A Protodenickelation of oxidative addition complexes B Reactivity of the oxidative addition complex with boronate 2a and base With the accumulated data in hand, transmetalation was extensively investigated using DFT to rationalize the experimentally observed diastereoselectivity. The formation of Ni-hydroxo dimer species under similar Suzuki-Miyaura cross-coupling conditions to those employed in our study was recently reported by Grimaud and coworkers, among others. According to their detailed study, dimer formation post oxidative addition was shown to depend on the reaction conditions. Hence, it is likely that catalytically active Ni-hydroxo dimers are formed, especially under conditions C1 in THF/H 2 O. These structures are also supported by DFT calculations, where they are calculated to be the lowest ground state structures preceding transmetalation (see the SI). We modeled our transmetalation calculations for the (Z)-selective conditions using dimer structures as starting points and assumed boronic acid induced dissociation and association of pinacol boronate to form transient transmetalation complexes analogous to transmetalation in Pd-catalyzed Suzuki-Miyaura cross-coupling reported by Denmark and coworkers. After conducting a comprehensive search of transition state structures, we arrived at transition states 7 and 8 (Figure ). With L1 as the ligand (Figure ), it was found that a cis-configuration of the alkene and aryl ligands on the Ni-center is the energetically preferred geometry. The energy gap between the two transition state structures (Z)-and (E)-7 is ΔΔG ‡ = 2.0 kcal/mol and thus in favor of (Z)-alkene complex (Figure ). This agrees with the experimentally observed (Z)-selectivity using L1 in THF/H 2 O (conditions C1). However, with L2 as the ligand (Figure ), the analogous transmetalation transition structures (Z)-and (E)-8 place the alkene and aryl fragments in a trans-configuration. The energy difference between the diastereomeric transition states (Z)-and (E)-8 in this case is ΔΔG ‡ = 1.8 kcal/mol, in favor of (E)-8, which is again in good agreement with the selectivity observed when employing conditions C2. However, more detailed insight into the transmetalation step and its corresponding ground states is obfuscated by its complicated nature. For example, it is unclear whether the boronate ester undergoes partial or full hydrolysis prior to transmetalation under the reaction conditions. Computational results support both boronic acid and pinacol boronate giving rise to a similar selectivity, which is in alignment with experimental results wherein comparable diastereoselectivities of product 3aa were observed upon replacing pinacol boronate 2a with its boronic acid analog 2a' under both conditions C1 and C2 (see SI, Table -2). Although the precise details of the transmetalation step and the corresponding ground states remain unclear, the combination of experimental and computational observations described above support the mechanistic rationale that a facile oxidative addition precedes an isomerization equilibrium, followed by a selectivity-determining transmetalation.