Is cerocene really a Ce(III) compound? All-electron spin-orbit coupled CASPT2 calculations on M(eta(8)-C8H8)2 (M = Th, Pa, Ce).

Spin-orbit free CASPT2 wave functions and energies are presented for the ground and 31 excited states of three f element sandwich molecules; thorocene (ThCOT(2)), protactinocene (PaCOT(2)), and cerocene (CeCOT(2)). Ground-state metal-ring centroid distances are optimized at this level and show excellent agreement with experiment. The effects of spin-orbit coupling are included and are found to be negligible for the ground states of ThCOT(2) and CeCOT(2), for which comparison of the electronic excitation energies is made with experimental data. For PaCOT(2), spin-orbit coupling is found to alter significantly the energies and nature of the ground and low-lying excited states, and good agreement is obtained with previous computational data. The ground state of CeCOT(2) is found to be strongly multiconfigurational, though not in the same way as previously reported. The relationship of this result to previous computational and experimental data is discussed, as is the most appropriate way to view the electronic structure of CeCOT(2). It is concluded that the occupations of the natural orbitals produce a more reliable description of the CeCOT(2) ground state than does the configurational admixture.