Valance Bond Theory Preparation and Properties
Valence Bond Theory :
→ It was developed by Pauling.
→ This theory mainly deals with electronic structure of central metal ion, geometry, magnetic properties of complex, and type of bonding.
→ It describes the bonding in terms of hybridized orbitals of the central metal atom/ion. Salient features of theory are :
→ The central metal atom/ion makes available a number of empty 8, p and d-atomic orbitals equal to its coordination number. These vacant orbitals hybridized together to form hybrid orbitals which are same in the number as the atomic orbitals hybridising together. They are vacant, equivalent in energy and have definite geometry.
→ These hybrid orbitals will arrange in geometry which will be the geometry of complex
→ These hybrid orbitals now overlap with l.p. containing orbital of ligand to form coordinate bond.
→ The non-bonding electrons of the metal occupy the inner orbitals. These are grouped in accordance with Hund’s rule. However, under the influence of some ligands there may be some rearrangement of electrons in the atomic, orbitals (against Hund’s rule).
→ A strong field ligand (CN–,CO, NH,,en, Py, NO–2, etc.) is capable of forcing the electron of the metal atom to pair up if required.
→ Pairing is done only to the extent which is required to cause the hybridisation possible for that coordination number.
→ A weak field ligand (X–, OH–, H2O etc.) is incapable of making the electrons of the metal to pair up.
→ The d-orbitals involved in the hybridisation may be either inner orbitals or outer orbitals. The complexes formed in these two ways are referred to as inner orbital or outer orbital complexes respectively.
Example : (n – 1)d + ng + np inner orbital complexes/low spin complexes. ns + np+nd outer orbital complexes/high spin complexes.
→ If the complex contains unpaired electrons, it is paramagnetic in nature, while if it does not contain unpaired electrons, it is diamagnetic in nature.
→ On the basis of VBT, the number of unpaired electrons and the geometries of the complexes ion having central metal ion with configuration d1 to d10 are related to each other as shown below:
Examples of VBT
[Fe3(NH6)]3+
Atomic no. of iron = 26
Electronic configuration = 3d6 4s2
Oxidation no. of iron = + 3
Here, six ammonia molecules form six coordinate bonds with iron.
Shape: Octahedral
Colour: Coloured complex due to unpaired electron.
Magnetic Property: Paramagnetic
It is a inner orbital complex.
[Ni(CN)4]2-
Atomic no. of nickel = 28
Electronic configuration = 3d8 4s2
Oxidation no. of Ni = + 2
- Shape: Square planar
- Colour: Colourless (No unpaired electron)
- Magnetic Property : Diamagnetic
Few examples to illustrate the VBT
Limitations of valence bond theory
- The theory could not explain colour of complexes.
- The theory could not explain formation of high spin and low spin complexes very well.
- Destorted geometries of many complexes cannot be explained by this theory.
- Relative stability of complexes cannot be explained by this theory.