Correlation between Ligand Coordination Number and the Shift of the ⁷F₀−⁵D₀ Transition Frequency in Europium(III) Complexes

The relative shift of the Eu(III) 7 F 0 − 5 D 0 transition toward lower energies due to the formation of inner-sphere Eu(III) complexes is shown to be linearly proportional to the total ligand donor number of the ligand bound to Eu(III) for 40 organic ligand complexed species in both water and DMSO solvents. Such a correlation allows estimation of the donor number, CN L, of organic ligands coordinated to Eu(III) from the relative frequency, Δν, of the Eu(III) 7F 0 − 5 D 0 transition by the equation of CN L = 0.237Δν + 0.628. The validity of the method has been confirmed by using shift data of other Eu(III) complex systems from the literature. The nature of the shift of 7 F 0 − 5 D 0 band upon complexation is discussed as possibly due to a small degree of covalency in the Eu−L bond of the complexes with organic ligands.