He@Mo(6)Cl(8)F(6): a stable complex of helium.

The electronic structure and chemical stability of the endo helium cluster, He@Mo(6)Cl(8)F(6), were investigated carefully by using density function theory. The results show that the cluster is significantly different from typical van der Waals systems: the bond distance between helium and molybdenum is only about 1.89 A. Moreover, the bonding analysis clearly reveals considerable charge and bond order on the helium atom and bond order for He-Mo. The dissociation of He@Mo(6)Cl(8)F(6) to He + Mo(6)Cl(8)F(6) is prohibited by a barrier of 0.86 eV (19.8 kcal/mol), indicating that the cluster is chemically stable. However, no covalent He-Mo bonding was found so it is an analogue of He@adam. Comparison was also made with the isoelectronic system of [Mo(6)Cl(8)F(6)](2-).