Rational approach of the stoichiometries of lanthanide complexes with $alpha;2-6P2W17O61910$minus; heteropolytungstate in aqueous solution*1

Abstract

An extended series of lanthanides (Ln) in their LnIII oxidation state was reacted with [α2-P2W17O61]10− (HPA) and the formation constants were determined for the complexes. The technique was the competition between this lacunary heteropolyanion and Arsenazo III for complexation of the same lanthanide. Unlike the usually featureless spectra of the Ln–HPA derivative, the visible spectra of the Arsenazo III complexes, in the presence of HPA, remain sharp, well-defined and could be recorded without interference from the Ln:HPA complexes formed simultaneously. These spectra were compared to those obtained with the same concentration of lanthanide in the absence of the heteropolyanion. Previous knowledge of the formation constants for Arsenazo III complexes with each lanthanide was necessary, so that those pertaining to the heteropolyanion complexes were left as the only unknowns in the set of equations. The 1:1 and 1:2 types of Ln:HPA complexes were found to coexist easily. Typically, values of logβ1=9.66 and logβ2=16.26 were found for the apparent formation constants corrected for acidity and other complexation effects in the case of the Ce3+ cation. Such values explain the coexistence of the two types of complexes in dilute solutions, with the possibility to keep a majority of a single species in solution under drastic concentration conditions. The same observation holds for all the series of LnIII species examined in this work, because the formation constants are close from one lanthanide to the next. The comparison with the few values of formation constants available in the literature is satisfactory.