Density Functional Investigation of Metal-Metal Interactions in d ⁴d⁴ Face-Shared [M₂Cl₉] ^3- (M = Mn, Tc, Re) Systems

The molecular and electronic structures of the d⁴d⁴ 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 d⁴d⁴ 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 [Re₂Cl ₉]^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 (σ and δ_π) bond with a metal-metal separation of 2.57 Å. In contrast, high-spin configurations and electron localization are favored in [Mn₂Cl₉] ^3-, the global for this species being the ferromagnetic S = 4 state with a rather long metal-metal separation of 3.43 A. 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 d⁴d⁴ systems in this study parallel those previously found for the analogous d²d² complexes of V, Nb, and Ta, and can be rationalized on the basis that the d ⁴d⁴ dimer configuration is the hole equivalent of the d²d² configuration.