

Valence bond theory assumes that the valence electrons occupy atomic orbitals rather than molecular orbitals. It also states that one atom’s nucleus in a molecule is attracted to the electrons of the other atoms.

Q3. Why is VBT superior to VSEPR?

Both VSEPR and VBT are applied to compounds with covalent bonds. VBT is considered superior to VSEPR because VSEPR theory only explains the shape of a molecule, while VBT tells about the creation of covalent bonds between atoms of a molecule.