Notes On Valence Bond Theory - CBSE Class 12 Chemistry
The valence bond theory satisfactorily explains the structure and magnetic properties of a large number of coordination compounds. The valence bond theory was proposed and developed by Linus Pauling. Salient features of the theory: The central metal atom (or) ion has the required number of vacant orbitals for accommodating the electrons donated by the ligands. The number of vacant orbitals is equal to the coordination number of the metal ion for a particular complex. The vacant orbitals of the metal atom (or) ion undergo suitable hybridisation to yield a set of equivalent hybrid orbitals of definite geometry. A ligand orbital containing a lone pair of electrons forms a coordinate covalent bond by overlapping with the hybrid orbitals of the metal ion. The magnetic moment of the complex can be used to determine its geometry. In the formation of a complex, if the metal utilises its inner d -orbitals, then such a complex is called an inner orbital complex (or) low-spin (or) spin-paired complex. If the metal utilises its outer d- orbitals, then such a complex is called an outer orbital complex (or) high-spin (or) spin-free complex. Drawbacks of the valence bond theory: a) Does not explain the colour exhibited by coordination compounds. b) Does not provide a quantitative interpretation of the stability of the complexes. c) Does not give the quantitative interpretation of magnetic data. d) Fails to predict the exact geometry of the complexes with the coordination number four.

#### Summary

The valence bond theory satisfactorily explains the structure and magnetic properties of a large number of coordination compounds. The valence bond theory was proposed and developed by Linus Pauling. Salient features of the theory: The central metal atom (or) ion has the required number of vacant orbitals for accommodating the electrons donated by the ligands. The number of vacant orbitals is equal to the coordination number of the metal ion for a particular complex. The vacant orbitals of the metal atom (or) ion undergo suitable hybridisation to yield a set of equivalent hybrid orbitals of definite geometry. A ligand orbital containing a lone pair of electrons forms a coordinate covalent bond by overlapping with the hybrid orbitals of the metal ion. The magnetic moment of the complex can be used to determine its geometry. In the formation of a complex, if the metal utilises its inner d -orbitals, then such a complex is called an inner orbital complex (or) low-spin (or) spin-paired complex. If the metal utilises its outer d- orbitals, then such a complex is called an outer orbital complex (or) high-spin (or) spin-free complex. Drawbacks of the valence bond theory: a) Does not explain the colour exhibited by coordination compounds. b) Does not provide a quantitative interpretation of the stability of the complexes. c) Does not give the quantitative interpretation of magnetic data. d) Fails to predict the exact geometry of the complexes with the coordination number four.

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