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Covalency in Lanthanides. An X‑ray Absorption Spectroscopy and Density Functional Theory Study of LnCl6x (x = 3, 2)

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posted on 2015-02-25, 00:00 authored by Matthias W. Löble, Jason M. Keith, Alison B. Altman, S. Chantal E. Stieber, Enrique R. Batista, Kevin S. Boland, Steven D. Conradson, David L. Clark, Juan Lezama Pacheco, Stosh A. Kozimor, Richard L. Martin, Stefan G. Minasian, Angela C. Olson, Brian L. Scott, David K. Shuh, Tolek Tyliszczak, Marianne P. Wilkerson, Ralph A. Zehnder
Covalency in Ln–Cl bonds of Oh-LnCl6x (x = 3 for Ln = CeIII, NdIII, SmIII, EuIII, GdIII; x = 2 for Ln = CeIV) anions has been investigated, primarily using Cl K-edge X-ray absorption spectroscopy (XAS) and time-dependent density functional theory (TDDFT); however, Ce L3,2-edge and M5,4-edge XAS were also used to characterize CeCl6x (x = 2, 3). The M5,4-edge XAS spectra were modeled using configuration interaction calculations. The results were evaluated as a function of (1) the lanthanide (Ln) metal identity, which was varied across the series from Ce to Gd, and (2) the Ln oxidation state (when practical, i.e., formally CeIII and CeIV). Pronounced mixing between the Cl 3p- and Ln 5d-orbitals (t2g* and eg*) was observed. Experimental results indicated that Ln 5d-orbital mixing decreased when moving across the lanthanide series. In contrast, oxidizing CeIII to CeIV had little effect on Cl 3p and Ce 5d-orbital mixing. For LnCl63– (formally LnIII), the 4f-orbitals participated only marginally in covalent bonding, which was consistent with historical descriptions. Surprisingly, there was a marked increase in Cl 3p- and CeIV 4f-orbital mixing (t1u* + t2u*) in CeCl62–. This unexpected 4f- and 5d-orbital participation in covalent bonding is presented in the context of recent studies on both tetravalent transition metal and actinide hexahalides, MCl62– (M = Ti, Zr, Hf, U).

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