Electrophilic, d0 transition-metal complexes have found use as effective catalysts for a number of chemical transformations, most prominently olefin polymerization. The reactivity associated with this catalysis involves rapid migratory insertions of the olefin substrate into d 0 metal-carbon bonds. Recently, other fundamental reaction steps have been identified for d 0 metal complexes, and some of these are useful in new catalytic reactions. For example, a novel CÀH activation process, s-bond metathesis, enables catalytic additions of C À H bonds to olefins.[1, 2] Similar s-bond metathesis steps have been found for activation of E À H bonds (E = maingroup element), and this reactivity is important in catalytic element-element (e.g., SiÀSi, SnÀSn, PÀP, SbÀSb, etc.) bond formations with d 0 metal catalysts. [3][4][5][6][7][8][9] Recent studies in d 0 transition-metal-main-group chemistry have implicated a new type of fundamental reaction step: migratory deinsertion (or a-elimination) of a low-valent main-group fragment ER n from a M À ER n R' derivative. This process was first observed for zirconocene and hafnocene stannyl derivatives such as [CpCp*(Cl)HfÀSnPh 3 ] (Cp = C 5 H 5 , Cp* = C 5 Me 5 ), which decomposes to [CpCp*(Cl)HfÀ Ph] and SnPh 2 , [7] and [CpCp*(Cl)Hf À SnHMes 2 ] (Mes = 2,4,6-Me 3 C 6 H 2 ), which eliminates SnMes 2 to form [CpCp*(Cl)Hf À H].[5] Eliminations of this type appear to operate in the catalytic dehydropolymerization of secondary stannanes R 2 SnH 2 to polystannanes H(SnR 2 ) n H. A likely mechanism for the latter process involves dehydrocoupling of the stannane with a metal hydride by s-bond metathesis to form H 2 and a Hf À SnHR 2 complex, with subsequent elimination of the stannylene SnR 2 to regenerate the metal hydride. The stannylene is then polymerized by rapid insertions into Hf À Sn or HÀSn bonds (Scheme 1).[6] Similar reactivity leads to SbÀSb bond formation by a-stibinidene elimination from a HfÀSbHR complex, [8] and an analogous process is implicated for the formation of As À As bonds via a Zr À AsHMes derivative. [9] In parallel, the a-elimination of methylene from [Cp' 2 Ce(CH 2 X)] (Cp' = 1,2,4-tBu 3 C 5 H 2 ; X = F, Cl, Br, I, OMe, NMe 2 ) in the reaction of [Cp' 2 CeH] with CH 3 X to form [Cp' 2 CeX] and CH 4 shows that the formation of carbene occurs in the presence of electron-withdrawing X groups . [10] Interestingly, then, there appears to be two fundamental processes associated with dehydrocoupling of main-group compounds as catalyzed by early transition metals: s-bond metathesis and migratory deinsertion of low-valent species (and subsequent reinsertion into the appropriate bond). Experimentally, the mechanism of Scheme 1 may be difficult to distinguish from a mechanism involving only s-bond metathesis steps (as proposed for silanes).[ ]), with free-energy barriers that are lower than those associated with a purely s-bond metathesis mechanism for SnÀSn bond formation. Scheme 1. Mechanism proposed for the dehydrocoupling of stannanes in the presence of a hafnium hyd...