Variable photon energy photoelectron spectroscopy of tris-cyclopentadienyl lanthanides
The gas phase photoelectron (PE) spectra of LnCp3 (Cp = η-C5H5; Ln = Pr, Nd, Sm), measured with a wide range of photon energy, are reported.
M. Coreno, et al. Dalton Trans (2014).
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For d block transition metal complexes, gas phase photoelectron (PE) spectroscopy has had considerable success in revealing their electronic structure. For closed shell molecules in the valence region, there is normally a close correspondence between the photoelectron bands and the orbital structure predicted by density functional theory (DFT). Lanthanide complexes are not so straightforward. The f-electrons are energetically part of the valence region, and for some elements are active in redox chemistry, but spatially are part of the core, the f-orbitals lacking significant overlap with ligand orbitals. Core ionization may result in significant perturbation of a molecule's electronic structure, which manifests itself in satellite structure accompanying the primary core PE bands, often referred to as shake-up or shake-down bands. These bands are assigned to many electron processes in which a valence excitation or transition accompanies the core ionization.
Ionization of the 4f electrons of lanthanides gives very characteristic final state structure that depends on the electronic ground state adopted by the 4fn configuration. The relative intensities of the ion states arising from ionization of the 4f electrons have been predicted using a fractional parentage description of the ground state molecule and these predictions are on the whole consistent with data obtained from solid state photoemission studies. Although consideration of spin orbit coupling leads to modification of the relative intensity predictions the pattern of the bands associated with a particular 4fn−1 configuration are characteristic and may be assigned by consideration of the electronic spectrum of the Ln3+ ion of the preceding element, i.e. the lanthanide with an atomic number of one less and a 4fn−1 configuration. Here we present the variable photon energy PE spectra of PrCp3, NdCp3 and SmCp3 and compare the results with those reported previously for Ce, Yb and Lu.. |
The gas phase PE spectra of LnCp3 (Cp = η-C5H5; Ln = Pr, Nd, Sm) have been measured using synchrotron radiation. Variation of band intensity with photon energy, including in the region of 4d to 4f excitation, has enabled unambiguous assignment of the ion states. In all three cases, ion states associated with both 4fn and 4fn−1 configurations are identified, where n is the number of f electrons in the molecular ground state. The data are discussed in the context of the previous results for Ln = Ce, Yb and Lu. The cationic ground state of LnCp3 for all members of the series except Ce has a fn configuration with a hole in the ligand shell, specifically in an orbital of a′ symmetry. Ce is exceptional in having an intermediate valence ground state comprised of a mixture of f0a′2 and f1a′1 configurations, where a′ is the highest occupied orbital of the Cp3 ligand set. The f electron in the latter occupies the cos 3Φ component of the Ln 4f±3 orbitals giving a singlet state overall. The 4f band intensities are similar to those observed previously in solid state studies but in general are better resolved. The 4fn−1 states are accessed by direct ionization or by a 4d–4f resonance process coupled with super Koster–Kronig decay. The more intense of the 4fn states are observable outside the resonance region but become most clearly visible within the resonance region. The mechanisms of their access are discussed. SmCp3 is the most likely candidate to have an intermediate valence ground state as has been established for YbCp3. However, the intensity of the 4fn bands suggest that an alternative mechanism for their access on 4f ionization must be present.
Variable photon energy photoelectron spectroscopy of tris-cyclopentadienyl lanthanides M. Coreno, M. de Simone, J. C. Green, N. Kaltsoyannis, R. Coates, C. Hunston, N. Narband, A. Sella DOI: 10.1039/C3DT53512F (Paper) Dalton Trans., 2014, 43, 5134-5141
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