.sigma.-Bond metathesis mechanism for dehydropolymerization of silanes to polysilanes by d0 metal catalysts

The σ‐CAM Mechanism: σ Complexes as the Basis of σ‐Bond Metathesis at Late‐Transition‐Metal Centers

“…[112][113][114] In 1992, Tilley et al had already suggested that "a transient intermediate which may form prior to a four-center transition state is a complex with coordinated SiÀH, HÀH, or Si À Si s bonds." [115] There are now a few examples of C À C agostic complexes, [65,[116][117][118][119][120] but intermolecular s-C À C complexes are not known as far as we are aware. We look forward to the discovery of evidence for s-EÀE' complexes and their reactivity.…”

Organosilicon Polymers?Synthesis, Architecture, Reactivity and Applications

“…8 Tilley has postulated a -bond metathesis mechanism involving four-center transition states in which -bonds are broken and formed (Scheme 6). 8,93 M-H represents the coordinatively unsaturated metal hydride which acts as the active catalyst (d 0 metal hydride). The proposed mechanism is based on two steps: the dehydrometalation of the silane with the metal hydride (A) and the coupling of the metal silyl complex with the hydrosilane resulting in Si-Si bond formation (B).…”

Metallocenes

“…Group IVB metallocenes catalyze the stepwise polymerization (dehydrocoupling) of primary silanes, yielding H-(SiHR)n-H and n H 2 (272). The mechanism is proposed to be strictly metathetical for such molecules as (Cp)(C 5 (CH 3 ) 5 )Hf(R)(X) (R = H, alkyl; X = H, alkyl, chloride), wherein the entire process takes place on neutral hafnium (Scheme 8) (273). A different pathway, probably involving the coupling of free silyl radicals, appears to operate in polymerizations catalyzed by group IVB complexes (eg, Cp 2 ZrCl 2 ) activated by noncoordinating ionizing agents such as [(C 6 H 5 ) 3 C][B(C 6 F 5 ) 4-n (n-Bu) n ] (n = 0-2) following pretreatment with n-BuLi (274).…”