Infrared and Raman spectroscopic and theoretical studies of nonaaqua complexes of trivalent rare earth metal ions

Abstract

Infrared (IR), far-infrared (far-IR) and Raman spectra have been recorded of the hydrated lanthanide(III) trifluoromethanesulfonates, [Ln(OH 2) 9](CF 3SO 3) 3, (Ln = La, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu), and also for the deuterated [La(OD 2) 9](CF 3SO 3) 3 compound. This has enabled complete assignments of all seven metal–oxygen stretching modes for the first time. Force constant calculations have been performed for an LnO 9 3+ moiety, with a tricapped trigonal prismatic oxygen atom arrangement in D 3h point group symmetry. The lanthanide contraction in reflected in continuously increasing trends for the averaged Ln O stretching frequencies and force constants with increasing inverse of ionic radii. The stretching force constants increased from 0.81 to 1.16 Ncm −1 for the LnO 6 prism in the series from La to Lu, while those for the capping LnO 3 bonds increased from 0.49 to 0.65 Ncm −1. The Ln O stretching force constants showed a smooth correlation with the Ln O bond distances to the prism oxygen atoms, while the correlation to the Ln O capping oxygen atoms has a different shape, reflecting the increased ligand–ligand repulsion with decreasing lanthanide ion size.