Crystal field theory treats the ligands as point charges. Which of the following observations shows a weakness of this model?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Anionic ligands are found at the low end of the spectrochemical series.
If ligands acted only as point charges, a negatively charged ligand should repel the d electrons most and so give the largest splitting.
In fact, anions such as I−, Br−, Cl− and F− are weak field ligands at the low end of the spectrochemical series, while neutral CO is at the top.
The model also ignores the covalent character of the metal–ligand bond.
Why the other options are incorrect
Option (1) The five d orbitals of an isolated gaseous metal ion have the same energy. — Incorrect. This degeneracy of the free ion is the starting point of the theory, not a weakness of it.
Option (3) The complex [Ti(H2O)6]3+ is violet in colour. — Incorrect. CFT explains this colour well: the single d electron absorbs blue-green light and jumps from t2g to eg (a d–d transition).
Option (4) In an octahedral field, the dx²−y² and dz² orbitals are raised in energy. — Incorrect. CFT explains this too: these orbitals point towards the ligands and are repelled more.
NCERT (2026-27 edition):
5.5.6 Limitations of Crystal Field Theory – “…However, from the assumptions that the ligands are point charges, it follows that anionic ligands should exert the greatest splitting effect. The anionic ligands actually are found at the low end of the spectrochemical series. Further, it does not take into account the covalent character of bonding between the ligand and the central atom.”
5.5.4 Crystal Field Theory – “…ligands can be arranged in a series in the order of increasing field strength as given below: I– < Br– < SCN– < Cl– < S2– < F– < OH– < C2O42– < H2O < NCS– < edta4– < NH3 < en < CN– < CO”