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9.2.4: Electronic Spectra - Ultraviolet and Visible Spectroscopy - Lanthanides and Actinides

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  • The ions of most lanthanide and actinide ions absorb light in the UV and visible regions.  The transitions involve only a redistribution of electrons within the 4f orbitals (--> f' transitions) are orbitally-forbidden by the quantum mechanical selection rules.  Consequently, the compounds of lanthanide ions are generally pale in color.

    Unlike transition metal complexes, the crystal/ligand field effects for the lanthanide 4f orbitals are virtually insignificant.  This is a result of the extensive shielding of the 4f electrons by the 5s and 5p orbitals.  Consequently the  f --> f' absorption bands are very sharp (useful fingerprinting and quantitation of LnIII)  and the optical spectra are virtually independent of environment.



    Figure\(\PageIndex {1}\): The spectrum of a neodymium(III) complex over the range of 25,000 cm-1 to 9100 cm-1 (400 nm to 1100 nm).  The sharp "atom-like" spectral features for Ln3+ ion complexes are the same be thay be in the gas, solid or solution phase. 

    The insensitivity of the f --> f' transitions leads to limited use in study of lanthanide materials.  CeIII and TbIII complexes have the highest intensity bands in the UV due to 4f--> 4fn-15d1 transitions.  The f -->d are not orbitally forbidden (n-1 = 0 (empty sub-shell) for CeIII = 7 (half-filled sub-shell) for TbIII).