Covalency in Lanthanides. An X-ray Absorption Spectroscopy and Density Functional Theory Study of LnCl_6~(x-) (x = 3, 2)

摘要

Covalency in Ln-Cl bonds of O_h-LnCl_6~(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 CeCl_6~(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 LnCl_6~(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 CeCl_6~(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, MCl_6~(2-) (M = Ti, Zr, Hf, U).