Ab initio SCF, two-configuration (TC) SCF, and CI calculations have been carried out to investigate the bonding potentialities of Re(VI) in trioxorhenium with Re(O) in Re(CO)(5). Calculations show that the electron-withdrawing character of ReO3 makes the hypothetic O3Re-Re(CO)(5) binuclear complex a highly polar molecule. The charge transfer toward trioxorhenium (0.534 e) is not so large, however, as in O3Re-ReCl2(dmpm)(2), the only bimetallic complex characterized to date with,ReO3. In both O3Re-Re(CO)(5) and O3Re-ReCl2(dmpm)(2), the metal-metal coupling can be expressed in terms of a sigma donation from the most oxidized rhenium atom to the least oxidized one. The topological analysis of the Laplacian distribution of density shows that the bonding of the ReO3 fragment is mostly ionic. The critical points of the Laplacian distribution characterized along the metal-metal bond axis are typical of a a donation, but also evidence some covalent character. The energy associated with a homolytic dissociation is computed to be 271 kJ mol(-1), accounting for the geometrical relaxation of the ligands. This value is larger than the Re-L bond energies previously computed for various types of O3Re-L molecules. An investigation carried out on O3Tc-Tc(CO)(5) leads to similar results.