Evaluating the electronic structure of formal Ln(II) ions in Ln(II)(C(5)H(4)SiMe(3))(3) (1–) using XANES spectroscopy and DFT calculations

The isolation of [K(2.2.2-cryptand)][Ln(C(5)H(4)SiMe(3))(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(C(5)H(4)SiMe(3))(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(C(5)H(4)SiMe(3))(3) (1–) (Ln = Sm, Tm, Yb, Lu, Y) are determined to contain 4f(6) 5d(0) (Sm(II)), 4f(13) 5d(0) (Tm(II)), 4f(14) 5d(0) (Yb(II)), 4f(14) 5d(1) (Lu(II)), and 4d(1) (Y(II)) electronic configurations. Additionally, our results suggest that Ln(C(5)H(4)SiMe(3))(3) (1–) (Ln = Pr, Nd, Gd, Tb, Dy, Ho, and Er) also contain Ln(II) ions, but with 4f(n) 5d(1) configurations (not 4f(n+1) 5d(0)). In these 4f(n) 5d(1) complexes, the C (3h)-symmetric ligand environment provides a highly shielded 5d-orbital of a′ symmetry that made the 4f(n) 5d(1) electronic configurations lower in energy than the more typical 4f(n+1) 5d(0) configuration.