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

Abstract

Covalency in Ln-Cl bonds of Oh-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 L3,2-edge and M5,4-edge XAS were also used to characterize CeCl6(x-) (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 Ce(III) and Ce(IV)). 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 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 (t1u* + t2u*) 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).

A Macrocyclic Chelator That Selectively Binds Ln4+ over Ln3+ by a Factor of 1029

“…For comparison, a ground state composition of 75% 3d 10 4f 0 and 25% 3d 10 L4f 1 was found previously to model the LMCT energy (3.3 eV) for (Et4N)2CeCl6 as defined by ΔEgs = 2.5 eV, ΔEfs = -1.5 eV, and Tgs = Tfs = 0.70. 57 Hence, the CTM4XAS calculations suggest that there is slightly more 4f 1 character in the ground state of CeLK4 (38%) than in (Et4N)2CeCl6 (25%). The relatively low percentages of 4f 1 character observed for these two compounds suggested that the LMCT configuration has a smaller contribution to the ground state than previously reported for (C8H8)2Ce, based on L3-edge XANES spectroscopy (89%) and multiconfiguration interaction calculations (80%).…”

“…7,12,[17][18][19]22,[24][25][26][154][155][156][157] However, the role of final state effects including core-hole induced changes in 4f occupancy, and other physical phenomena, remains controversial and is the subject of a persistent debate. 18,33,[35][36] Figure 8 compares the background-subtracted and normalized Ce L3-edge XANES spectra of CeLK4 and reference compounds CeO2, 158 [Et4N]2[CeCl6], 57 and (C8H8)2Ce. 99 Each of the L3-edges exhibited a double-peak structure that is characteristic of published data for formally Ce 4+ compounds such as CeO2, Ce(SO4)2, and CeF4.…”

“…[55][56] Molecular cerium compounds are usually stabilized in the 4+ formal oxidation state by halides 57 and hard, oxygen donor ligands such as alkoxides [58][59][60][61][62][63][64][65][66][67] and acetylacetonates. [68][69] Many "non-classical" tetravalent cerium molecules have also been prepared, including organometallic compounds, [70][71][72] amide, 65,70,[73][74][75][76][77][78] porphyrins [79][80][81][82][83] and a variety of other multidentate N-donor ligands.…”

“…96,98 More recently, an in-depth chlorine K-edge X-ray absorption spectroscopy (XAS) and DFT study showed that Cl 3p and Ln 4f covalency was significant in tetravalent CeCl6 2-but virtually nonexistent in the trivalent analog, CeCl6 3-. 57 In spite of this and other notable synthetic progress, 57,72,87,98,[104][105][106] molecular Ce compounds are seldom the subject of emerging methods for physical and theoretical characterization. Molecular solids are promising candidates for further study because they can often be prepared as pure, crystalline compounds, can be designed with high symmetry, and can be modeled as a complete system without requiring periodic boundary conditions or other approximations.…”

4f chemistry towards rare earth materials science and engineering

“…The forces of electrostatic attraction between ions in a compound cause the chemical bonding of ions with the surrounding ions of opposite charge. However, it has been a longstanding mystery concerning 4f electrons of RE elements when evaluating their chemical bonding ability [4][5][6]. Very recently, we discovered that RE elements can form three types of chemical bonding, namely sp hybridization, spd hybridization, and spdf hybridization [7].…”