Our initial mechanistic proposal was that a facile oxidative addition of the enol tosylate 1 to the Ni-complex is followed by an isomerization event, stereodetermining transmetalation, and reductive elimination (Figure ). However, as the exact order of the described elementary steps was unclear, detailed computational studies were performed. Geometry optimizations were conducted using the B3LYP-D3BJ functional, and energies were refined with M06L using a larger basis set and THF as an implicit solvent. Oxidative addition calculations were modeled after extensive work reported by Neufeldt and co-workers. The oxidative addition step was calculated to proceed through different transition state structures depending on the ligand on the metal (Figure ). A concerted five-centered pathway was favored with L1 (ΔG ‡ = 12.1 and 3.5 kcal/mol for the (Z)-and (E)-alkene complex, respectively), while a S N 2-type pathway was favored with L2 (ΔG ‡ = 9.3 and 3.0 kcal/mol for the (Z)and (E)-alkene complex, respectively) as the ligand. In contrast, reductive elimination was found to proceed through the same concerted three-centered transition state with both ligands (L1: (Z)-complex ΔG ‡ = 13.7 kcal/mol, (E)-complex ΔG ‡ = 7.1 kcal/mol. L2: (Z)-complex ΔG ‡ = 11.2 kcal/mol, (E)-complex ΔG ‡ = 9.1 kcal/mol). The calculated energy barriers for these steps are readily surmountable under the reaction conditions and thus should not have an influence on the observed selectivity. We then turned our attention towards elucidating a potential isomerization event and the role of transmetalation in the stereodetermining step. The precise nature of these processes was much more ambiguous. For the present reaction, the mechanistic rationale of the isomerization event was of particular interest. For Pd-catalyzed carbometalation reactions, previous reports have postulated that the isomerization of the vinyl-Pd complex occurs via zwitterionic Pd-carbenoid intermediates, which form after the carbometalation step. Alternatively, Crabtree and co-workers have proposed that the key isomerization step proceeds via formation of an η 2 -vinyl complex for Pdcatalyzed hydrosilylation of alkynes. Nevertheless, a deeper mechanistic understanding of this widely occurring isomerization event in cross-coupling reactions has remained elusive, especially for Ni-catalyzed reactions. (20-25 kcal/mol) could be achieved through an η 2 -vinyl complex at various points after oxidative addition and before transmetalation (Figure ). The proposed η 2 -vinyl complex can be formed after oxidative addition (Figure ) by dissociation of the tosylate, which allows the alkene to isomerize via an η 2vinyl cationic transition state 5-TS with two phosphine ligands coordinated. As an alternative, the isomerization was calculated to be equally facile prior to transmetalation via the hydroxocomplex 6 (Figure ). For both complexes 5 and 6, the isomerization barriers were favored by 2-3 kcal/mol with L2 as the ligand. Scheme 4. Mechanistic investigation of the isomerization: (A) stoichiometric oxidative addition and (B) reaction of oxidative addition complex 1a'-L1 with boronate 2a and base.