| Abstract
| - All lanthanoid(III) ions were found to coordinate eight dimethylsulfoxide (DMSO) oxygen atoms in a distorted square antiprism. The transition from an orthorhombic to a monoclinic space group occurring between Pr and Nd with less-efficient packing for the smaller ions seems induced by the increasing polarization of the Ln−O bonds. Up to three of the DMSO ligands were disordered in alternative configurations related by a twist around the Ln−O bond.
- The octakis(DMSO) (DMSO = dimethylsulfoxide) neodymium(III), samarium(III), gadolinium(III), dysprosium(III),erbium(III), and lutetium(III) iodides crystallize in the monoclinic space group P21/n (No. 14) with Z = 4, while theoctakis(DMSO) iodides of the larger lanthanum(III), cerium(III), and praseodymium(III) ions crystallize in theorthorhombic space group Pbca (No. 61), Z = 8. In all [Ln(OS(Me2)8]I3 compounds the lanthanoid(III) ions coordinateeight DMSO oxygen atoms in a distorted square antiprism. Up to three of the DMSO ligands were found to bedisordered and were described by two alternative configurations related by a twist around the metal−oxygen (Ln−O) bond. To resolve the atomic positions and achieve reliable Ln−O bond distances, complete semirigid DMSOmolecules with restrained geometry and partial occupancy were refined for the alternative sites. This disordermodel was also applied on previously collected data for the monoclinic octakis(DMSO)yttrium(III) iodide. At ambienttemperature, the eight Ln−O bond distances are distributed over a range of about 0.1 Å. The average valueincreases from Ln−O 2.30, 2.34, 2.34, 2.36, 2.38, 2.40 to 2.43 Å (Ln = Lu, Er, Y, Dy, Gd, Sm, and Nd) for themonoclinic [Ln(OSMe2)8]I3 structures, and from 2.44, 2.47 to 2.49 Å (Ln = Pr, Ce, and La) for the orthorhombicstructures, respectively. The average of the La−O and Nd−O bond distances remained unchanged at 100 K, 2.49and 2.43 Å, respectively. Despite longer bond distances and larger Ln−O−S angles, the cell volumes are smallerfor the orthorhombic structures (Ln = Pr, Ce, and La) than for the monoclinic structure with Ln = Nd, showing amore efficient packing arrangement. Raman and IR absorption spectra for the [Ln(OS(CH3)2)8]I3 (Ln = La, Ce, Pr,Nd, Gd, Tb, Dy, Er, Lu, and Y) compounds, also deuterated for La and Y, have been recorded and analyzed bymeans of normal coordinate methods. The force constants for the Ln−O and S−O stretching modes in the complexesincrease with decreasing Ln−O bond distance and show increasing polarization of the bonds for the smaller andheavier lanthanoid(III) ions.
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