The existence of a novel octahedral UO6 complex had been suggested by Pyykko et al [Pyykko, P.; Runeberg, N., Straka, M; Dyall, K. G Chem. Phys Lett 2000, 328, 415]. We have now investigated the stability, the geometric and electronic structures, and the vibrations of various UO6 molecules, using spin-orbit density functional and scalar-relativistic coupled-cluster approaches We find four different (meta-)stable species, namely D-3(2h)-UO2(eta(2)-O-2(center dot))(2) at lowest energy, C-3(2 nu)-UO4 center dot(eta(2)-O-2(center dot)) and D-1(3)-U(eta(2)-O-2)(3) at medium energies, and O-1(h)-UO6 at highest energy The decay of O-h-UO6 occurs via an activated spin-flip mechanism. The UO6 species correspond to local minima on singlet and triplet energy surfaces and might be trapped in noble gas matrices Experimentally, the four species might be identified through their vibrational spectra Uranium is best described as coordinated by oxygen atoms in various oxidation states as oxo O2-, oxido(I) O center dot-, peroxido O-2(2-), and superoxido O-2(center dot-) ligands. The occurrence of monovalent oxygen is remarkable The resulting characterization of the central ion as U-VI in all four cases does not fully reflect the electronic differences, nor the "valence-activity" of the U-6p(6) semicore shell