Lanthanide‐nitrate interaction in anhydrous acetonitrile and coordination numbers of the lanthanide ions: FT‒IR study

Abstract 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 = [NO ] 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(ClO 4 ) 3 . When nitrate is progressively added to the Ln(ClO 4 ) 3 solutions, the formation of [Ln(NO 3 ) n ] (3− n )+ species is clearly evidenced by the FT‐IR spectra. All the NO 3 − ions are coordinated and bidentate. A quantitative study was performed using the v 1 and v 6 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 (±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 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 3 ) 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 v 2 , v 4 and v 6 vibrations. There is a considerable difference between Ln III ions, the nitrate inequivalences being larger in the middle of the series.