Valence Bond Theory was first introduced by " Linnus Pauling ". This theory not only explains the formation of Complex but also explains the geometry and magnetic properties of Complexes.
Pauling explained the formation of metal ligand bond on the basis of overlapping of atomic orbitals but only difference is that, metal orbital is empty and ligand orbital have pair of electron. In this bond formation only ligand contribute their both electron and hence such type of bond is called as Covalent Co-ordinate bond or dative bond.
Postulates of V.B.T
Postulates of valence bond theory are as follows—
•Central metal: Each central metal ion or atom makes available empty orbitals, so that electron pair from ligands are accepted, thus central metal is called as electron acceptor species.
•Ligands: ligands are negativity charged or neutral molecules having a pair of electron which they can donate to central metal. Thus ligands are called as electron donar species.
•Bond formation: A co-ordinate bond between metal and ligand is formed by the overlapping of empty orbital of central metal with orbitals of ligands having pair of electron. Both electrons are contributed by only ligands and thus the bond formed is called as Covalent Co-ordinate bond or dative bond.
•Hybridization: When Complex have all similar ligands, then all metal bonds have equal energy.
To explain this, it is suggested that the different vaccent orbitals of central metal mixed together to form same number of equivalent hybrid orbitals, This process is called as hybridisation.
In four Co-ordinated Complexes, the central metal undergoes sp³ or dsp² hybridisation.
In six Co-ordinated Complexes, the central metal undergoes spd² or d²sp³ hybridisation.
•Geometry: Hybrid orbitals are directional in nature. The relation between Coordination number, hybridisation and geometry of molecule is given by —
•Magnetic properties: The magnetic properties of Complexe depends upon the number of unpaired electrons present in central metal.
If the central metal ion of Complex does not contain unpaired electron , the complex should be diamagnetic in nature.
If the central metal of Complex contain unpaired electrons then then the Complex should be paramagnetic in nature.
The magnetic strength of Complex measured in the terms of unpaired electrons of central metal as —
μ= √n(n+2) B.M.
Where, n= number of unpaired electron
For diamagnetic Complexes, magnetic moment is zero wheras for paramagnetic Complexes, the magnetic moment value is small positive .
Limitations of V.B.T.
Valence Bond Theory suffers from following limitations—
1. Valence bond theory gives much more importance to the role of central metal but it ignores the role of ligands.
2. It fail to explain the absorption Spectra and thus colour of Complex.
3. It fails to explain the kinetics and mechanism of Complex formation.
4. It fails to explain the distorted octahedral geometry of [Cu (H2O)6 ]²+ Complex, even all ligands are same.
5.VBT fails to explain the pairing of unpaired electrons of central metal which is against Hunds rule.
Pauling explained the formation of metal ligand bond on the basis of overlapping of atomic orbitals but only difference is that, metal orbital is empty and ligand orbital have pair of electron. In this bond formation only ligand contribute their both electron and hence such type of bond is called as Covalent Co-ordinate bond or dative bond.
Postulates of V.B.T
Postulates of valence bond theory are as follows—
•Central metal: Each central metal ion or atom makes available empty orbitals, so that electron pair from ligands are accepted, thus central metal is called as electron acceptor species.
•Ligands: ligands are negativity charged or neutral molecules having a pair of electron which they can donate to central metal. Thus ligands are called as electron donar species.
•Bond formation: A co-ordinate bond between metal and ligand is formed by the overlapping of empty orbital of central metal with orbitals of ligands having pair of electron. Both electrons are contributed by only ligands and thus the bond formed is called as Covalent Co-ordinate bond or dative bond.
•Hybridization: When Complex have all similar ligands, then all metal bonds have equal energy.
To explain this, it is suggested that the different vaccent orbitals of central metal mixed together to form same number of equivalent hybrid orbitals, This process is called as hybridisation.
In four Co-ordinated Complexes, the central metal undergoes sp³ or dsp² hybridisation.
In six Co-ordinated Complexes, the central metal undergoes spd² or d²sp³ hybridisation.
•Geometry: Hybrid orbitals are directional in nature. The relation between Coordination number, hybridisation and geometry of molecule is given by —
•Magnetic properties: The magnetic properties of Complexe depends upon the number of unpaired electrons present in central metal.
If the central metal ion of Complex does not contain unpaired electron , the complex should be diamagnetic in nature.
If the central metal of Complex contain unpaired electrons then then the Complex should be paramagnetic in nature.
The magnetic strength of Complex measured in the terms of unpaired electrons of central metal as —
μ= √n(n+2) B.M.
Where, n= number of unpaired electron
For diamagnetic Complexes, magnetic moment is zero wheras for paramagnetic Complexes, the magnetic moment value is small positive .
Limitations of V.B.T.
Valence Bond Theory suffers from following limitations—
1. Valence bond theory gives much more importance to the role of central metal but it ignores the role of ligands.
2. It fail to explain the absorption Spectra and thus colour of Complex.
3. It fails to explain the kinetics and mechanism of Complex formation.
4. It fails to explain the distorted octahedral geometry of [Cu (H2O)6 ]²+ Complex, even all ligands are same.
5.VBT fails to explain the pairing of unpaired electrons of central metal which is against Hunds rule.
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