DFT calculations performed on a slightly truncated model support the mechanistic scenario outlined in Figure (details in SI section 4), which accounts for the experimental observations. The semihydrogenation cycle starts with the endergonic (+16.7 kcal/mol) displacement of diphenylacetylene for H2 to form complex 2 from resting state 4. Rate-limiting H2 cleavage via LLHT (see above) yields complex 3 with an overall activation energy of 29.9 kcal/mol, consistent with a slow reaction at 70 °C. Endergonic coordination of diphenylacetylene yields complex 6, followed by insertion (∆G ‡ = 25.8 kcal/mol) to form the vinyl complex 7. To complete the catalytic cycle, transfer of a hydrogen atom from the CH2 backbone to the vinyl ligand forms Z-stilbene and regenerates the olefin backbone with an overall barrier of 30.9 kcal/mol. Interestingly, two nearly isoenergetic pathways were identified for the C-H formation step: a stepwise b-hydride elimination/reductive elimination sequence involving a high-lying nickel hydride or a concerted LLHT-like step akin to that found for H-H cleavage (details in SI section 4.1). The associated barriers are ca. 1 kcal/mol higher than that for H2 cleavage, but this step will be additionally favored by concentration effects in the presence of an excess of alkyne substrate. A s-bond metathesis pathway cleaving the Ni-C bond of complex 7 with an incoming H2 molecule to form 3 directly was also considered but was found energetically inaccessible with an overall barrier of 38.7 Kcal/mol (see SI section 4.5).