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Valence Bond Theory & Magnetic Properties of Complexes Notes | Study Chemistry for JEE - JEE

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Table of contents
What is Valence Bond (VB) Theory?
History of Valence Bond Theory
Postulates of Valence Bond Theory
Number of Orbitals and Types of Hybridization
Limitations of Valence Bond Theory
Applications of Valence Bond Theory
Magnetic Properties of Complexes
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What is Valence Bond (VB) Theory?

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.

Valence Bond Theory & Magnetic Properties of Complexes Notes | Study Chemistry for JEE - JEE

The metal bonding is essentially covalent in origin and the metallic structure involves the resonance of electron-pair bonds between each atom and its neighbours.

History of Valence Bond Theory

The Lewis approach to chemical bonding failed to shed light on the formation of chemical bonds. Also, valence shell electron pair repulsion theory (or VSEPR theory) had limited applications (and also failed in predicting the geometry corresponding to complex molecules).

In order to address these issues, the valence bond theory was put forth by the German physicists Walter Heinrich Heitler and Fritz Wolfgang London. The Schrodinger wave equation was also used to explain the formation of a covalent bond between two hydrogen atoms. The chemical bonding of two hydrogen atoms as per the valence bond theory is illustrated below.

Valence Bond Theory & Magnetic Properties of Complexes Notes | Study Chemistry for JEE - JEE

This theory focuses on the concepts of electronic configuration, atomic orbitals (and their overlapping) and the hybridization of these atomic orbitals. Chemical bonds are formed from the overlapping of atomic orbitals wherein the electrons are localized in the corresponding bond region.

The valence bond theory also goes on to explain the electronic structure of the molecules formed by this overlapping of atomic orbitals. It also emphasizes that the nucleus of one atom in a molecule is attracted to the electrons of the other atoms.

Postulates of Valence Bond Theory

It was developed by Pauling. The salient features of the theory are summarised below :

  1. Under the influence of strong-field ligands, the electrons of the central metal ion can be forced to pair up against Hund's rule of maximum multiplicity.
  2. Under the influence of weak field ligands, the electronic configuration of central metal atom and ion remains the same.
  3. If the complex contains unpaired electrons, it is paramagnetic in nature, whereas if it does not contain unpaired electrons, then it is diamagnetic in nature and magnetic moment is calculated by spin only formula.
    Valence Bond Theory & Magnetic Properties of Complexes Notes | Study Chemistry for JEE - JEEwhere n is the number of unpaired electrons in the metal ion.
    Relation between unpaired electrons and magnetic moment

    Magnetic moment (Bohr magnetons)

    0

    1.73

    2.83

    3.87

    4.90

    5.92

    Number of unpaired electrons

    0

    1

    2

    3

    4

    5


    Thus, the knowledge of the magnetic moment can be of great help in ascertaining the type of complexity.

  4. When ligands are arranged in increasing order of their splitting power then an experimentally determind series is obtained named as spectrochemical series.

    Valence Bond Theory & Magnetic Properties of Complexes Notes | Study Chemistry for JEE - JEE
    Valence Bond Theory & Magnetic Properties of Complexes Notes | Study Chemistry for JEE - JEE
  5. The central metal ion has a number of empty orbitals for accommodating electrons donated by the ligands.
    The number of empty orbitals is equal to the coordination number of the metal ion for a particular complex.
  6. The atomic orbital (s, p or d) of the metal ion hybridise to form hybrid orbitals with definite directional properties. These hybrid orbitals now accept e- pairs from ligands to form coordination bonds.
  7. The d-orbitals involved in the hybridisation may be either inner (n - 1) d orbitals or outer n d-orbitals. The complexes formed in these two ways are referred to as inner orbital complexes and outer orbital complexes, respectively.

Valence Bond Theory & Magnetic Properties of Complexes Notes | Study Chemistry for JEE - JEE

Question for Valence Bond Theory & Magnetic Properties of Complexes
Try yourself:According to VBT, the direction of a bond which is formed due to overlapping will be _____________
View Solution

Number of Orbitals and Types of Hybridization

According to VBT theory the 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 of definite geometry such as octahedral, tetrahedral, square planar and so on. These hybrid orbitals are allowed to overlap with ligand orbitals that can donate electron pairs for bonding.

Coordination NumberType of HybridisationDistribution of Hybrid Orbitals in Space
4sp3Tetrahedral
4dsp2Square planar
5sp3dTrigonal bipyramidal
6sp3d2Octahedral
6d2sp3Octahedral

Limitations of Valence Bond Theory

  1. Correct magnetic moment of complex compounds can not be theoretically measured by Valence bond theory.
  2. The theory does not offer any explanation about the spectra of complex (i.e., why most of the complexes are coloured).
  3. Theory does not offer any explanation for the existence of inner -orbital and outer -orbital complexes.
  4. In the formation of [Cu(NH3)4]2+, one electron is shifted from 3d to 4p orbital. The theory is silent about the energy availability for shifting such an electron.
    Such an electron can be easily lost then why does not [Cu(NH3)4]2+  complex show reducing properties.

Question for Valence Bond Theory & Magnetic Properties of Complexes
Try yourself:The formation of odd electron molecules such as H2+, NO, O3 is also explained in VBT.
View Solution

Applications of Valence Bond Theory

  1. The maximum overlap condition which is described by the valence bond theory can explain the formation of covalent bonds in several molecules.
  2. This is one of its most important applications. For example, the difference in the length and strength of the chemical bonds in H2 and F2 molecules can be explained by the difference in the overlapping orbitals in these molecules.
  3. The covalent bond in an HF molecule is formed from the overlap of the 1s orbital of the hydrogen atom and a 2p orbital belonging to the fluorine atom, which is explained by the valence bond theory.

Question for Valence Bond Theory & Magnetic Properties of Complexes
Try yourself:Which orbital would form a stronger bond if both of them have identical stability?
View Solution

Magnetic Properties of Complexes

The complex in which central transition metal ion has unpaired electrons is Paramagnetic.

The complex in which central transition metal ion has no unpaired electrons is diamagnetic.

The magnetic moment of a complex is calculated by the spin only formula

M = √[n(n+2)] BM

BM = Bohr Magneton


The magnetic moment of complex compounds depends upon:

  1. Type of hybridization.
  2. The oxidation state of central transition metal ion.
  3. The number of unpaired electrons.
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