Periodic trends in metal-metal bonding in edge-shared [M₂Cl₁₀]^4- systems

Periodic trends in metal-metal interactions in edge-shared [M₂Cl₁₀]^4- systems, involving the transition metals from groups 4 through 8 and electronic configurations ranging from d¹d¹ through d⁵d⁵, have been investigated by calculating metal-metal bonding and spin-polarization (exchange) effects using density functional theory. The trends found in this study are compared with those for the analogous face-shared [M₂Cl₉]^3- systems reported in earlier work. Strong linear correlations between the metal-metal bonding and spin-polarization terms have been obtained for all groups considered. In general, spin polarization and electron localization are predominant in 3d-3d species whereas electron delocalization and metal-metal bonding are favoured in 5d-5d species, with more variable results observed for 4d-4d systems. As previously found for face-shared [M₂Cl₉]^3- systems, the strong correlations between the metal-metal bonding and spin polarization energy terms can be related to the fact that both properties appear to be similarly affected by the changes in the metal orbital properties and electron density occurring within the dⁿdⁿ groups. A significant difference between the face-shared and edge-shared systems is that while the 4d metals in the former show a strong tendency for delocalized metal-metal bonded structures, the edge-shared counterparts display much greater variation with both metal-metal bonded and weakly coupled complexes observed. The tendency for weaker metal-metal interactions can be traced to the inability of the edge-shared bridging structure to accommodate the smaller metal-metal distances required for strong metal-metal bonding. Crown