Low-lying electronic states of lanthanocenes and actinocenes M(C8H8)(2) (M=Nd, Tb, Yb, U)

Large-scale state-averaged multiconfiguration self-consistent field, multireference configuration interaction, and averaged coupled-pair functional calculations, including relativistic effects by means of energy-consistent quasirelativistic pseudopotentials, have been carried out for the ground and low-lying excited states of the di-pi-cyclooctatetraene (or bis[8]annulene) metal sandwich complexes M(C8H8)(2) (M=Nd, Tb, Yb, U). It is found that the ground state configurations for the lanthanocenes are 4f(n) pi(3), while for the actinocenes they art: 5f(n-1)pi(4). The ground states for the lighter and heavier lanthanocenes are, respectively, the lower and higher multiplicity states resulting from the coupling between the highest possible spin-multiplicity of the central metal 4f(n) subshell and the unpaired ligand rr electron, whereas they always have the highest possible multiplicity of 5f(n-1)pi(4) for the actinocenes. The metal-ring distances and symmetric metal-ring stretching frequencies are reported. The special characteristics of the uranium 5f orbitals in uranocene are described. Extensive spin-orbit configuration interaction calculations were performed for uranocene and confirm the assignment of the ground state and first excited state of uranocene made previously by other authors. However, a different ordering is obtained for the higher states. The calculated term energy for the first excited state of uranocene is in excellent agreement with the experimental value. (C) 1997 American Institute of Physics.