Two Major Limitations Of Molecular Orbital Theory (MOT)

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Molecular Orbital Theory (MOT):

The VBT has two most serious limitations that electrons in molecules are treated as though they are localised and behave almost as they did in isolated atoms. This means that the VBT retains the individuality of the atoms composing molecule. The problem can be resolved by introducing the resonance theory, but with the loss of the original valence bond model. Hund[ 173], Mulliken[ 174], Van Vleck[175], Helsenberg[176], Jones[ 177] and others suggested an alternate treatment which is entirely different and more satisfactory.

According to MOT, the valency electrons are considered to be associated with all the nuclei concerned. The atomic orbitals from different atoms must be combined and produce a resultant orbital
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Therefore, the electrons may be described as occupying an atomic orbital, or by a wave function  which is a solution to the Schrodinger wave equation. The atomic orbitals must satisfy the following conditions to interact to form molecular orbitals. Atomic orbitals must (i) have roughly the same energy (ii) overlap one another as much as possible (iii) have the same symmetry with respect to bonding molecular axis. 4. Ligand Field Theory (LFT):

LFT is a more comprehensive and sophisticated theory is now evolved from the combination of CFT and MOT. Cotton defined LFT as the theory of the origin and the consequence of the splitting of the inner orbitals of transition metal ions and their chemical environment. The CFT assumes that the orbitals of the ligand atom do not overlap with the orbitals of the central metal ion under consideration. However, sufficient experimental evidence shows that there always some overlapping between ligand and metal orbitals.
It has been proved that the metal complexes have some degree of covalency in them as against what was thought by the CFT modelists. Evidence of covalency in the metal-ligand bond can be seen from PMR and ESR spectra of the
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