Structure and Bonding in [Sb@In8Sb12]3- and [Sb@In8Sb12]5-

We report the characterization of the compound [K([2.2.2]crypt)](4)[In8Sb13], which proves to contain a 1:1 mixture of [Sb@In8Sb12](3-) and [Sb@In8Sb12](5-). The tri-anion displays perfect T-h symmetry, the first completely inorganic molecule to do so, and contains eight equivalent In3+ centers in a cube. The gas-phase potential energy surface of the penta-anion has eight equivalent minima where the extra pair of electrons is localized on one In+ center, and these minima are linked by low-lying transition states where the electron pair is delocalized over two adjacent centers. The best fit to the electron density is obtained from a model where the structure of the 5- cluster lies close to the gas-phase transition state.