Electronic structure calculations, including relativistic core potenti
als and the spin-orbit interaction, have been carried out on the C-28,
Pa@C-28, and U@C-28 species. Excitation energies, spin-orbit splittin
gs, the electron affinity, and the ionization potential are computed f
or C-28. The ground state of C-28 is described well by the Hartree-Foc
k wave functions, but other states are not. The computed electron affi
nity and ionization potential are similar to those of C-60. Strong met
al-cage binding is found for Pa@C-28 and U@C-28, similar to that in U(
C8H8)(2). The ground electronic states depend on the order of the lowe
st-energy cage pi and metal 5f orbitals, with (pi*)(1) and (pi*)(1)(5
f)(1) found to be the ground electronic configurations for the two com
plexes. U@C-28 is found to be diamagnetic.