Explain Valence Bond Theory.

(N/A) The metal atom or ion,under the influence of ligands,makes available $(n-1)d, ns, np$ or $ns, np, nd$ orbitals for hybridisation to yield a set of equivalent orbitals of definite geometry such as octahedral,tetrahedral,square planar,etc. The number of available metal ion orbitals is equal to its coordination number.
Table: Number of Orbitals and Types of Hybridisations
| Coordination number | Type of hybridisation | Distribution of hybrid orbitals in space |
| :--- | :--- | :--- |
| $4$ | $sp^3$ | Tetrahedral |
| $4$ | $dsp^2$ | Square planar |
| $5$ | $sp^3d$ | Trigonal bipyramidal |
| $6$ | $sp^3d^2$ | Octahedral |
| $6$ | $d^2sp^3$ | Octahedral |
The hybrid orbitals are allowed to overlap with ligand orbitals that can donate electron pairs for bonding.
Each hybrid orbital of the metal ion receives an electron pair from a ligand.
The complex is said to be inner orbital or low spin if $(n-1)d, ns, np$ orbitals participate in hybridisation and is said to be outer orbital or high spin if $ns, np, nd$ orbitals participate in hybridisation.
It is possible to predict the geometry of a complex from the knowledge of its magnetic behaviour on the basis of valence bond theory. If all the electrons are paired,the complex is diamagnetic and if electrons are unpaired,it is paramagnetic.
It is important to note that the hybrid orbitals do not actually exist. In fact,hybridisation is a mathematical manipulation of the wave equation for the atomic orbitals involved.