Formation of Al 13 I - : Evidence for the Superhalogen Character of Al 13

Al 13 – is a cluster known for the pronounced stability that arises from coincident closures of its geometric and electronic shells. We present experimental evidence for a very stable cluster corresponding to Al 13 I – . Ab initio calculations show that the cluster features a structurally unperturbed Al 13 – core and a region of high charge density on the aluminum vertex opposite from the iodine atom. This ionically bound magic cluster can be understood by considering that Al 13 has an electronic structure reminiscent of a halogen atom. Comparisons to polyhalides provide a sound explanation for our chemical observations.