Which of the following cannot be explained by crystal field theory?

🧠 What CFT Can and Cannot Do

Crystal Field Theory treats ligands as point charges that electrostatically split the metal d-orbitals into $t_{2g}$ and $e_g$ (for octahedral geometry). With this framework, CFT successfully explains:

• ✓ Magnetic properties: HS vs LS, unpaired count from filling.
• ✓ Colour of complexes: d–d transitions across $\Delta_o$.
• ✓ Stability (in part): CFSE contributes to thermodynamic stability.

What CFT cannot explain:

• ✗ The order of the spectrochemical series — specifically, why CN⁻ and CO sit at the strong-field end and why F⁻ sits at the weak-field end. CFT can't distinguish ligands by their bonding character because it treats them all as point charges of the same type.

🗺️ Why the Spectrochemical Series Needs MOT, Not CFT

The actual spectrochemical ordering is driven by π-bonding effects that CFT ignores:

• π-acceptor ligands (CO, CN⁻, NO): empty $\pi^*$ orbitals accept electron density from filled metal $t_{2g}$ → stabilises $t_{2g}$ → effectively increases $\Delta_o$.
• π-donor ligands (F⁻, OH⁻, O²⁻, Cl⁻): filled p orbitals donate into metal $t_{2g}$ → destabilises $t_{2g}$ → decreases $\Delta_o$.

CFT, being purely electrostatic, has no machinery for this — only Molecular Orbital Theory / Ligand Field Theory captures it.

⚡ CFT vs LFT in One Line

• CFT: ligands = point charges, electrostatic only.
• LFT/MOT: ligands have orbitals; σ + π bonding is real.

For practical JEE problems, CFT is enough for predicting magnetism, colour, and CFSE — but it can't justify the spectrochemical series, only use it as input.

⚠️ Don't Confuse "Predict" with "Explain"

CFT can use the spectrochemical series (as an empirical input) to predict spin state and colour. But it cannot derive why CN⁻ > NH₃ > F⁻ — that requires bonding theory.

$\boxed{\text{Answer: (1) The order of the spectrochemical series}}$