Electronic structure calculations, including relativistic core potentials 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 splittings, the electron affinity, and the ionization potential are computed for C-28. The ground state of C-28 is described well by the Hartree-Fock wave functions, but other states are not. The computed electron affinity and ionization potential are similar to those of C-60. Strong metal-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 lowest-energy cage pi* and metal 5f orbitals, with (pi*)(1) and (pi*)(1)(5f)(1) found to be the ground electronic configurations for the two complexes. U@C-28 is found to be diamagnetic.