(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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