The molecular and electronic structures of mixed-valence d(1)d(2) (V, Nb, Ta) and d(4)d(5) (Fe, Ru, Os) face-shared [M2Cl9](2-) dimers; have been calculated by density functional methods in order to investigate metal-metal bonding in this series. General similarities are observed between d(1)d(2) and d(4)d(5) systems and can be considered to reflect the electron-hole equivalence of the individual d(1)-d(5) and d(2)-d(4) configurations. The electronic structures of the dimers have been analyzed using potential energy curves for the broken-symmetry and other spin states resulting from the d(1)d(2) and d(4)d(5) coupling modes. In general, a spin-doublet (S = 1/2) state, characterized by delocalization of the metal-based electrons in a metal-metal bond with a formal order of 1.5, is favored in the systems containing 4d and 5d metals, namely, the Nb, Ta, Ru, and Os dimers. In contrast, the calculated ground structures for [V2Cl9](2-) and [Fe2Cl9](2-) correspond to a spin-quartet (S = 3/2) state involving weaker coupling between the metal centers and electron localization. In the case of [Ru2Cl9](2-), both the spin-doublet and spin-quartet states are predicted to be energetically favored suggesting that this species may exhibit double-minima behavior. A comparison of computational results across the (d(1)d(1), d(1)d(2), d(2)d(2)) [Nb2Cl9](z-) and [Ta2Cl9](z-) and (d(4)d(4), d(1)d(5), d(5)d(5)) [Ru2Cl9](z-) and [Os2Cl9](z-) series has revealed that, in all four cases, the shortening of the metal-metal distances correlates with an increase in formal metal-metal bond order.