Theoretical Characterization of Bis(pentaisopropylcyclopentadienyl) Rare-Earth Metallocenes

We report ground states, equilibrium structures, and both electronic and electron paramagnetic resonance (EPR) spectra of divalent rare-earth metallocenes Ln(Cp^iPr)₂ as obtained from scalar and exact two-component (X2C) relativistic density functional theory (DFT) calculations. While the results for metallocenes with Ln = Nd, Sm, Eu, Tm, and Yb are consistent with a conventional (4f) configuration of the metal atom, metallocenes with Ln = La, Ce, Gd, and Lu are best described as having (4f)(5d)¹ ground states. Strong configuration mixing is observed for Ln = Pr; for Ln = Ho and Er, two distinct states of (4f) and (4f)(5d)¹ character are energetically close, and comparison with available experimental data slightly favors a (4f)(5d)¹ ground-state assignment. Our results show that the ground states of the Nd and Pr compounds have strong or partial (4f) character, suggesting that previous assignments based solely on experimental data and atomic f-d splittings [McClain, K. R. J. Am. Chem. Soc. 2022, 144, 22193-22201. 10.1021/jacs.2c09880] should be revised. X2C calculations of the EPR parameters show that the magnitude of the isotropic hyperfine coupling (HFC) relative to an atomic reference is a poor descriptor of s/d mixing, and ground-state assignments based on this approach are unreliable for rare-earth compounds. A massive HFC constant of 4401 MHz is predicted for the Lu compound.