Ligand-polarization contributions to the intensity of hypersensitive trivalent lanthanide transitions

The oscillator strengths of the hypersensitive f-electron transitions in lanthanide(III) complexes, the quasi-quadrupolar transitions with intensities sensitive to the ligand environment, are found to arise principally from the coulombic correlation between transient induced electric dipoles in the ligands and the f-electron quadrupole moments of the metal ion. The correlation gives a non-zero resultant electric dipole transition moment in complexes belonging to the point groups, Dp and Cpv , with p unrestricted, C 3h , D 3h , Td , and their subgroups, the mechanism being forbidden where there is inversion symmetry or an Sp axis with p ⩾ 5. The theory accounts quantitatively for the hypersensitive oscillator strengths of the lanthanide trihalides, which range over two orders of magnitude, and the substituent effects observed in the lanthanide tris-β-dicarbonyl chelate complexes. The restriction of hypersensitivity to the qudrupolar f-electron transitions is discussed.