In a multiple-bond metathesis reaction, the triazacydononane (tacn)-anchored methyl- and neopentyl (nP)-substituted tris(aryloxide) U-III complex [(((ArO)-Ar-nP,Me)(3)tacn)U-III] (1) reacts with mesityl azide and CO2 to form mesityl isocyanate and the dinuclear bis(mu-oxo)-bridged U-V/U-V complex [{(((ArO)-Ar-nP,Me)(3)tacn)U-V}(2)(mu-O)(2)] (3). This reaction proceeds via the mononuclear U-V imido intermediate [(((ArO)-Ar-nP,Me)(3)tacn)U-V(NMes)] (2), which has been synthesized and fully characterized independently. The dimeric U-V oxo species shows rich redox behavior: complex 3 can be reduced by one and two electrons, respectively, yielding the mixed-valent U-IV/U-V bis(mu-oxo) complex [K(crypt)][{(((ArO)-Ar-nP,Me)(3)tacn)U-IV/V}(2)(mu-O)(2)] (7) and the U-IV/U-IV bis(mu-oxo) complex K-2[{(((ArO)-Ar-nP,Me)(3)tacn)U-IV}(2)(mu-O)(2)] (6). In addition, complex 3 can be oxidized to provide the mononuclear uranium(VI) oxo complexes [(((ArO)-Ar-nP,Me)(3)tacn)U-VI(O)(eq)(OTf)(ax)] (8) and [(((ArO)-Ar-nP,Me)(3)tacn)U-VI(O)(eq)]SbF6 (9). The unique series of bis(mu-oxo) complexes also shows notable magnetic behavior, which was investigated in detail by UV/vis/NIR and EPR spectroscopy as well as SQUID magnetization studies. In order to understand possible magnetic exchange phenomena, the mononuclear terminal oxo complexes [(((ArO)-Ar-nP,Me)(3)tacn)U-V(O)(O-pyridine)] (4) and [(((ArO)-Ar-nP,Me)(3)tacn)U-V(PO)(O-NMe3)] (5) were synthesized and fully characterized. The magnetic study revealed an unusually strong antiferromagnetic exchange coupling between the two U-V ions in 3. Examination of the O-18-labeled bis(mu-oxo)-bridged dinuclear complexes 3, 6, and 7 allowed for the first time the unambiguous assignment of the vibrational signature of the [U(mu-O)(2)U] diamond core structural motif.