Bonding-Valence Bond Approach of Crystal Field Theory

Introduction 

One of the theories of quantum mechanics that can explain the bonding between different atoms is termed Valence Bond Theory. The theory was proposed by Heitler and London in which the unpaired electrons of other atoms combined to fill an empty orbital. The bonds thus formed were categorized as sigma and pi bonds. The two bonds formed by the theory differ. Meaning, sigma bond is formed when the electron sharing orbitals overlap head to head. On the other hand, the formation of a pi bond is when both the overlapping orbitals are parallel. Let’s discuss and explain the Valence Bond Theory  in detail! 

History Of Valence Bond Theory 

When the Lewis theory and valence shell electron pair repulsion theory failed to explain the chemical bonding in certain molecules, scientists researched to find answers related to it. Although VSEPR theory shed light on the shape of simple molecules, it failed to explain the geometry of complex compounds. Owing to the limited applications of both theories, certain geometries of complex compounds lacked explanation. To shed some light on the issues, Heitler and London put forth the Valence Bond Theory. They took the help of the Schrodinger wave equation to explain the geometry of complex molecules. 

Valence Bond Theory 

The electronic structure of the molecules was carefully explained by the theory. As per the theory, electrons were filled in the atomic orbitals within the molecule. Main areas that were focused on by VBT theory were:- 

• Electronic configuration of different elements 

• Atomic orbitals and they’re overlapping 

• The hybridization of atomic orbitals. 

When the atomic orbitals are overlapped, it results in the formation of chemical bonds. The electronic structure of the molecules is the second thing that is explained by the Valence Bond Theory. Additionally, it emphasized on how electrons of one atom are attracted to the nucleus of another atom. 

Postulates Of Valence Bond Theory 

Here are the important postulates of valence bond theory. Check out! 

1. When two half filled valence orbitals overlap each other, it gives rise to the chemical bonds. As the result of overlapping between the atoms, the electron density between the binding atoms rises. This, in turn, increases the stability of the molecule that is formed. 

2. The unpaired electrons present in the last shell of the atom participates in the chemical bond formation. However, the paired electrons don’t take part in the bond formation. 

3. The bonds formed as per the Valence Bond Theory are directional and parallel to the overlapping atomic orbitals. 

4. The two bonds of the VBT theory – pi bond and sigma bond differ from each other. That is, a pi bond is formed when there is sidewise overlapping (parallel overlapping). In contrast, a sigma bond is formed when atoms overlap along the axis containing the nuclei (head to head overlapping). 

Applications Of Valence Bond Theory 

The Valence Bond Theory is a way to explain chemical bonding that takes place in different molecules. One of the best examples that is formed under the Valence Bond Theory is the fluorine molecule, F2. During the formation of the fluorine molecule, F2, the Pz orbitals overlap and lead to the development of a single bond. Similarly, the hydrogen molecule is formed when two hydrogen atoms are overlapped. Although the two bonding structures are similar yet the bond strengths of both the molecules are different. In contrast, the third example of overlapping is HF. In this type of bonding, the unpaired electron of 1s hydrogen orbital and 2pz fluorine orbital is overlapped. Here are a few applications of Valence Bond Theory.

1. The extent of overlapping in different molecules is explained by valence bond theory. 

2. The difference in bond length and bond strength of H2 and F2 molecules are explained by valence bond theory. 

3. The chemical bonding in HF molecules is completely explained by the proposed theory. 

Limitations Of Valence Bond Theory 

Although valence bond theory was successful in explaining different complex compounds yet it failed to explain certain aspects. Here are some of the limitations of valence bond theory. 

1. The tetravalency of carbon was not explained by Valence Bond Theory.

2. The energies of the electrons was another aspect on which VBT theory shed no light. 

3. One of the main facts about Valence Bond Theory that electrons are localized in different locations was just an assumption. 

4. The kinetic or thermodynamic stabilities of various coordination compounds were not interpreted by the theory. 

5. It failed to differentiate between strong and weak ligands. 

6. The colour exhibited by the coordination compounds was not explained under the theory. 

Different Hybrid Orbitals In Valence Bond Theory 

There are three main hybrid orbitals that can be formed as per the valence bond theory. The three hybrid orbitals are explained below:-

• When the hybridization of one s and one p atomic orbitals are overlapped, it is known as sp hybrid orbital. 

• When one s and two p orbitals are hybridized, it leads to the formation of sp2 hybrid orbital. 

1. sp3 hybrid orbital is formed when one s and three p orbitals are hybridized. The shape of such a structure is tetrahedral. 

Aside from these three major hybrid orbitals, there are other hybrid orbitals that are formed by the mixing of different atomic orbitals. First is dsp2 with coordination number 4. In this type of hybridization, one d, one s and two p orbitals are overlapped. The same goes for sp3d in which one s, three p and one d orbitals are hybridized. The difference between the two hybridizations is the coordination number and the structure. dsp2 is square planar, whereas sp3d is trigonal bipyramidal. The two octahedral shapes formed as per the valence bond theory are sp3d2 and d2sp3. 

Conclusion 

Undoubtedly, Valence Bond Theory was effective in explaining numerous unsolved concepts. But the colour exhibition was not explained. Though the theories CFT and VBT explained the atomic orbitals, there was a major difference in the two. CFT, also known as Crystal Field Theory, explained about the orbital splitting. On the other hand, VBT, also known as Valence Bond Theory, explained about the orbital mixing.