Spectroscopic evidence for equilibrium between eight- and nine-coordinate Eu3+(aq) species in 0.1 M EuCl3(aq)

Crystalline europium bromate, [Eu(H2O)(9)][(BrO3)(3)], europium ethyl sulfate, [Eu(H2O)(9)][(C2H5SO4)(3)], and europium decavanate, [Eu(H2O)(8)](2)[(V10O28)].8H(2)O, are used as models for nine- and eight-coordinate Eu3+ aquo species in solution. A comparison of the D-5(1) --> F-7(0) laser excitation spectra and D-0 --> F-7(1,2) emission spectra of the model crystal systems with that of 0.1 M EuCl3(aq) clearly indicates a significant presence of the Eu(H2O)(8)(3+) species in aqueous solution. Modeling the 0.1 M EuCl3(aq) spectra using weighted sums of the eight and nine-coordinate crystal spectra suggest that approximately two-thirds of Eu3+ ions exist as Eu(H2O)(8)(3+) in aqueous solution. The current study takes advantage of the symmetry of the Eu(H2O)(n)(3+) complexes and lends further support, from this perspective, that an equilibrium exists between Eu(H2O)(8)(3+) and Eu(H2O)(9)(3+) with significant representation from both species.