Tetrahedral homonuclear organoelement clusters and subhalides of aluminium, gallium and indium.

This review is focused on the synthesis and the reactivity of tetrahedral organoelement clusters of the heavier elements of third main-group aluminium, gallium, and indium, which have been known for about a decade. They possess the elements in an unusually low oxidation state of +1 and have direct element--element interactions between their four constituents. Each cluster atom is further attached to one terminal and in most cases a bulky organic substituent, which prevents disproportionation by steric shielding. The synthesis of these compounds succeeds by different methods such as the reduction of suitable organoelement(III) halides with alkali metals and magnesium or the treatment of element(I) halides with lithium organyls. They are deeply coloured, and their bonding situation may best be described by delocalized molecular orbitals. They show a singular chemical reactivity, which results in the formation of many secondary products possessing unprecedented structures and properties. The synthesis of organoelement subhalides still containing the elements in low oxidation states is discussed in more detail in the second part of this review. These compounds are easily accessible by the careful oxidation of the clusters with halogen donors such as hexachloroethane or with AlX(3)/X(2) mixtures. They produce dimers via halogen bridges, but in certain cases monomers were observed even for the solid state. They are very effective starting compounds for secondary reactions and the generation of new products containing the elements in unusual oxidation states by salt-elimination reactions, for instance.