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

Single-crystal X-ray characterization of cationic (-diimine)Ni-ethyl and isopropyl -agostic complexes, which are key intermediates in olefin polymerization and oligomerization, are presented. The sharp Ni-C-C angles (75.0(3)degrees and 74.57(18)degrees) and short C-C distances (1.468(7) and 1.487(5)angstrom) provide unambiguous evidence for a -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 -hydride elimination-olefin rotation-reinsertion is constructed from variable-temperature NMR spectra with C-13-labeled agostic complexes. The enthalpy of activation (H) for -H elimination is 13.2kcalmol(-1). These results offer important mechanistic insight into two critical steps in polymerization: ligand association upon cleavage of the -agostic bonds and chain-migration via beta-H elimination.