Covalency in Lanthanides. An X‑ray Absorption Spectroscopy and Density Functional Theory Study of LnCl₆^<i>x</i>– (<i>x</i> = 3, 2)

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).本研究针对具有Oh点群对称性的LnCl₆^x-阴离子中Ln-Cl键的共价性展开了系统探究：当Ln为CeIII、NdIII、SmIII、EuIII、GdIII时，x=3；当Ln为CeIV时，x=2。实验主要采用Cl K边X射线吸收光谱（X-ray absorption spectroscopy, XAS）与含时密度泛函理论（time-dependent density functional theory, TDDFT）开展分析，同时还借助Ce L₃,₂边与M₅,₄边XAS对CeCl₆^x-（x=2、3）进行了表征。M₅,₄边XAS光谱采用构型相互作用计算进行建模。 本研究结果从两个维度进行评估：一是镧系金属（lanthanide, Ln）的种类，选取了从Ce到Gd的一系列镧系元素；二是镧系金属的氧化态（在可行条件下，即形式上的CeIII与CeIV）。 研究观测到Cl 3p轨道与Ln 5d轨道（t₂g*与e_g*轨道）之间存在显著的轨道混合。实验结果表明，随着镧系元素序列的推移，Ln 5d轨道的轨道混合程度逐渐降低。与之相反，将CeIII氧化为CeIV对Cl 3p与Ce 5d轨道的混合几乎无影响。对于LnCl₆^3-（形式上为LnIII），其4f轨道仅少量参与共价成键，这与既往的研究描述相符。 令人意外的是，在CeCl₆^2-中，Cl 3p轨道与CeIV的4f轨道（t₁u* + t₂u*轨道）的混合程度显著提升。本文结合近期针对四价过渡金属与锕系六卤化物MCl₆^2-（M=Ti、Zr、Hf、U）的相关研究，对这一4f与5d轨道意外参与共价成键的现象进行了讨论。