LOW-LYING ELECTRONIC STATES OF LANTHANOCENES AND ACTINOCENES M(C8H8)(2) (M=ND, TB, YB, U)

Large-scale state-averaged multiconfiguration self-consistent field, m ultireference configuration interaction, and averaged coupled-pair fun ctional calculations, including relativistic effects by means of energ y-consistent quasirelativistic pseudopotentials, have been carried out for the ground and low-lying excited states of the di-pi-cyclooctatet raene (or bis[8]annulene) metal sandwich complexes M(C8H8)(2) (M=Nd, T b, Yb, U). It is found that the ground state configurations for the la nthanocenes 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 a re, respectively, the lower and higher multiplicity states resulting f rom the coupling between the highest possible spin-multiplicity of the central metal 4f(n) subshell and the unpaired ligand rr electron, whe reas they always have the highest possible multiplicity of 5f(n-1)pi(4 ) for the actinocenes. The metal-ring distances and symmetric metal-ri ng stretching frequencies are reported. The special characteristics of the uranium 5f orbitals in uranocene are described. Extensive spin-or bit configuration interaction calculations were performed for uranocen e and confirm the assignment of the ground state and first excited sta te of uranocene made previously by other authors. However, a different ordering is obtained for the higher states. The calculated term energ y for the first excited state of uranocene is in excellent agreement w ith the experimental value. (C) 1997 American Institute of Physics.