Q3(b). Colourless Ce³⁺ (4f¹) and red Ce⁴⁺ (4f⁰)

1. Colour needs absorption between 400 and 700 nm. Lanthanoid colours usually come from f–f transitions; these are Laporte forbidden (f and f are both u), so they are weak and sharp, and the 4f orbitals, shielded by 5s and 5p, hardly feel the ligands.
2. Ce³⁺, 4f¹: one electron gives only the term ²F, split by spin–orbit coupling into two levels, ²F_(5/2) (ground) and ²F_(7/2). The only f–f transition connects these two levels and lies in the infrared.
3. 4f → 5d in Ce³⁺ is Laporte allowed (Δ l=1) and intense, but it needs more energy: it lies in the ultraviolet. With nothing absorbed in the visible, Ce³⁺ salts are colourless.
4. Ce⁴⁺, 4f⁰: there is no f electron, so no f–f or f–d transition is possible. Colour must come from another source.
5. Charge transfer: Ce⁴⁺ is a strongly oxidising, high-charge ion (it is readily reduced to 4f¹). An electron is easily moved from a filled ligand orbital (O of water or nitrate, or a halide) into the empty 4f orbital: ligand-to-metal charge transfer (LMCT). The transition is Laporte and spin allowed, so it is intense.
6. Position: because Ce⁴⁺ is so easily reduced, the LMCT band lies low, in the near UV and running into the violet–blue. Blue light is removed and the transmitted light looks yellow–orange to red, as in ammonium hexanitratocerate(IV), (NH₄)₂[Ce(NO₃)₆].

Parallel: MnO₄⁻ (3d⁰, purple) and CrO₄²⁻ (3d⁰, yellow) owe their colours to LMCT in the same way; an ion with no d or f electrons is coloured only when it is strongly oxidising.

Application. the fading of the orange Ce⁴⁺ colour is used in cerimetry: Ce⁴⁺ titrations (for example of Fe²⁺) need only a redox indicator such as ferroin, and the reagent is stable in acid.