Computational study of endohedral IrSi9+ isomers

Recent ion- trap experiments ( See Ref. 2) have demonstrated the existence of endohedral transition metal - silicon clusters of composition MeSiN+ ( Me = Hf, Ta, W, Re, Ir) with a characteristic number of Si cage atoms for each of the Me atom species. In this series, IrSi9+ was the smallest of the reported systems. In view of its moderate size, this cluster appears particularly suitable for a systematic study of the bonding and stabilization mechanism prevailing in this newly discovered class of metal - Si-N units. We present a discussion of two highly symmetric pure Si-9 cage structures, namely a trigonal prism and a tricapped trigonal prism, and identify a stable IrSi9+ ( C-3h) unit with a cage structure intermediate between these two D-3h geometries. An additional endohedral IrSi9+ species of lower symmetry ( C-s), but higher stability than the C-3h alternative, is found from insertion of the Ir impurity into the C-2v ground- state geometry of Si-9. Comparison is made with an exohedral isomer derived from a substitutional structure based on Si-10. The role of electron transfer from the Si-9 cage to the Ir impurity in the stabilization of the endohedral complexes is emphasized.