LANTHANIDE-NITRATE INTERACTION IN ANHYDROUS ACETONITRILE AND COORDINATION NUMBERS OF THE LANTHANIDE IONS - FT-IR STUDY

The interaction between lanthanide ions Ln(III)(Ln = La, Nd, Sm-Dy, Er, Yb) and nitrate ions is investigated by FT-IR spectroscopy in dilute anhydrous MeCN solution. The work is performed for ratios R = [NO3-]t/[Ln(III)]t ranging from 0 to 8 and for solutions generally 0.05 M in Ln(III), prepared from anhydrous lanthanide perchlorates Ln(ClO4)3. When nitrate is progressively added to the Ln(ClO4)3 solutions, the formation of [Ln(NO3)n](3-n)+ species is clearly evidenced by the FT-IR spectra. All the NO3- ions are coordinated and bidentate. A quantitative study was performed using the nu1 and nu6 vibrational modes for coordinated NO3- ions. The average coordination numbers estimated for Nd, Eu, Th, and Er in solutions of trinitrates arc 9.0, 9.1, 8.3, and 8.2, respectively (+/- 0.3 unit). In presence of an excess NO-, these numbers become 9.8, 10.2, 10.0, 9.8, 9.9, and 9.9 (+/- 0.3 unit) for La, Nd, Eu, Tb, Er, and Yb, respectively. No hexanitrate species forms under the experimental conditions used (R up to 8). The structural aspect of the various nitrate species is also investigated. In the pentanitrate species, all the ligands appear to be equivalent, while large inequivalences are observed for Ln(NO3)3 solutions. Since for the latter most of the absorption bands assigned to nitrate vibrations contain several components, a curve-fitting procedure has been used for decomposing the nu2, nu4, and nu6 vibrations. There is a considerable difference between Ln(III) ions, the nitrate inequivalences being larger in the middle of the series.