Valence Bond Theory: Overlapping of Atomic Orbitals

The bonding of coordination compounds has been the subject of several theories. One such theory is the Valence Bond Theory. The theory is based on the Lewis concept of the electron-pair bond. We can also say that the chemical bonding of elements depends largely on the ability of atoms to share electron-pair bonds. 

Hence, the theory explains the chemical bonding of elements using quantum mechanics. It focuses mainly on the electronic configuration, orbitals, hybridization, and overlap of orbitals.

What does the Valence Bond Theory say?

According to the Valence Bond Theory, “Electrons in a molecule occupy atomic orbitals rather than molecular orbitals. The atomic orbitals overlap on the bond formation, and the larger the overlap, the stronger the bond.”

The original metal bonding is covalent, and the metallic structure consists of the resonance of electron pairs between atoms and their neighbors.

History of Valence Bond Theory:

The Lewis approach to chemical bonding did not explain how chemical bonds are formed. On the other hand, the Valence Shell Electron Pair Repulsion (VSEPR) theory had limited application, since it could not account for the geometry of complex compounds. The theory could only predict geometrically for simple combinations. 

Hence, German physicists Walter Heinrich Heitler and Fritz Wolfgang London came up with the valence bond theory to resolve both the above problems. They also used Schrödinger’s wave equation to throw light on the formation of covalent bonds between two hydrogen atoms.

The theory primarily focused on the concepts of electronic configuration, overlapping of atomic orbitals, and their hybridization.

Postulates of Valence Bond Theory:

The major postulates of the Valence Bond Theory are:

• The overlapping of two half-filled valence orbitals of two dissimilar atoms influences the formation of covalent bonds. The electron density between the two bonded atoms rises due to the overlap. Atoms acquire this stabilizing property as a result.
• Atomic orbitals possessing more than one unpaired electron can form one bond. Paired electrons in the valence shell cannot participate in such a bond.
• Covalent bonds are directional and parallel to corresponding areas of intersecting atomic orbitals.

Covalent bonds can be classified into two types according to the design of the overlap, viz., a pi bond and a sigma bond. Covalent bonds created by the sideways overlap of atomic orbitals are called pi bonds. On the other hand, bonds formed by the overlapping of atomic orbitals along the inter nuclear axis are called Sigma bonds.

Coordination Numbers and Hybridization:

The valence bond theory for coordination compounds mentions that a metal atom or ion, under the influence of ligands, can use its (n-1) d, ns, np, or ns, np, nd orbitals for hybridization to yield a set of equivalent orbitals.

This could also result in definite geometrical forms such as octahedral, tetrahedral, square planar, etc. These hybrid orbitals then overlapped with the ligand orbitals to donate electron pairs for bonding.

Applications of Valence Bond Theory:

• The Valence Bond Theory for coordination compounds describes the maximum overlap condition, which explains the creation of covalent bonds in several molecules.

• We can explain the difference in the length and strength of the chemical bonds in H2 and F2 molecules, considering the difference in their overlapping orbitals.

• In an HF molecule, both Hydrogen and Fluorine have an unpaired electron each. Hence, the 1s orbital of Hydrogen(H+) overlaps with the 2pz orbital of Fluorine(F-) to form a covalent bond. This entire process can be explained using the Valence Bond Theory for coordination compounds.

Drawbacks of the Valence Bond Theory:

Following are certain limitations to the Valence Bond Theory for Coordination Compounds:

• Tetravalency that is exhibited by carbon cannot be explained using this theory.
• It does not talk about the energies of the electrons.
• By default, the theory assumes the electrons to be localised and restricted to certain areas.
• The kinetic and thermodynamic stabilities of coordination compounds do not have quantitative interpretations.
• The theory does not differentiate the weak and strong ligands.
• Is unable to explain the colors of coordination compounds.

Conclusion:

• Lewis’ approach for chemical bonding and the VSEPR theory had certain limitations. So, the Valence Bond Theory was proposed to explain the covalent bonding between atoms.
• The VB theory for coordination compounds focuses on the sharing of unpaired electrons.
• There are two kinds of bonds- the Sigma bond and the Pi bond.
• The head-on overlap of atomic orbitals results in sigma bonds.
• The lateral overlap of atomic orbitals results in pi bonds.