Once diphenylacetylene has been consumed, the isomerization catalytic cycle starts. In agreement with experiment, the associated barriers would be prohibitively high (32.5 kcal/mol) with the diphenylacetylene complex 4 as resting state but become readily accessible when the hydride compound 3 can accumulate in solution. Endergonic (17.9 kcal/mol) coordination of Zstilbene to hydride complex 3 to form complex 8 is followed by hydride insertion (∆G ‡ = 24.2 kcal/mol) yielding trans-dialkyl nickel complex 9. Facile C-C bond rotation to form conformer 10 followed by b-hydride elimination (∆G ‡ = 21.9 kcal/mol) releasing E-stilbene to regenerate 3 complete the isomerization cycle (details in SI 4.2). Because the catalyst deactivation is in competition with the isomerization process according to the experimental data, we also investigated its mechanism. A possible pathway starts with the rotation of the Ni-C bond of complex 10 to yield conformer 12 (-3.7 kcal/mol). b-hydride elimination (∆G ‡ = 24.6 kcal/mol) takes place yielding complex 13 that subsequently undergoes reductive elimination resulting in complex 5. Another possible pathway involves a concerted hydrogen transfer transition state: a hydrogen atom is transferred from one alkyl ligand to the alkyl ligand part of the pincer backbone in complex 12 with energy barrier of ∆G ‡ = 25.0 kcal/mol yielding complex 5 (see SI 4.6). The overall barrier for self-hydrogenation is slightly higher than the isomerization process (2.7 Kcal/mol), consistent with the catalyst performing a few isomerization turnovers before decaying via the thermodynamically favored self-hydrogenation pathway. Other pathways considered were the reductive elimination from hydrides 3, 8 or 11, which were all prohibitively high in energy, in accord with the difficulty of such a process for trans substituents in a square-planar structure. Concerted hydrogen transfer from complex 10 (no rotation) were also not feasible to yield complex 5 (see SI 4.6).