Limitations of Crystal Field Theory

Crystal Field Theory is, to a large extent, genuinely successful — it gives a workable account of the formation, structures, colour, and magnetic properties of coordination compounds, and it does all of this with a comparatively simple electrostatic picture. Even so, the model has real weaknesses that follow directly from its founding assumption of treating ligands as simple point charges:

• It gets the spectrochemical ordering of anionic ligands wrong in principle. If ligands really behaved as point charges, then anionic ligands — carrying an actual negative charge — should produce the greatest electrostatic repulsion with the metal $d$ electrons, and so should sit at the strong-field end of the spectrochemical series. Experimentally, the opposite is found: anionic ligands are generally found at the low (weak-field) end of the series, not the high end.
• It ignores covalency entirely. By construction, CFT treats the metal–ligand interaction as purely ionic/electrostatic, so it takes no account whatsoever of the genuine covalent character present in many metal–ligand bonds. Real bonding is not purely electrostatic, and a theory that assumes it is will inevitably mispredict some properties. …