Periodic trends in metal-metal interactions in face-shared [M ₂Cl₉]^z- systems

The periodic trends in metal-metal interactions in even-electron and mixed-valence [M₂Cl₉]^z- face-shared systems, involving transition metals in Groups 4 to 8 and electronic configurations ranging from d¹d¹ through to d⁵d⁵ and from d¹d² through to d⁴d⁵, have been investigated by calculating metal-metal bonding and spin polarization (exchange) effects using density functional theory. These two terms are in opposition to one another and their relative difference determines the extent to which the metal-based electrons are delocalized and thus the degree of metal-metal bonding. Remarkably strong linear correlations between the two terms, and between each term and the square of the spin density on the metal centres, have been obtained for all group and period series considered, and are discussed in terms of their dependence on the metal orbital properties and electron density.