In order to establish the best reaction conditions for the semi-hydrogenation of alkynes, diphenyl acetylene (1a) was chosen as model substrate in combination with MeOH and KBH4 as hydrogen source. When 1a was treated with 1 mol % of Mn1 at 60°C with MeOH and 0.5 equiv of KBH4, E-stilbene (1b) was obtained in 99% isolated yield (Table , entry 1). Only traces of Z-stilbene (1c) were observed and over-hydrogenation did not take place.