Evaluating the electronic structure of formal Ln^II ions in Ln^II(C₅H₄SiMe₃)₃^1- using XANES spectroscopy and DFT calculations

The isolation of [K(2.2.2-cryptand)][Ln(C₅H₄SiMe₃)₃], formally containing Ln^II, for all lanthanides (excluding Pm) was surprising given that +2 oxidation states are typically regarded as inaccessible for most 4f-elements. Herein, X-ray absorption near-edge spectroscopy (XANES), ground-state density functional theory (DFT), and transition dipole moment calculations are used to investigate the possibility that Ln(C₅H₄SiMe₃)₃^1- (Ln = Pr, Nd, Sm, Gd, Tb, Dy, Y, Ho, Er, Tm, Yb and Lu) compounds represented molecular Ln^II complexes. Results from the ground-state DFT calculations were supported by additional calculations that utilized complete-active-space multi-configuration approach with second-order perturbation theoretical correction (CASPT2). Through comparisons with standards, Ln(C₅H₄SiMe₃)₃^1- (Ln = Sm, Tm, Yb, Lu, Y) are determined to contain 4f⁶ 5d⁰ (Sm^II), 4f¹³ 5d⁰ (Tm^II), 4f¹⁴ 5d⁰ (Yb^II), 4f¹⁴ 5d¹ (Lu^II), and 4d¹ (Y^II) electronic configurations. Additionally, our results suggest that Ln(C₅H₄SiMe₃)₃^1- (Ln = Pr, Nd, Gd, Tb, Dy, Ho, and Er) also contain Ln^II ions, but with 4fⁿ 5d¹ configurations (not 4fⁿ⁺¹ 5d⁰). In these 4fⁿ 5d¹ complexes, the C_3h-symmetric ligand environment provides a highly shielded 5d-orbital of a′ symmetry that made the 4fⁿ 5d¹ electronic configurations lower in energy than the more typical 4fⁿ⁺¹ 5d⁰ configuration.