= Solution
Write $L$ for the imidazolylidene <N-heterocyclic carbene> bearing the two mesityl groups shown. The proposed active unit is \b[$\mathrm{LNi(H)(SiPh_3)}$], with H and SiPh$_3$ 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.<Modular nickel-silane catalytic system for the isomerization of alkenes by Kiana E. Kawamura and colleagues>{ref}
\Image[/past-exam-of-the-chemistry-course-of-the-year-3-of-the-natural-sciences-course-of-the-university-of-cambridge/2022/paper-3-q38-nickel-cycle.png]
{title=Hydride insertion and beta elimination with a shared intermolecular H/D pool}
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