Discuss the main postulates of valence bond theory.

Explanation

Main postulates of valence band theory are as follows:
1. The central metal ion in the complex makes available a number of empty orbitals for the formation of coordination bonds with suitable ligands. The number of empty orbitals made available for this purpose is equal to coordinm number of the central metal ion.
2. The appropriate atomic orbitals (s, p and d) of the metal hybridise to give of equivalent orbitals of definite geometry such as square planar, tetrahedral, hedral and so on.
The following types of hybridisation are involved for different geometries of the complexes.
`4" "sp^3" "`Tetrahedral
`4" "dsp^2" "`Square planar
`5" "sp^3 d" "`Trigonal bipyramidal
`6" "sp^3 d^2" "`Octahedral
`" "" or "d^2 sp^3" "`Octahedral
3. The chorbitals involved in the hybridisation may be either inner d.orbitals (n-1) dor outer d-orbitals i.e., nd. For example, in case of octahedral hybridisation, the orbitals. may be two 3d, one 4s and three 4p (`d^2 sp^3`) or one 4s, 4p and two 3d (`sp^3 d^2` hybridisation).
4. Each ligand has at least one orbital (of donor atom) containing a lone of electrons.
5. The empty hybrid orbitals of metal ion overlap with the filled orbital of the ligand to form metal-ligand coordinate covalent bonds.