Correlation of the intensity of the hypersensitive transitions of complex ions in solution with ligand polarizability

The change in the intensity of the 4f spectral bands of lanthanide (Ln) complex ions in solution, the so-called hypersensitive transitions (hst), has been subject of a series of investigations. It is of interest to relate the intensification of the hst observed on complex ion formation with the ligand properties. It has been assumed recently that the intensity increase for the hst is determined by the ligand polarization. Thus it was shown that ligand polarizability makes a significant contribution to the intensity of hst and they indicated how to calculate to a good degree of approximation the intensities of the hst for the gas phase trihalides and for complex ions in solution with β-diketone ligands. It was found that relating hypersensitivity to the ligand polarizability is formally equivalent to the model assumed earlier, i.e., dielectric inhomogeneity. It was concluded that the intensity of the hst is determined by the polarizability of the solvent molecules. The calculation of the intensity of the absorption spectrum of the Nd3+ ion in the crystal fields of Y2O3 and CaWO4 was made by using the polarizability of the surrounding ions. In the present work we will relate the intensities of the hst of the ions Nd3+, Ho3+ and Er3+ as indicated by their oscillator strengths (P) to the ligand polarizabilities as measured by their molar refractions