Insight into the Electronic Structure of Formal Lanthanide(II) Complexes using Magnetic Circular Dichroism Spectroscopy

Magnetic circular dichroism (MCD) spectroscopy has been utilized to evaluate the electronic structure of the tris(cyclopentadienyl) rare-earth complexes [K(2.2.2-cryptand)][LnCp′₃] (Ln = Y, La, Pr, Eu, Gd; Cp′ = C₅H₄SiMe₃), which contain ions in the formal +2 oxidation state. These complexes were chosen to evaluate the 4fⁿ5d¹ electron configuration assignments of the recently discovered La(II), Pr(II), and Gd(II) ions versus the traditional 4fⁿ⁺¹ configuration of the long-known Eu(II) ion. The 4d¹ Y(II) complex provided another benchmark in the MCD study. Transitions with f-orbital character were observed in the NIR MCD spectra of the 4f²5d¹ complex [PrCp′₃]^-. This study provides the first direct observation of f-f transitions in such Ln(II) species. The broadening of these transition for Pr(II) provides further confirmation of the 4fⁿ5d¹ versus 4fⁿ⁺¹ electronic configurations previously proposed and supported by restricted active-space (RAS) calculations. For further insight into the electronic structure of these [LnCp′₃]^- complexes, experimental UV-vis MCD spectroscopy was coupled with spectral calculations, which allowed for the assignment of transitions. The sensitivity of UV-vis MCD to spin-orbit coupling (SOC) and the increased spectral resolution in comparison to electronic absorption spectroscopy enabled identification of low-energy nd to (n + 1)p transitions in this class of complexes. Combined, these studies provide further insight into the electronic transitions and overall electronic structure of low-valent lanthanide(II) organometallic complexes.