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LIMITATIONS OF VALENCE BOND THEOTY (VBT):


(1) Although valence bond theory provides a satisfactory representation of the complex compound based upon the concept of orbital hybridisation, it cannot account for the relative stabilities for different shapes and coordination numbers in metal complexes.
(2) VBT cannot explain as to why Cu2+ forms only distorted octahedral complexes even when all the six ligands are identical.
(3) The valence bond theory does not provide any satisfactory explanation for the existence of inner orbital and outer orbital complexes.
(4) Sometimes the theory requires the transfer of electron from lower energy to the higher energy level, which is very much unrealistic in absence of any energy supplier (for example, this happens in the case of [CuX4]2- .
(5) The changes in the properties of the metal ion along with the ligands and the simple metal ions cannot be explained. For example, the colour changes associated with electronic transition within d orbitals are affected on formation of complex, but the valence bond theory does not offer any explanation.
(6) Sometimes the same metal acquires different geometry when formation of complex takes place with different ligands. The theory does not explain as to why at one time the electrons must be rearranged against the Hund’s rule while, at other times the electronic configuration is not disturbed.
(7) The energy change of the metal orbitals on formation of complex is difficult to be calculated mathematically.
(8) VBT fails to explain the finer details of magnetic properties including the magnitude of the orbital contribution to the magnetic moments.
(9) The VBT does not explain why certain complexes are more labile than the others.
(10) It does not give quantitative interpretation of thermodynamic or kinetic stabilities of coordination compounds.
(11) It does not make exact predictions regarding the tetrahedral and square planar structure of 4-coordinate complexes.
(12) It does not tell about the spectral properties of coordination compounds.

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