Write for the imidazolylidene N-heterocyclic carbene bearing the two mesityl groups shown. The proposed active unit is , with H and SiPh both bonded to nickel, rather than a nickel-free hydrosilane or a bare NHC-Ni fragment.
Displacement of coordinated styrene and formal oxidative addition of Si-H to Ni(0) give a low-coordinate nickel(II) hydride-silyl unit; substrate binding fills a vacant site. The productive cycle is alkene coordination, migratory insertion into Ni-H, then beta-hydride elimination from the other beta carbon, releasing the shifted alkene and regenerating Ni-H.
The strongly donating NHC stabilizes a reactive low-coordinate metal and assists Si-H activation. Its steric bulk discourages unproductive saturated bis-alkene species and promotes product departure, but excessive crowding can hinder substrate binding/insertion. The silyl ligand also tunes the metal electronically: changing its aryl substituents changes donation and the barriers for hydride transfer/elimination. Product release and reversible insertion determine whether further double-bond migration occurs. Bulky ligands can suppress competing alkyl-silyl reductive elimination, which would instead consume substrate by hydrosilylation.
The hydride-silyl formulation and insertion/elimination interpretation are the experimentally proposed mechanism; the individual catalyst resting states need not all be uniquely fixed by the activation recipe.[ref]
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