To get more insight into this dependency and into the reaction mechanism in general, we monitored the reaction with 10 mol% catalyst loading by 1 H NMR (Figure ), which revealed two well-separated regimes. First, as long as diphenylacetylene is present in excess of the catalyst, clean conversion to Z-stilbene occurs with an apparent zero order with respect to diphenylacetylene (Figure , a). Then, rapid isomerization of Z-stilbene to E-stilbene starts taking place reaching a Z:E ratio of 6:3. After a short period of time (ca. 1h), the process considerably slows down and does not reach thermodynamical equilibrium. Concomitantly, small amounts of the overhydrogenation product diphenylethane are formed. To rule out simultaneous semihydrogenation and isomerization, a substrate competition experiment was performed using equal amounts of diphenylacetylene and Z-stilbene at the beginning of the reaction with 10 % mol catalyst. The results showed that semihydrogenation takes place first and after isomerization of stilbene (see SI section 1.3). Concentrations were determined by 1 H NMR using mesitylene as internal standard. A) On top, kinetic profile at 10 % mol catalyst loading at 70 ºC heated and monitored by 1 H NMR. Bottom: profile of detected catalyst speciation during semihydrogenation of diphenylacetylene (resting state 4 and self-hydrogenated species 5 and 5'). The green dots represent the total concentration of detected species plotted in a separate graphic for clarity. The red arrow marks a temporary decrease in the total concentration of detected species, suggesting a buildup of the hydride mixture 2 ⇄ 3 that cannot be detected under these conditions because it gives broad features. B) Molecular structure of self-hydrogenated product 5 determined by X-ray crystallography with selected bond lengths. Only one of two independent molecules is shown. Hexane solvent molecules and the phenyl groups of the phosphines and of the (E)-stilbene coligand are omitted for clarity. c) Formation of partially deuterated self-hydrogenated species 5 in catalytic conditions under D2 atmosphere. d) Deuterium scrambling of d 2 -complex 1 in catalytic conditions under H2 atmosphere.