Closed-shell interactions (CSI) range from extremely weak van der Waals forces, as for the helium dimer (dissociation energy, E d 9.06 Â 10 À2 kJ mol À1 ), [1, to strong aurophilic interactions (E d up to 30 kJ mol À1 ) [3] . The strength of the latter lie almost in the order of covalent bonds, and strong CSI have recently been reviewed by Pyykkö. [4] The bonding situation in these compounds is described by an equal number of electrons in bonding and antibonding molecular orbitals. This gives rise to a bond order of zero and explains why CSI are generally so weak.
Unusually strong ªclosed-shell interactionsº have recently been reported for the d 10 -s 2 system Au ± Hg (E d 179 kJ mol À1 , Table ), [5] and for the related cationic gold rare gas (Rg) compounds RgAu (E d 126 kJ mol À1 ). These cationic dimers fulfill the formal criterion for CSI since they consist of two interacting closed-shell fragments, the d 10 gold cation and a closed-shell atom. However, significant charge transfer from the neutral atom to the gold cation at the equilibrium distance and low-lying alternative dissociation channels indicate a contribution from open-shell configurations to the bonding. Even though the degree of covalency in RgAu and related species remains subject to discussion, [7] these compounds are clearly different from the aforementioned examples. Their highest occupied molecular orbitals the current transport by trivalent charge carriers. This challenge can be managed, for example, with ªTubandt electrolysisº experiments.