The formation and high reactivity of transition-metal-element multiple bonds plays an important role in transitionmetal-catalyzed reactions, in particular, by facilitating the cleavage and formation of usually robust bonds. Olefin metathesis is a typical and very useful example of this type of reaction, in which carbene complexes, which have a metalcarbon double bond, are not only key intermediates but may also act as high-performance catalysts. In contrast to metal-carbon multiple bonds, metal-element multiple bonds, where the element is from the third or subsequent row of the periodic table, have been much less widely investigated. Among them, silylene complexes, which possess a metal-silicon double bond, have been the most extensively studied, but the mechanisms of their reactions remain rather unclear.
Both ourselves and Pannell's group have insisted, through the generation of silyl(silylene) complexes with transition metals from groups 6 to 9 and the preparation of their donorstabilized forms, that 1,2-and 1,3-group migrations of these systems (Scheme 1) occur very easily under mild conditions, and cause the metal-catalyzed oligomerization/deoligomerization, isomerization, and redistribution of organosilicon Scheme 1. Illustrating the 1,2-and 1,3-group migrations in silyl(silylene) complexes with metals of groups 6 to 9.