Explain crystal field splitting in an octahedral complex with a diagram. Use it to explain (i) why [Fe(CN)₆]³⁻ has one unpaired electron while [FeF₆]³⁻ has five, and (ii) why [Ti(H₂O)₆]³⁺ is coloured.
How to lay out the answer. Follow the frame, then write it in your own words.
1 · State the basis. Crystal field theory treats metal-ligand bonding as electrostatic; ligands act as point charges that split the degenerate d orbitals.
2 · Draw the splitting diagram. Five degenerate d orbitals split into t₂g (lower, −0.4Δₒ) and eg (upper, +0.6Δₒ); label the gap Δₒ.
3 · Explain the two iron complexes.
Fe³⁺ is 3d⁵.
CN⁻ is a strong field ligand: Δₒ > P, so t₂g⁵ eg⁰, 1 unpaired electron.
F⁻ is a weak field ligand: Δₒ < P, so t₂g³ eg², 5 unpaired electrons.
4 · Explain the colour. Ti³⁺ (d¹, t₂g¹) absorbs blue-green light (about 498 nm) and the electron jumps to eg: a d-d transition. The complementary colour, violet, is seen.
5 · Link to real life. Ruby and emerald owe their colours to d-d transitions of Cr³⁺ ions.
6 · Close in one line. So Δₒ compared with P decides the spin, and the size of Δₒ decides the colour.
Scoring tip: Draw the dotted average line and mark 0.4Δₒ and 0.6Δₒ. These labels carry a mark.
