The molecular and electronic structures of the d(4)d(4) face-shared [M2Cl9](3-) (M = Mn, Tc, Re) dimers have been calculated by density functional methods in order to investigate metal-metal bonding in this series. The electronic structures of these systems have been analyzed using potential energy curves for the broken-symmetry and other spin states arising from the various d(4)d(4) coupling modes, and closed energy cycles have been utilized to identify and quantify the parameters which are most important in determining the preference for electron localization or delocalization and for high-spin or low-spin configurations. In [Tc2Cl9](3-) and [Re2Cl9](3-), the global minimum has been found to be a spin-triplet state arising from the coupling of metal centers with low-spin configurations, and characterized by delocalization of the metal-based electrons in a double (delta and delta(pi)) bond with a metal-metal separation of 2.57 Angstrom. In contrast, high-spin configurations and electron localization are favored in [Mn2Cl9](3-), the global minimum for this species being the ferromagnetic S = 4 state with a rather long metal-metal separation of 3.43 Angstrom. These results are consistent with metal-metal overlap and ligand-field effects prevailing over spin polarization effects in the Tc and Re systems, but with the opposite trend being observed in the Mn complex. The ground states and metal-metal bonding observed for the d(4)d(4) systems in this study parallel those previously found for the analogous d(2)d(2) complexes of V, Nb, and Ta, and can be rationalized on the basis that the d4d4 dimer configuration is the hole equivalent of the d(2)d(2) configuration.