[Co@Sn Sb ] : An Off-Center Endohedral 12-Vertex Cluster.

We report on the asymmetric occupation of a 12-vertex cluster centered by a single metal atom. Three salts of related intermetalloid cluster anions, [Co@Sn Sb ] (1), [Co @Sn Sb ] (2), and [Ni @Sn Sb ] (3) were synthesized, which have pseudo-C -symmetric or pseudo-D -symmetric 12-vertex Sn/Sb shells and interstitial Co ions or Ni atoms. Anion 1 is a very unusual single-metal-"centered" 12-atom cluster, with the inner atom being clearly offset from the cluster center for energetic reasons. Quantum chemistry served to assign atom types to the atomic positions and relative stabilities of this cluster type. The studies indicate that the structures are strictly controlled by the total valence electron count-which is particularly variable in ternary intermetalloid cluster anions. Preliminary Sn NMR studies in solution, supported by quantum-chemical calculations of the shifts, illustrate the complexity regarding Sn:Sb distributions of such ternary systems.