FT-IR and fluorometric investigation of rare-earth and metal ion solvation. Part 11. Interaction between DMSO and lanthanide perchlorates in anhydrous acetonitrile

Abstract

The interaction between DMSO and lanthanide perchlorates in anhydrous acetonitrile has been investigated by FT-IR and Raman spectroscopy. A quantitative study was performed on 0.05 M solutions with R=[DMSO] t/ [Ln] t=7−20. The v 7(SO) vibration of DMSO was used to determine the concentration of free DMSO in solution. None of the investigated systems shows any interaction with either perchlorate ions or acetonitrile molecules. The average coordination number ( CN) of the Ln(III) ions is therefore equal to the average number of bonded DMSO molecules. Changes in coordination numbers are observed both with increasing atomic number and with the solvent composition. The data may be interpreted in terms of equilibria between six-, seven-, eight-, nine- and ten-coordinate species. For R=8, a gradual change is observed between 7.8 (La) and 6.6 (Lu). For R=12 and 15, the overall change amounts to 1.3 (8.7−7.4) and 1.7 units (9.7−8.0), respectively, but in addition, a clear discontinuity is observed at Gd (c. 1 unit). For R=15, additional breaks (c. 0.5 unit) occur around Nd and Er. For a single ion, changes in coordination number with increasing DMSO composition ( R=7−15) amounts to almost 2 units for lighter Ln ions and to 1.0−1.2 units for heavier ions. All the DMSO molecules in the inner-coordination sphere appear to be equivalent, so that the increase in CN is interpreted in terms of an expansion of the first coordination sphere to accommodate additional donor molecules. Equilibrium constants have been estimated from the average CNs. They vary between log K 8=2.2 (La) to 0.6 (Lu), log K 9=1 (La) to 0.4 (Lu) and log K 10=0.9 (La) to 0.3 (Sm).