Open Shells in Endohedral Clusters: Structure and Bonding in the [Fe 2 @Ge 16 ] 4- Anion and Comparison to Isostructural [Co 2 @Ge 16 ] 4 .

The anionic cluster [Fe ₂ @Ge ₁₆ ] ^4- has been characterized and shown to be isostructural to the known D _{2 h} -symmetric α isomer of the cobalt analogue [Co ₂ @Ge ₁₆ ] ^4- . Together with the known pair of compounds [Co@Ge ₁₀ ] ^3- and [Fe@Ge ₁₀ ] ^3- , the title compound completes a set of four closely related germanium clusters that allow us to explore how the metal-metal and metal-cage interactions evolve as a function of size and of the identity of the metal. The results of spin-unrestricted density functional theory (DFT) and multiconfigurational self-consistent field (MC-SCF) calculations present a consistent picture of the electronic structure where transfer of electron density from the metal to the cage is significant, particularly in the Fe clusters where the exchange stabilization of unpaired spin density is an important driving force.