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

Infrared (IR), far-infrared (far-IR) and Raman spectra have been recorded of the hydrated lanthanide(III) trifluoromethanesulfonates, [Ln(OH2)9](CF3SO3)3, (Ln=La, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu), and also for the deuterated [La(OD2)9](CF3SO3)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 LnO9(3+) moiety, with a tricapped trigonal prismatic oxygen atom arrangement in D3h point group symmetry. The lanthanide contraction in reflected in continuously increasing trends for the averaged LnO 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 LnO6 prism in the series from La to Lu, while those for the capping LnO3 bonds increased from 0.49 to 0.65 Ncm-1. The LnO stretching force constants showed a smooth correlation with the LnO bond distances to the prism oxygen atoms, while the correlation to the LnO capping oxygen atoms has a different shape, reflecting the increased ligand-ligand repulsion with decreasing lanthanide ion size.