The molecular and electronic structures of mixed-valence face-shared (Cr, Mo, W) d(2)d(3) and (Mn, Tc, Re) d(3)d(4) [M2Cl9](2-) 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 d(2)d(3) and d(3)d(4) coupling modes. In (d(2)d(3)) [Mo2Cl9](2-) and [W2Cl9](2-), the global minimum has been found to be a spin-doublet state characterized by delocalization of the metal-based electrons in a multiple metal-metal bond (with a formal bond order of 2.5). In contrast, weak coupling between the metal centers and electron localization are favored in (d(2)d(3)) [Cr2Cl9](2-), the global minimum for this species being a ferromagnetic S = 5/2 state with a relatively long Cr-Cr separation. The (d(3)d(4)) [Re2Cl9](2-) system also exhibits a global minimum corresponding to a metal-metal bonded spin-doublet state with a formal bond order of 2.5, reflecting the electron-hole equivalence between d(2)d(3) and d(3)d(4) configurations. Double minima behavior is predicted for (d(3)d(4)) [Tc2Cl9](2-) and [Mn2Cl9](2-) due to two energetically close low-lying states (these being S = 3/2 and S = 5/2 states for the former, and S = 5/2 and S = 7/2 states for the latter). A comparison of computational results for the d(2)d(2), d(2)d(3), and d(3)d(3) [W2Cl9](2-) Series and the d(3)d(3), d(3)d(4), and d(4)d(4) [Re2Cl9](2-) series indicates that the observed trends in metal-metal distances can only be rationalized if changes in both the strength of a bonding and metal-metal bond order are taken into consideration. These two factors act conjointly in the W series but in opposition to one another in the Re series. In the case of the [Cr2Cl9](z-) and [Mn2Cl9](z-) climers, the metal-metal bond lengths are significantly shorter for mixed-valence (d(2)d(3) or d(3)d(4)) than d(3)d(3) systems. This result is consistent with the fact that some degree of metal-metal bonding exists in the former (due to partial delocalization of a single a electron) but not in the latter (where all metal-based electrons are completely localized).