Abstract

A series of di- and polymetal complexes involving closed-shell, heavy main-group atoms and ions shows a selection of special physical properties. These involve short metal-metal contacts, low entropies of formation and, most interestingly, strong Raman bands at low wavenumbers. These results together with the constitution of the coordination compounds, where the majority of electrons are assembled on the highly polarizable metal atoms and ions, experimental results have been interpreted in terms of direct, partial covalent metal-metal bonding. Previous theoretical studies have challenged this view and instead attributed the obvious attractive forces involved to secondary-type of interactions, such as dispersion. The current study utilizes a multitude of theoretical tools, such as NBO, NEDA, NCI, ELF and AIM to characterize the interactions in models comprising closed-shell dimers, as well as experimentally studied ring and cage systems, constituting the main reason for the hypotheses on metal-metal interactions. The results show that all experimental results can be attributed to the covalent interactions between the electron-rich, metal centres and the bridging anions in ring and cage coordination compounds of high symmetry, where the experimentally observed effects can be traced to the combination of covalent interactions between the metal centres and the anions along the edges of the ring and cage systems in combination with the cooperative effects generated by the high symmetry of these ring and cage systems.