Computational Study of Endohedral IrSi9+Isomers

Abstract

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–SiN units. We present a discussion of two highly symmetric pure Si9 cage structures, namely a trigonal prism and a tricapped trigonal prism, and identify a stable IrSi9 + (C3h) unit with a cage structure intermediate between these two D 3h geometries. An additional endohedral IrSi9 + species of lower symmetry (Cs), but higher stability than the C 3h alternative, is found from insertion of the Ir impurity into the C 2v ground-state geometry of Si9. Comparison is made with an exohedral isomer derived from a substitutional structure based on Si10. The role of electron transfer from the Si9 cage to the Ir impurity in the stabilization of the endohedral complexes is emphasized.