Valence Bond Theory
Metals & Non-Metals > Coordination Compounds
| Coordination Number | Orbitals | Type of hybrid | No. of hybrid orbitals | Geometry of the hybrid orbitals |
|---|---|---|---|---|
| 4 | s(1), p(3) | sp3 | 4 | Tetrahedral |
| 4 | d(1), s(1) p(2) | dsp2 | 4 | Square planar |
| 5 | s(1), p(3), d(1) | sp3d | 5 | Trigonal bipyramidal |
| 6 | s(1), p(3), d(2) | sp3d2 | 6 | Octahedral |
| 6 | d(2), s(1), p(3) | d2sp3 | 6 | Octahedral |
Valence Bond (VB) Theory, proposed in large part by the American scientists Linus Pauling and John C. Slater. The valence bond theory is used to explain the hybridization, the geometry and magnetic properties of metal complexes .
The important features of this theory are:
- The central metal ion in the complex makes available a number of empty orbitals for the formation of coordinate bonds with suitable ligands. The number of empty orbitals made available for bonding purposes is equal to the coordination number of the central ion.
- The available orbitals on the metal ion hybridize to give an equal number of new hybrid orbitals of the equivalent energy. The hybrid orbitals are described according to the geometry of the complex.
- Each ligand has at least one sigma orbital containing a lone–pair of electrons.
- The empty hybrid orbitals of the metal ion overlap with the filled sigma orbitals of the ligand forming ligand–metal coordinate bond.
- The d–orbitals involved in the hybridization may be inner d–orbitals, (n−1) d–orbitals, or the outer d–orbitals, nd–orbitals. The complexes formed in these two ways are referred to as low spin and high spin complexes or, inner and outer orbital complexes respectively.
- The non–bonding electrons on metal atom/ion occupy the inner d–orbitals which do not participate in hybridization.
Ni(CN)4 (Square planar)
Formation of [Ni(CN)4]2− complex ion in terms of VB theory is shown below Electronic configuration of Ni
Electronic configuration Of Ni2+ (ground state)
Pairing of 3d electrons (excited state)
Hybridization in Ni2+
Formation of [Ni(CN)4]2−
Defects of Valence bond theory
- The main defect of the simple Valence bond theory lies in its failure
- To include the anti bonding molecular orbitals produced during complex formation.
- It fails to offer an explanation for the striking colors of many complexes, which arise from their selective absorption of light of only certain wavelengths
[Ni(CN)4]2− is an Inner orbital complex When the central metal atom/ion uses its inner d–orbitals, (n−1) d orbitals for bonding, then the complex formed is called inner orbital complex.