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

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,L-2-edge and M-5,M-4-edge XAS were also used to characterize CeCl6x- (x = 2, 3). The M-5,M-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 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).