Raman spectra of the lanthanide oxalates

Abstract Raman and infrared spectral data are presented for the trivalent lanthanide sesquioxalates, M 2 (C 2 O 4 ) 3 · n H 2 O, except the promethium complex, in the region from 100 to 4000 cm −1 . These complexes were isolated by precipitation from aqueous solution using Na 2 C 2 O 4 . In addition, several commercially available oxalate complexes were examined. On the basis of nearly identical Raman spectral patterns, two distinct structural groups have been identified. The first group consists of the isostructural (monoclinic) decahydrates of La 3+ through Er 3+ . For this group, a linear correlation is found for the frequencies of several skeletal stretching modes versus the lanthanide ionic radius. The second group consists of the isostructural (triclinic) hexahydrates of Er 3+ through Lu 3+ . In agreement with existing x‐ray diffraction data, these vibrational data indicate structural differences between the hexa‐ and deca‐hydrates. In addition, unique vibrational spectral patterns have been obtained for what appear to be other hydrated forms of the oxalates of Ho 3+ , Er 3+ and Lu 3+ , possessing structures different from those of the above hexa‐ and deca‐hydrates. For all of these complexes, the Raman data suggest that the oxalate ligand is tetradentate coordinated to two lanthanide ions. Thus, the structural differences undoubtedly result from the lanthanide contraction and changes in the disposition of coordinated and interstitial water molecules.