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Coordination Compounds

Valence Bond Theory in Coordination Compounds

You must have studied that Werner’s theory for coordination compounds failed. Do you remember why Werner’s theory for coordination compounds failed? Well, it failed to explain many critical aspects of valence electrons and directions in the coordination compounds.

Then, how do we explain the structure of all these coordination compounds? Yes, we use the Valence bond theory which came on to replace the Werners theory. In this chapter, we will look at the valence bond theory and its important postulates. In the end, we will also look at the imitations of the valence bond theory.

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What is the Valence Bond Theory?

One of the major drawbacks of Werner’s theory is its failure to explain the coordination compounds’ directional properties. Valence bond theory became successful in explaining the structure and bond linkages in these coordination compounds.

So, what does the valence bond theory say? According to this theory, the metal atom or ion under the influence of ligands can use its (n-1)d, ns, np, nd orbitals for hybridisation. This would lead to a yield a set of equivalent orbitals of definite geometry.

These include octahedral, tetrahedral, square planar and other such geometrical arrangements. The hybridised orbitals can overlap with the ligand orbitals. These orbitals are ready to donate valence electrons / electron pairs for bonding.

Valence electrons

Explanation of Valence Electrons in Valence Bond Theory

We will understand the valence bond theory with the help of an example. Let us consider the diamagnetic octahedral complex [Co(NH3)6]3+. Here, the cobalt ion has the electronic configuration of 3d6. The hybridization scheme is as follows,

Orbitals of Co+3ion:

Valence electrons

d2sp3 hybridised orbitals of Co3+ is as follows,

Valence electrons

d2sp3 hybrid,

Valence electrons

We can see that the compound does not contain any unpaired valence electrons. Therefore, it is diamagnetic in nature. All the six pairs of electrons from NH3 molecules occupy the six hybridized orbitals.

Since, the inner d orbital (3d) takes place in hybridisation, the complex, [Co (NH3)6]3+ is what we call the inner orbital or low spin or spin paired complex. The paramagnetic octahedral complex generally uses outer orbital (4d) in hybridisation (sp3d2). It is known as outer orbital or high spin or spin-free complex.

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 Crystal Field Theory

Solved Example for You

Q: State the limitations of the valence bond theory.

Ans: The limitations of the valence bond theory are as follows:

  • With the valence bond theory, we can not have a quantitative interpretation of magnetic data. This is one major drawback of the valence bond theory.
  • The valence bond theory fails to explain the various colors that the coordination compounds exhibit. This is also relatable to the Werner theory.
  • The valence bond theory does not give a quantitative interpretation of the thermodynamic stabilities of the coordination compounds. It does not also speak of the kinetic stabilities of these compounds as well.
  • The various predictions that the valence bond theory makes regarding the tetrahedral and square planar structures of 4-coordinate complexes are not completely accurate.
  • It does not draw any distinctive line between the weak and strong ligands.
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