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.