Covalency
in Lanthanides. An X‑ray Absorption
Spectroscopy and Density Functional Theory Study of LnCl<sub>6</sub><sup><i>x</i>–</sup> (<i>x</i> = 3, 2)
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Abstract
Covalency
in Ln–Cl bonds of <i>O</i><sub><i>h</i></sub>-LnCl<sub>6</sub><sup><i>x</i>–</sup> (<i>x</i> = 3 for Ln = Ce<sup>III</sup>, Nd<sup>III</sup>, Sm<sup>III</sup>, Eu<sup>III</sup>, Gd<sup>III</sup>; <i>x</i> = 2 for Ln = Ce<sup>IV</sup>) anions has been investigated, primarily
using Cl K-edge X-ray absorption spectroscopy (XAS) and time-dependent
density functional theory (TDDFT); however, Ce L<sub>3,2</sub>-edge
and M<sub>5,4</sub>-edge XAS were also used to characterize CeCl<sub>6</sub><sup><i>x</i>–</sup> (<i>x</i> =
2, 3). The M<sub>5,4</sub>-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 Ce<sup>III</sup> and Ce<sup>IV</sup>). Pronounced mixing
between the Cl 3p- and Ln 5d-orbitals (<i>t</i><sub>2<i>g</i></sub>* and <i>e</i><sub><i>g</i></sub>*) was observed. Experimental results indicated that Ln 5d-orbital
mixing decreased when moving across the lanthanide series. In contrast,
oxidizing Ce<sup>III</sup> to Ce<sup>IV</sup> had little effect on
Cl 3p and Ce 5d-orbital mixing. For LnCl<sub>6</sub><sup>3–</sup> (formally Ln<sup>III</sup>), 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 Ce<sup>IV</sup> 4f-orbital mixing (<i>t</i><sub>1<i>u</i></sub>* + <i>t</i><sub>2<i>u</i></sub>*) in CeCl<sub>6</sub><sup>2–</sup>. 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, MCl<sub>6</sub><sup>2–</sup> (M = Ti, Zr, Hf, U)