The geometric structure and electronic properties of the 5f(1) complex protactinocene, Pa(COT)(2) (COT = eta(8)-C8H8), have been investigated using gradient-corrected density functional methods with the inclusion of spin-orbit coupling. The calculated structure of Pa(COT)(2) with scalar relativistic corrections is intermediate between those of Th(COT)(2) and U(COT)(2). Spin-orbit effects are essential for the calculation of state energies of Pa(COT)(2). Under D-8h* double-group symmetry, the ground state is found to be an E-5/2u state that corresponds to an (f phi)(1) electron configuration. The first excited state (E-1/2u) lies only about 0.05 eV above the ground state. The low-lying states follow the ordering of E-5/2u (f phi) < E-1/2u (f sigma+f pi) much less than E-3/2u (f pi) similar to E-7/2u (f phi) < 2E(1/2u) (f pi+f sigma) < E-1/2g (d sigma) much less than 2E(3/2u) (f delta) < 2E(5/2u) (f delta). The lowest-energy electric-dipole-allowed f --> d electronic transition is calculated to occur at 368 nm, which is in excellent agreement with the experimental estimation of 365 nm. The first 20 vertical ionization energies and the magnetic moment of Pa(COT)(2) have been predicted as based on the spin-orbit calculations. A comparison of the calculated infrared vibrational frequencies and absorption intensities of Pa(COT)(2) with the available experimental data is presented, and the vibrational spectra are assigned.