The interaction between lanthanide ions Ln"' (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 = [NO;],/[Ln"'], ranging from 0 to 8 and for solutions generally 0 . 0 5 ~ in Ln"', prepared from anhydrous lanthanide perchlorates Ln(CIO,),. When nitrate is progressively added to the Ln(C10,), solutions, the formation of [Ln(N03),](3 -n)t species is clearly evidenced by the FT-IR spectra. All the NO; ions are coordinated and bidentate. A quantitative study was performed using the vI and v6 vibrational modes for coordinated NO; ions. The average coordination numbers estimated for Nd, Eu, Tb, and Er in solutions of trinitrates are 9.0, 9.1, 8.3, and 8.2, respectively (M.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 hexanitrato species forms under the experimental conditions used ( R up to 8). The structural aspect of the various nitrato species is also investigated. In the pentanitrato species, all the ligands appear to be equivalent, while large inequivalences are observed for Ln(NO,), 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 vz, v,, and v6 vibrations. There is a considerable difference between Ln"' ions, the nitrate inequivalences being larger in the middle of the series.
Introduction. -During the last decades, there has been a growing interest in the use of lanthanide ions as probes in biochemical systems [I]. This has prompted numerous studies about the coordination properties of these ions, particularly in solution. In the case of trivalent lanthanide ions, this information is difficult to acquire in view of their high kinetic lability, their nondirectional chemical bonding, and the range of large coordination numbers they can display. A recent paper reviews the subject in aqueous solutions [2]. The coordination numbers of Ln"' ions and their interaction with anions and neutral molecules in organic solvents have been systematically investigated in our laboratory by means of UVjVIS absorption spectroscopy [3], luminescence , and Fourier -transform infrared spectroscopy (FT-IR) [6] [7] [lo-141. Studies of the lanthanide-nitrate interaction as well as the determination of stability constants and thermodynamic parameters have been performed in various solvents, e.g. aqueous and anhydrous alcohols [15], dimethylformamide [3] [4] [7] [16] [17], dimethylacetamide [ 181, dimethylsulfoxide [ 191, and H,O/acetone mixtures [20]. In anhydrous MeCN, conductimetric measurements on dilute solutions of Nd, Eu, or Tb nitrates indicate that no dissociation occurs [3] [6] [lo]. Moreover, the nitrato group frequencies are consistent with bidentate anions having C,, local symmetry.
') Part 13 of the series 'FT-IR and Fluorometric Investigation of Rare-Earth and Metal Ion Solvation'; Part 12: ~4 1 .