Metal clusters edge-localized bonding

The rare gas rule is based on the assumption that bonding interactions are localized, or, more exactly, localizable, along the directions of minimum distance, namely, the edges of the polyhedron therefore, M—M bonds can be represented by two center-one electron pair bonds. For a metal cluster the rule then takes the form... 

It became necessary to develop new approaches for predicting electron closed-shell structures, and ideas came from Williams, Wade, Mingos, and Rudolph, which now make up the polyhedral skeletal electron pair theory (PSEPT). The bonding in clusters can sometimes be described in terms of edge-localized, two-center two-electron bonds. Thus, through the formation of element-element bonds, the atoms of a polyhedron can acquire an effective inert gas configuration (8 valence electrons for a main group element and 18 for a transition metal atom). 

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