Structure and Isotope Effects of the β-H Agostic (α-Diimine)Nickel Cation as a Polymerization Intermediate

Single-crystal X-ray characterization of cationic (α-diimine)Ni-ethyl and isopropyl β-agostic complexes, which are key intermediates in olefin polymerization and oligomerization, are presented. The sharp Ni-C_α -C_β angles (75.0(3)° and 74.57(18)°) and short C_α -C_β distances (1.468(7) and 1.487(5) Å) provide unambiguous evidence for a β-agostic interaction. An inverse equilibrium isotope effect (EIE) for ligand coordination upon cleavage of the agostic bond highlights the weaker bond strength of Ni-H relative to the C-H bond. An Eyring plot for β-hydride elimination-olefin rotation-reinsertion is constructed from variable-temperature NMR spectra with ¹³ C-labeled agostic complexes. The enthalpy of activation (ΔH^≠ ) for β-H elimination is 13.2 kcal mol^-1 . These results offer important mechanistic insight into two critical steps in polymerization: ligand association upon cleavage of the β-agostic bonds and chain-migration via β-H elimination.