Analysis of the Detailed Configuration of Hydrated Lanthanoid(III) Ions in Aqueous Solution and Crystalline Salts by Using K- and L3-Edge XANES Spectroscopy

• Published in: Chemistry : a European journal Vol. 16; no. 2; pp. 684 - 692
• Main Authors: D'Angelo, Paola, Zitolo, Andrea, Migliorati, Valentina, Persson, Ingmar
• Format: Journal Article
• Language: English
• Published: Weinheim WILEY-VCH Verlag 11-01-2010; WILEY‐VCH Verlag; Wiley Subscription Services, Inc
• Subjects: Chemistry; hydrates; Hydration; lanthanides; Spectrum analysis; structure elucidation; X-rays; XANES spectroscopy
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.200900122

The structural properties of the hydrated lanthanoid(III) ions in aqueous solution and in the isostructural trifluoromethanesulfonate salts have been investigated by a quantitative analysis of the X‐ray absorption near‐edge structure (XANES) spectra at the K‐ and L3‐edges. The XANES analysis has provided a clear description of the variation of lanthanoid(III) hydration properties across the series. It was found that all of the lanthanoid(III) hydration complexes retain a tricapped trigonal prism (TTP) geometry, and along the series two of the capping water molecules become less and less strongly bound, before finally, on average, one of them leaves the hydration cluster. This gives rise to an eight‐coordinated distorted bicapped trigonal prism with two different LnO capping distances for the smallest lanthanoid(III) ions. This systematic study has shown that for lanthanoid compounds more accurate structural information is obtained from the analysis of the L3‐edge than from K‐edge XANES data. Moreover, whereas the second hydration shells provide a detectable contribution to the L3‐edge XANES spectra of the lighter lanthanoid ions, the K‐edge spectra are insensitive to the more distant coordination spheres.
A smooth transition: X‐ray absorption near‐edge structure (XANES) spectroscopy has been found to be the ideal experimental technique to shed light on the structure of the hydration complexes of the LnIII ions in aqueous solution. The analysis of the XANES spectra has provided definitive experimental proof of the retention of the tricapped trigonal prism geometry (see figure) of the LnIII hydrated ions along the series.