On the basis of relativistic density functional theory Calculations, homo- and heterovalent binuclear,uranium :complexes of a polypyrrolic macrocycle in a U-O-U bridging fashion have been investigated. These complexes show a Variety of oxidation states for uranium ranging from III to VI, which have been confirmed by the calculated electron-spin density on each metal center. An equatorially 5-fold uranyl coordination mode is suitable for hexavalent uranium complexes, while silylation of the uranyl oxo is favored by pentavalent uranium. Uranyl oxo ligands are not required anymore for the coordination environment of tetra- and trivalent uranium because of their replacement by strong donors such as tetrahydrofuran and iodine. Optimization, of binuclear U-VI-U-III complexes with various,coordinating modes of U-III, donor and-donor types reveals that 0.5-1.0 electron his been transferred from U-III to U-VI. Consequently, U-V-U-IV complexes are more favorable. Electronic structures and formation reactions of several representative uranium complexes were calculated. For example, a 5f-based sigma(U-U) bonding Orbital is found in the diuranium(IV) complex, rationalizing that fact that it shows the shortest U-U distance (3.82 angstrom) among the studied binuclear complexes.