Metal-Metal Interactions in Mixed-Valence [M2Cl9]2- Species: Electronic Structure of d^1d^2 (V, Nb, Ta) and d4d5 (Fe, Ru, Os) Face-Shared Systems

The molecular and electronic structures of mixed-valence d^1d^2 (V, Nb, Ta) and d^4d^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^1d^2 and d^4d^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 d1d2 and d4d5 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 (d1d1, d1d2, d2d2) [Nb2Cl9]^z- and [Ta2Cl9]^z- and (d^4d^4, d^4d^5, d^5d^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.