X-ray Absorption Fine Structure Spectroscopic Studies of Octakis(DMSO)lanthanoid(III) Complexes in Solution and in the Solid Iodides

Octakis(DMSO)lanthanoid(III) iodides (DMSO = dimethylsulfoxide), [Ln(OS(CH3)(2))(8)]I-3, of most lanthanoid(III) ions in the series from La to Lu have been studied in the solid state and in DMSO solution by extended X-ray absorption fine structure (EXAFS) spectroscopy. L-3-edge and also some K-edge spectra were recorded, which provided mean Ln-O bond distances for the octakis(DMSO)lanthanoid(III) complexes. The agreement with the average of the Ln-O bond distances obtained in a separate study by X-ray crystallography was quite satisfactory. The crystalline octakis(DMSO)lanthanoid(III) iodide salts have a fairly broad distribution of Ln-O bond distances, ca. 0.1 angstrom, with a few disordered DMSO ligands. Their EXAFS spectra are in excellent agreement with [hose obtained for the solvated lanthanoid(III) ions in DMSO solution, both of which show slightly asymmetric distributions of the Ln-O bond distances. Hence, all lanthanoid(III) ions are present as octakis(DMSO)lanthanoid(ill) complexes in DMSO solution, with the mean Ln-O distances centered at 2.50 (La), 2.45 (Pr), 2.43 (Nd), 2.41 (Sm), 2.40 (Eu), 2.39 (Gd), 2.37 (Tb), 2.36 (Dy), 2.34 (Ho), 2.33 (Er), 2.31 (Tm), and 2.29 angstrom (Lu). This decrease in the Ln-O bond distances is larger than expected from the previously established ionic radii for octa-coordination. This indicates increasing polarization of the Ln(III)-O(DMSO) bonds with increasing atomic number. However, the S(1s) electron transition energies in the sulfur K-edge X-ray absorption near-edge structure (XANES) spectra, probing the unoccupied molecular orbitals of lowest energy of the DMSO ligands for the [Ln(OS(CH3)(2))(8)](3+) complexes, change only insignificantly from Ln = La to Lu. This indicates that there is no appreciable change in the sigma-contribution to the S-O bond, probably due to a corresponding increase in the contribution from the sulfur lone pair to the bonding.