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

Covalency in Ln–Cl bonds of O_h-LnCl₆^x– (x = 3 for Ln = Ce^III, Nd^III, Sm^III, Eu^III, Gd^III; x = 2 for Ln = Ce^IV) anions has been investigated, primarily using Cl K-edge X-ray absorption spectroscopy (XAS) and time-dependent density functional theory (TDDFT); however, Ce L_3,2-edge and M_5,4-edge XAS were also used to characterize CeCl₆^x– (x = 2, 3). The M_5,4-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 Ce^III and Ce^IV). Pronounced mixing between the Cl 3p- and Ln 5d-orbitals (t_2g* and e_g*) was observed. Experimental results indicated that Ln 5d-orbital mixing decreased when moving across the lanthanide series. In contrast, oxidizing Ce^III to Ce^IV had little effect on Cl 3p and Ce 5d-orbital mixing. For LnCl₆^3– (formally Ln^III), 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^IV 4f-orbital mixing (t_1u* + t_2u*) in CeCl₆^2–. 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, MCl₆^2– (M = Ti, Zr, Hf, U).