The transition between those two regimes is accompanied by a change in catalyst speciation evidenced by operando 1 H NMR and 31 P NMR data. In the first regime, the only observable Nicontaining species is the diphenylacetylene π-complex 4 (Figure , a), which also could be independently prepared by treating complex 1 with equivalent of diphenylacetylene. An olefinic 1 H signal at ¶ 5.72 ppm indicates that the olefin backbone decoordinates from Ni when the πacidic alkyne binds, as has been observed for a related ketone pincer ligand (see SI section 2). Once diphenylacetylene has been consumed, complex 4 gradually disappears, which coincides with the onset of (E)-(Z) isomerization and overhydrogenation. At this time, a temporary decrease is observed in the total concentration of detected Ni species: the hydride mixture 2 ⇄ 3 likely accumulates in the reaction medium, but it cannot be detected at these concentrations because it gives broad signals (see Figure , a. Catalyst mass balance, red arrow). Supporting this idea, neither (Z)-nor (E)-stilbene displaced the N2 ligand in 1 in stoichiometric experiments, showing that these olefins cannot occupy the same binding site as diphenylacetylene in 4 (see SI section 1.4). In the same time period, two new complexes gradually appear. The main species could be isolated after reaction and identified both spectroscopically and crystallographically as the Estilbene complex 5 (see Figure ), in which the olefin backbone has been hydrogenated (selfhydrogenation). In C6D6 solution, it displays a characteristic deshielded aliphatic multiplet 1 H signal at ¶ 8.38-8.52 ppm corresponding to the central C-H unit engaging in an anagostic interaction with the nickel center (H73). The minor species was identified in situ as the selfhydrogenated complex but with diphenylacetylene as coligand, 5' (see SI sections 1.3, 1.5). When 5 and 5' reach their final concentration, overhydrogenation stops and (E)-(Z) isomerization becomes exceedingly slow, suggesting that catalyst self-hydrogenation shuts down both pathways. Indeed, the isolated complex 5 revealed a sluggish isomerization catalyst under relevant conditions (10 mol%, 70ºC, 4.6 atm H2): after 24h, the isomerization reaction has not completed (~20 %) and only trace amounts of diphenylethane are observed. Complex 5 is also inactive for the hydrogenation of diphenylacetylene when only ligand exchange between diphenylacetylene and (E)-stilbene is observed (see SI section 1.5). To assess whether the allylic H atom of the olefin pincer ligand participates in the catalytic reaction, we resorted to isotope labeling experiments. Performing the hydrogenation with D2 expectedly results in rapid deuteration of the olefinic proton of 4, but no deuterium incorporation at the allylic C-H position is observed (Figure ). The experiment using a complex 1 deuterated in alkyl and olefinic positions results in scrambling of hydrogen uniquely in the olefin position (Figure ). These experiments rule out any direct involvement of this allylic position in the hydrogenation mechanism (see SI section 1.4) The observations above suggest that the final product distribution is largely determined by the competition between (E)-(Z) isomerization and catalyst self-hydrogenation. This competition could be studied separately by exposing (Z)-stilbene to 1 (10 mol %) under an H2 atmosphere at 70ºC (see SI 1.3). At early times, rapid catalytic isomerization to (E)-stilbene was observed together with gradual appearance of the self-hydrogenation product 5; the 1 H NMR signal of free H2 was broad, suggesting the presence of hydride mixture 2 ⇄ 3 in addition to complex 5. After ca. 2h (ca. 77% conversion), isomerization considerably slowed down, the 1 H NMR signal of H2 became sharp, and complex 5 was still the only Ni-containing species detected by 1 H NMR. A maximum of 82 % conversion (incl. 2 % diphenylethane) was reached after 10h. In contrast, no isomerization was observed under an N2 atmosphere, showing that complex 1 alone is inactive. These results confirm that the hydride mixture 2 ⇄ 3 generated from 1 under H2 catalyzes olefin isomerization, presumably via a hydride mechanism, and decays to complex 5 by selfhydrogenation. Interestingly, the same reaction at 25 ºC reached 93 % conversion to E-stilbene after 10h with barely any overhydrogenation (1%), suggesting that temperature can be used to control the relative rates of self-hydrogenation vs. catalytic isomerization. While the self-hydrogenation process forming the inactive complex 5 could formally be initiated by C-H reductive elimination from hydride 3, such a reaction is unlikely for trans substituents in a square-planar structure. Accordingly, heating either 1 or its benzonitrile analogue (benzonitrile is a weaker ligand than phenylacetylene) under an H2 atmosphere does not result in any observable C-H bond formation (see SI section 1.4). In contrast, the clean formation of complex 5 from 1 and H2 in the presence of (E)-or (Z)-stilbene suggests that an olefin acting as a "hydride shuttle" takes part in this process.