We postulated that if aniline could protonate the vinyl fragment of 3a, the system would regenerate 1 and one equivalent of (Z)-1,2-diphenylethylene. This was realized in experiments. From these stoichiometric reactions, it is easy to envisage a catalytic cycle very similar to those put forward for copper alkoxide catalysis, 40 towards (Z)-selective alkyne semihydrogenation. DFT calculations were used to ascertain that each of the steps involved in catalytic turnover had an accessible Gibbs activation energy (Figure ). The global energy barrier for turnover is ΔG 298 ≠ = +35.0 kcal mol -1 , consistent with a reaction that requires forcing conditions. This barrier is associated with two steps, a reversible H2 splitting and subsequent diphenylacetylene hydrozincation. Hydrogenation reactions were performed at a substrate concentration of 0.17 M in 2-7 mL of solvent, with stirring (900 rpm) supplied by a magnetic stirrer bar. At 145 °C and 23 bar H2 pressure, the optimal yield and selectivity for the semi-hydrogenation of diphenylacetylene with 10 mol % 1 was obtained (Figure , entry 1). This reaction produces (Z)-1,2-diphenylethylene in >80% with >99:1 stereoselectivity and ~97:3 chemoselectivity with only small amounts of over hydrogenation to the alkane. A negative control using no catalyst showed no conversion. 3a is a poor catalyst leading to only a small amount of product (7%) likely from a stoichiometric reaction. 4 was also catalytically active for the semihydrogenation of diphenylacetylene to (Z)-1,2-diphenylethylene in high yield and selectivity (Figure ).