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^iPr₅)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)^<i>n</i>+1 configuration of the metal atom, metallocenes with Ln = La, Ce, Gd, and Lu are best described as having (4f)^<i>n</i>(5d)¹ ground states. Strong configuration mixing is observed for Ln = Pr; for Ln = Ho and Er, two distinct states of (4f)^<i>n</i>+1 and (4f)^<i>n</i>(5d)¹ character are energetically close, and comparison with available experimental data slightly favors a (4f)^<i>n</i>(5d)¹ ground-state assignment. Our results show that the ground states of the Nd and Pr compounds have strong or partial (4f)^<i>n</i>+1 character, suggesting that previous assignments based solely on experimental data and atomic f-d splittings [McClain, K. R. <i>J. Am. Chem. Soc.</i> 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.