The interaction of dimethylsulphoxide (DMSO) with neodymium, terbium and erbium nitrates is studied in dilute anhydrous acetonitrile solutions. The FT-IR difference spectra are measured for ratios R = [DMSO]t[Ln]tranging from 0 to 6 and for [LnIII] = 0.05 M and the results are compared with those obtained previously for europium solutions. The nitrate ions remain coordinated in a bidentate fashion; dissociation occurs only if R > 9. Bound acetonitrile molecules are present up to R = 2.9 (europium, terbium, erbium) or 4.1 (neodymium). The number of coordinated DMSO molecules per LnIIIion has been determined by the molar ratio method applied to both the v7(1008 cm-1) and v22(716 cm-1) absorptions. For R = 6, the values are 3.6 (neodymium), 3.1 (europium), 3.0 (terbium) and 3.0 (erbium), which leads to the following average coordination numbers: 9.6 (neodymium), 9.1 (europium), 9.0 (terbium) and 9.0 (erbium). These coordination numbers depend upon the DMSO concentration. Between R = 2 and R = 6 equilibria take place between eight- and nine-coordinate species (europium, terbium, erbium, lg K9= 1.8 ± 0.2) or nine- and ten-coordinate species (neodymium, lg K10= 1.1 ± 0.2). © 1986.
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