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Many theories have been proposed to explain the nature of coordination compound bonding. The Valence Bond Theory (VBT) is amongst one of them. The Valence Bond Theory was formulated in effort to use quantum mechanics to explain chemical bonding. This theory primarily focuses on the formation of individual bonds from the atomic orbitals of the contributing atoms.
What is Valence Bond Theory (VBT)?
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According to the Valence Bond Theory, electrons in a molecule occupy atomic orbitals rather than molecular orbitals. In the bond formation, the atomic orbitals overlap each other. The wider the overlap, the stronger the bond will be.
Metal bonding is generally covalent in nature, and metallic structure entails electron-pair bond resonance between each atom and its neighbours.
Need for Valence Bond Theory
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The structure of molecules was explained by Lewis' theory. However, it was unable to explain the formation of chemical bonds. Similarly, VSEPR theory explains the shape of simple molecules. However, it had a very restricted application. It also failed to explain complicated molecular geometry. As a result, scientists had to develop the valence bond theory to overcome these limitations.
History of Valence Bond Theory
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The Lewis approach towards chemical bonding was ineffective in explaining how chemical bonds occur. Furthermore, the valence shell electron pair repulsion theory (often referred to as VSEPR theory) had just a few applications and also failed in predicting the geometry corresponding to complex molecules. In order to resolve these issues Valence Bond Theory was introduced.
The valence bond theory was proposed by German physicists Walter Heinrich Heitler and Fritz Wolfgang London. The development of a covalent bond between two hydrogen atoms was also explained using the Schrodinger wave equation.
The chemical bonding of two hydrogen atoms, as defined by the valence bond is shown below:
The Valence Bond Theory primarily focuses on the ideas of electronic configuration, atomic orbitals (and their overlapping), and atomic orbital hybridization. Atomic orbitals overlapping and electrons being concentrated in the relevant bond region are responsible for the formation of chemical bonds.
The electronic structure of molecules formed by this overlapping of atomic orbitals is also explained by the valence bond theory. It also emphasises how the nucleus of one atom in a molecule is attracted to the electrons of the other atoms in a molecule.
Postulates of Valence Bond Theory
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- The overlapping of two half-filled valence orbitals of two different atoms forms the covalent bond. The electron density between two linked atoms increases due to the overlapping. The molecule thus gains the property of stability.
- More than one bond can be created if the atomic orbitals contain more than one unpaired electron. As per the valence bond theory, the paired electrons in the valence shell cannot participate in such bond formation.
- A covalent bond has a specific direction. The region of overlapping atomic orbitals is also parallel to such a bond.
- There are two types of covalent bonds based on the overlapping pattern: sigma bonds and pi bonds. The pi bond is formed by the overlapping of atomic orbitals sideways, whereas the sigma bond is created by the overlapping of atomic orbitals along the axis of the nucleus.
It's important to note that sigma bonds include atomic orbitals overlapping head-to-head, whereas pi bonds involve parallel overlapping.
Number of Orbitals and Types of Hybridization
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According to the Valence Bond Theory, a metal atom or ion can use its (n-1)d, ns, np, or ns, np, nd orbitals for hybridization under the influence of ligands. It results in a group of similar orbitals with specific geometry such as octahedral, tetrahedral, square planar, and so on. The ligand orbitals that can donate electron pairs for bonding are allowed to overlap with these hybrid orbitals.
| Coordination Number | Type of Hybridisation | Distribution of Hybrid Orbitals in Space |
|---|---|---|
| 4 | sp3 | Tetrahedral |
| 4 | dsp2 | Square planar |
| 5 | sp3d | Trigonal bipyramidal |
| 6 | sp3d2 | Octahedral |
| 6 | d2sp3 | Octahedral |
Applications of Valence Bond Theory
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- The maximum overlap conditions which is given by the valence bond theory can explain the formation of covalent bonds in many compounds.
- One of its most important applications is the variation in the length and strength of chemical bonds in H2 and F2 molecules can be explained by the difference in their overlapping orbitals.
- The overlap of the 1s orbital of the hydrogen atom and the 2p orbital of the fluorine atom makes the covalent bond in an HF molecule which is explained by the valence bond theory.
Limitations of Valence Bond Theory
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The Valence Bond theory has some limitations. They are as follows:
- It is unable to explain carbon's tetravalency.
- The energy of electrons is not discussed in this theory.
- According to the assumptions of this theory, the electrons are thought to be localised to specific regions.
- It does not provide a quantitative analysis of the thermodynamic or kinetic stabilities of coordination molecules.
- There is no difference between weak and strong ligands.
- There is no explanation behind the colour of coordination compounds.
