A mystery solved? Photoelectron spectroscopic and quantum chemical studies of the ion states of CeCp3+

Abstract

The electronic states of CeCp(3)(+) have been studied experimentally by variable photon energy photoelectron spectroscopy, and computationally using multi-configurational ab initio methods. Relative partial photoionisation cross section and branching ratio data are presented to confirm our previous conclusion that bands A and D in the valence photoelectron spectrum, despite their 3.2 eV separation, are produced by ionization of the single 4f electron of CeCp(3) [M. Coreno, M. de Simone, J. C. Green, N. Kaltsoyannis, N. Narband and A. Sella, Chem. Phys. Lett., 432, 2006, 17]. The origin of this effect is probed using the CASSCF/CASPT2 approach. While configurations based on the canonical CASSCF orbitals are found to be an unreliable description of the ground and excited states of CeCp(3)(+), the state-specific natural orbitals and their occupations yield greater insight, allowing us to characterize ion states in terms of the presence or otherwise of a Ce 4f-localised electron. Neither the CeCp(3)(+) ground state (assigned to band A), and two excited states ((1)A' and (1)A'', associated with band D), possess such a metal-based electron, as expected of f ionization. The (1)A' and (1)A'' states differ from the ground state in having a significant Ce 5d population, arising from Cp --> Ce charge transfer, which accompanies f ionization, and which is responsible for the energetic separation of bands A and D in the valence photoelectron spectrum.

All-electron CASPT2 study of Ce(η8–C8H6)2

“…2. CePn 2 ground state natural orbitals obtained from CASSCF (12,16) calculations, and CeCOT 2 ground state natural orbitals obtained from a CASSCF(8,14) calculation [11]. Only NOs with signifcantly non-integer occupation are shown.…”

“…1, where the p 2 -p 1 energy gap reduces from 2.97 eV (CeCOT 2 ) to 1.06 eV (CePn 2 , D 2h ) and 1.17 eV(CePn 2 , D 2d ), and led us to expand the active space in the present calculations to include the higher lying of the p 1 levels. We therefore present CASPT2 (12,16) calculations, using our adopted notation. It should be noted that this active space allows for occupation of any of the twelve 4f and 5d levels.…”

“…This is of particular importance for the cerium compounds, for whilst there is little doubt that the actinocenes synthesized to date contain a tetravalent actinide ion, opinion with regard to cerocene is divided. We have discussed this is in detail in recent publications [11,12], and so only briefly summarise here.…”

Intermediate-Valence Tautomerism in Decamethylytterbocene Complexes of Methyl-Substituted Bipyridines

“…Recent experimental and theoretical studies of certain Ce-and Yb-based organometallic complexes [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15] have highlighted the importance of multiconfigurational interactions in understanding their electronic, structural, and magnetic properties. The canonical examples include Ce(cot) 2 , also known as "cerocene," where cot = cyclooctatetraene = C 8 H 8 , and Cp * 2 Yb(bipy), where Cp * = pentamethylcyclopentadienyl, and bipy = 2,2 ′ -bipyridine.…”

“…Interaction (CI) calculations [1][2][3][12][13][14][15] indicate that a singlet ground state develops that is lower in energy than that of the triplet state because it is a mixture of various open-and closed-shell singlet configurations. These singlet states are therefore multiconfigurational, that is, quantum mechanical admixtures of configurations with different f occupations, and are intermediate valent, abbreviated below as IV, in the sense that the effective valence for each Ce or Yb atom is non-integral.…”