The cycloheptatrienyl actinide sandwich compounds An(eta(7)-C7H7)(2)(q) (An = Th-Am; q = 2-, 1-, 0, 1+) have been studied by using local and gradient-corrected density functional methods, with the inclusion of scalar (mass-velocity and Darwin) relativistic effects; It has been found that the staggered conformer of U(eta(7)-C7H7)(2)(-) is more stable than the eclipsed one by about 0.6 kcal/mol. The f delta orbitals not only participate in the bonding with the e(2) '' p pi orbitals of the C7H7 rings, but are as important as the d delta orbitals in stabilizing the frontier pn orbitals of the C7H7 rings. With increasing atomic number of the actinide, the 5f manifold and ligand based frontier e(2) '' MOs become considerably closer in energy. As a result, the actinide 5f percentage in the frontier e(2) '' MOs increases markedly, while the contribution by the 6d orbitals gradually decreases. The ground electron configurations, ionization energies, electron affinities, and An-C7H7 bond energies are-strongly affected by these effects. The bonding-analysis indicates that U(eta(7)-C7H7)(2)(-) and U(eta(7)-C7H7)(2) are best considered as complexes of U(III) and U(IV), respectively.