Atomic Clusters, agglomerations of a few to a few hundred atoms which are bound by delocalized bonding, exhibit size dependent properties which vary markedly depending on the size, shape, or composition. The stability of clusters may be understood within a variety of models, including the Jellium Model, Aromaticity, and Wade-Mingos Rules, depending on the geometry and metallicity of the the cluster. Because certain cluster sizes and electron counts are quite stable relative to others of similar size, these clusters' physical and chemical properties are dominated by their desire to reach a certain valence state. These clusters who have a strong propensity towards a well defined valence are called superatoms because they exhibit tendencies reminscent of an atom or group of the periodic table.
Al13 is one example of a superatom cluster. Al13 has 39 valence electrons, and within the jellium model, clusters with 40 electrons have large HOMO-LUMO Gaps and enhanced stability. Because of this, the cluster has a very high electron affinity of 3.4 eV, and the anion has a large HOMO-LUMO Gap of 1.87 eV. Because the chemical and electrostatic properties are dominated by the enhanced stability it recieves by moving to a valence of -1, the Al13 cluster is considered a Superhalogen Superatom. While the superatom behavior is determined by the electronic levels near the Fermi energy, in simple metal clusters the entire valence electronic structure is made of electronic shells, which are multiple highly degenerate states, much like atoms. For such clusters, the jellium model offers a simplified framework for explaining the observed behavior. Employing an extension of the independent nuclear shell model to a simple electron gas concept developed by Cohen and Chou, Knight and co-workers accounted for the observations. In this model, one imagines that the positive charge of all of the ions in the cluster is distributed uniformly over a sphere of the size of the cluster. The energy levels of electrons for such a charge distribution correspond to 1s2 1p6 1d10 2s2 1f14 2p6, The magic sizes thus correspond to filled electronic shells, suggesting the role of these on the stability.
The next chapter in superatoms arrived with the identification of Aluminum-Halide superatoms in the gas phase. It was found that while other cluster species readily reacted away, Al13I- and Al13I2-, which bear direct resemblance to well-known di- and tri-halide ions, were quite resistant to etching. Theoretical studies on Al13I- showed that the clusters stability resides in the ability of the Al13 moiety to retain its anionic characteristics, in terms of both geometry and charge state, even in the presence of an iodine atom. The aluminum cluster actually pulls charge away from the iodine atom. The discovery and characterization of Al13I- and Al13I2- showed that Al13 can truly be described as a superhalogen (recall that stable X2- and X3- complexes, where X denotes a halogen, are well-characterized). Additional work on the Al14Ix- clusters revealed that the Al14 also seeks a valence state of +2, which also corresponds to the 40 e- shell closing. Al7- was found to have Multiple Valence in which it may tend towards both +2 and +4 states depending on the reactant. Superatom have been identified experimentally in the gas phase using Oxygen Etching, as cluster with closed electronic shells are inert when exposed to Oxygen at room temperature, while open shell clusters react readily.
Work in other groups, including Prof. Whetten, Prof. Hakkinen, Prof. Murray, and Prof. Kornberg among others have studied a variety of gold based cluster assembled materials which are stabilized by thiol based ligands. The stability of the clusters which are cystallized are derived from the jellium model, in which the electrons from gold, and the electrons withdrawn by the electronegative linkers are considered.