Limitations of Valence Bond Theory

Valence Bond Theory does a reasonably good job of accounting for the formation, geometry, and magnetic behaviour of many coordination compounds. Even so, it carries a number of serious shortcomings that limit how far it can be trusted:

• It rests on a fairly large number of underlying assumptions about which orbitals hybridise and how, rather than deriving these from first principles.
• It provides no quantitative treatment of magnetic data — it can rationalise whether a complex is paramagnetic or diamagnetic, and roughly how many unpaired electrons are present, but it cannot predict magnetic moments as numbers with any precision.
• It has nothing to say about colour. Since VBT does not distinguish the energies of individual $d$ orbitals (it treats hybrid orbitals as equivalent), it cannot explain why coordination compounds absorb specific wavelengths of visible light and appear coloured.
• It gives no quantitative measure of stability — neither the thermodynamic stability (how favourable complex formation is energetically) nor the kinetic stability (how readily the complex reacts or exchanges ligands) can be predicted from the theory.
• It cannot reliably predict which geometry a four-coordinate complex will adopt — whether tetrahedral or square planar — without already knowing the answer from experiment. …