AbstractMolecules in which there are neighboring electrophilic and nucleophilic centers are unusually reactive. Oligomerization can be prevented only by bulky groups attached to the main group metal atom that would act as electron pair acceptor, or to the basic non‐metal atom. The basic and the acid centers behave as a single unit in chemical reactions; the system is similar to a “double bond” whose π‐electron density is largely concentrated at one atom. The unsaturated nature of these molecules can be seen in (for example) their addition reactions with hydrogen compounds of non‐metals, or in reactions that are distantly related to cycloadditions at homopolar double bonds. The selection of suitable reaction partners leads to polycyclic, cage‐like molecules containing metal atoms. If these atoms possess lone pairs (as is usual in the lower oxidation states of the third and fourth main groups), these can be utilized to form bonds to further (Lewis acid) metal centers. In some cases large assemblies can be built up from polycyclic systems in this way; a characteristic of these assemblies is a one‐dimensional array of metal atoms. Commonly occurring structural features of the polycyclic species are tetrahedra, trigonal bipyra