Metal-metal bonds in Zintl clusters: Synthesis, structure and bonding in [Fe2Sn4Bi8]3– and [Cr2Sb12]3–

We report here the synthesis and characterization of two new members of the M2E12 family of endohedral Zintl clusters, [Fe2Sn4Bi8]3– and [Cr2Sb12]3–, both of which contain open-shell metal dimers encapsulated inside a triple-decker cluster of main-group atoms. The 75-electron [Fe2Sn4Bi8]3– cluster has a D4h-symmetric structure, while [Cr2Sb12]3–, despite having the same 75-electron count, is strongly distorted to a geometry that resembles a CrSb8 crown capped by a CrSb4 unit. The structural differences between the two are driven by the increasing availability of 3d electron density in the earlier transition metal, which leads, ultimately, to different electronic configurations in the two clusters. The trends precisely mirror those observed in the ME10 and ME12 families containing a single transition metal ion.