Question

What are the limitations of VB theory?

Answer 1

1. This theory does not offer a quantitative explanation of the stability of complexes.
2. In this theory, the coupling of electrons against Hund's rule under the influence of a strong ligand is not adequately defined. The theory fails to address the reason why the electrons must be rearranged in opposition to Hund's rule at one point, while the electronic configuration remains unaltered at another. This generates superfluous complications when determining the geometry of a complex.
3. It is incapable of interpreting the absorption spectra of complexes.
4. It is unable to interpret the absorption spectra of complexes.
5. It fails to elucidate the behaviour of the d8 ion (Ni²⁺, Pb²⁺, Au³⁺, etc.) in the formation of the anticipated 4-coordination complexes. The theory's preference for square planar structures over tetrahedral or other structures is not entirely logical.
6. Sometimes, the theory necessitates the transfer of electrons from a lower energy level to a higher energy level, which is illogical in the absence of an energy supplier.
7. This theory fails to provide a satisfactory explanation for the reason why certain complexes are more labile than others (labile complexes are those in which one ligand can be readily replaced by another ligand).
8. It fails to account for the fracturing of d-orbitals of the central metal atom as ligands approach it.
9. It does not provide a quantitative explanation as to why certain complexes are inner orbital complexes and the others are outer orbital complexes for the same metal.
10. For example, [Fe(CN)₆]⁴⁻ is diamagnetic (low spin) whereas [Fe(H₂O)₆]⁴⁻is paramagnetic (high spin).