Q1(i). Electronic spectra of Eu³⁺ and Am³⁺ complexes
Account for the similar electronic spectra of Eu³⁺ complexes with various ligands and the variation of electronic spectra of Am³⁺ complexes as the ligand is varied.
Answer in brief
Eu³⁺ (4f⁶): the 4f orbitals are buried under filled 5s and 5p shells, so ligands hardly perturb them and the f–f bands are sharp and ligand-independent. Am³⁺ (5f⁶): the 5f orbitals extend further, interact with ligands and give broader bands that shift and change in intensity with the ligand.
Key points
- 4f shielding: the 4f shell lies inside 5s² 5p⁶. Crystal-field splitting is 100 cm⁻¹ (small); spin–orbit coupling dominates, so the spectrum resembles that of the free ion: line-like and ligand-insensitive.
- 5f exposure: 5f orbitals are less shielded, and have a larger radial extension; there is some covalency (nephelauxetic shift to lower energy) and larger crystal-field splitting ( 10²–10³ cm⁻¹), so the band positions and…
- Laporte-forbidden f–f bands are weak (ε<10) in both; in Am³⁺ they are stronger and more sensitive (hypersensitive transitions) to the ligand field.
The full model answer
Worked out step by step, as it should go in the answer booklet. The solution sheet follows it.
- 4f shielding: the 4f shell lies inside 5s² 5p⁶. Crystal-field splitting is 100 cm⁻¹ (small); spin–orbit coupling dominates, so the spectrum resembles that of the free ion: line-like and ligand-insensitive.
- 5f exposure: 5f orbitals are less shielded, and have a larger radial extension; there is some covalency (nephelauxetic shift to lower energy) and larger crystal-field splitting ( 10²–10³ cm⁻¹), so the band positions and intensities change with the ligand.
- Intensity: Laporte-forbidden f–f bands are weak (ε<10) in both; in Am³⁺ they are stronger and more sensitive (hypersensitive transitions) to the ligand field.
the difference is exploited in the separation of Am(III) from Eu(III) by soft-donor extractants (BTP, BTBP), which bind the more covalent Am³⁺ more strongly.
Coversspectra of Ln and An ionsElectronic spectra of Eu³⁺ and Am³⁺ complexes
All parts of Question 1
- 1(a) Zinc blende and wurtzite: radius-ratio rationale5 marks
- 1(b) Spin-forbidden bands in 4d and 5d complexes5 marks
- 1(c) Phosphate buffer at pH 7.405 marks
- 1(d) Onset of AgCl precipitation5 marks
- 1(e) Iodometry and iodimetry5 marks
- 1(f) Masking agents in the estimation of Ca²⁺5 marks
- 1(g) First-row metal in EAN-obeying organometallics5 marks
- 1(h) Radioactive displacement: ²¹⁰Pb and ²²⁶Ra5 marks
- 1(i) Electronic spectra of Eu³⁺ and Am³⁺ complexes5 marks
- 1(j) Urea required to precipitate Fe³⁺ as Fe(OH)₃5 marks