Orbital overlap concept
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According to the orbital overlap concept, a covalent bond is formed when two half-filled atomic orbitals and thus forming a lower energy state containing electrons with opposite spins partially overlap.
Due to the partial overlapping, a portion of each of the two half-filled atomic orbitals' electron clouds becomes similar. The chances of detecting electrons in the overlap zone are substantially higher than in other areas.
The extent of overlap depends on the two participating atoms, their size and the valence electrons. In general, the greater the overlap, the stronger is the bond formed between the two atoms.
Formation of hydrogen molecule
When two hydrogen atoms with opposite-spin electrons come near to each other, their s orbitals overlap, resulting in the two atoms joining to form a molecule.
Formation of HF molecule
When a fluorine atom with one unpaired electron in its p orbital approaches a hydrogen atom with an electron of opposite spin in its 1s orbital, the two half-filled orbitals combine together. This results in the formation of a chemical bond between the two atoms.
Read Also: Class 12 Group 18 Elements
Formation of fluorine molecule
When a fluorine atom approaches another fluorine atom with an electron of opposite spin in the 2p orbital, the half-filled orbitals overlap.It results in the formation of fluorine molecules.
Explanation of the existence of H2 and non-existence of H3 and H4
A hydrogen atom has just one half-filled atomic orbital (1s) that can overlap with another hydrogen atom's half-filled atomic orbital to form H2. There are no more half-filled atomic orbitals available, hence no more bonds can be formed. Due to this reason the H3 and H4 molecule does not exist.
Read More: Valency of Carbon (Tetravalency) Important Notes
Important Questions Based on Valence Bond Theory
Ques.1: Explain the formation of H2 molecules on the basis of valence bond theory. (3 Marks)
Answer: Let a molecule is formed between atoms of hydrogen HA and HB and eA and eB be their respective electrons.
As they tend to come closer, two different forces act between the nucleus of one atom and the electrons of the other atom and vice versa. The nuclei of the atoms and their electrons repel each other. To overcome the force of repulsion, a certain amount of energy is required. Although the number of new attractive and repulsive forces is the same, the magnitude of the attractive forces is more. Thus, when two hydrogen atoms approach each other, the overall potential energy of the system decreases. It results in the formation of a stable molecule of hydrogen.
Ques.2: What is the valence bond theory? (3 Marks)
Answer: Valence Bond Theory states that electrons in a molecule occupy atomic orbitals rather than molecular orbitals. These atomic orbitals overlap on the bond which results in a larger overlap. The larger the overlap, the stronger the bond. This bonding is generally covalent in nature.
Some application of valence bond theory are:
- The bond in an HF molecule is formed by the overlap of one s orbital of the hydrogen atom and two p orbital of the fluorine atom. This is explained in detail by the valence bond theory.
- The difference in the length and strength of chemical bonds in H2 and F2 molecules is explained by the overlapping orbitals in the molecules.
Ques.3: How many different types of covalent bonds are produced when orbitals overlap, and what are they? (3 Marks)
Answer: Two types of covalent bonds are produced by the overlapping of orbitals. Sigma(σ) and pi(π) bonds are the names for these types of bonding.
- Head-on or axial overlap is the end-to-end overlap of atomic orbitals along the internuclear axis that forms sigma bonds. The s-s overlapping, s-p overlapping, and p-p overlapping are the three varieties of end-on overlapping.
- When atomic orbitals overlap in such a way that their axes remain parallel to one another and perpendicular to the internuclear axis, a pi bond is produced.
Ques.4: What are the drawbacks of valence bond theory? (3 Marks)
Answer: The valence bond theory has some limitations. These limitations are:
- The valence bond theory is unable to explain the tetravalency exhibited by carbon.
- According to the assumptions of this theory, the electrons are thought to be localized to specific regions.
- The theory does not discuss the energies of electrons.
- It does not give a proper explanation about the difference between weak and strong ligands.
- It does not provide a quantitative analysis of the thermodynamic or kinetic stabilities of coordination molecules.
Ques.5: What are the assumptions of valence bond theory? (2 Marks)
Answer: The theory of Valence Bonds covers the creation of covalent bonds as well as the electronic structure of molecules. The hypothesis assumes that electrons occupy particular atoms' atomic orbitals inside a molecule, and that electrons from one atom are attracted to the nucleus of another atom.
Ques.6: What is the difference between sigma bonds and pi bonds? (2 Marks)
Answer: A sigma bond is a chemical bond which is formed by the linear or co-axial overlapping of the atomic orbitals of two atoms. The head-to-head overlapping takes place in the sigma bond.
The pi bond is a chemical bond which is formed by the overlapping of atomic orbitals sideways. Both of them are a sort of a covalent bond. Pi bonds involve the overlapping of atomic orbitals in a parallel manner.
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