In sum, an efficient protocol for the selective semihydrogenation of aryl-aryl, aryl-alkyl and terminal alkynes to afford E-alkenes is described. The pre-catalyst is the bench-stable alkyl complex fac-Mn(dippe)(CO)3(CH2CH2CH3) in combination with KBH4 and MeOH as hydrogen source. The hydrogen gas required for the hydrogenation is formed in situ upon alcoholysis of KBH4. Thus, high pressure equipment is not required. The reaction proceeds at 90 o C with catalyst loadings of 0.5 to 2.0 mol% and a reaction time of 20 h. This represents a rare example of a manganese catalyzed hydrogenation of alkynes to give selectively E-alkenes. High functional group tolerance including halides, phenols, nitriles, unprotected amines and heterocycles was observed. Even challenging substrates such as alkyl-alkyl alkynes and terminal alkynes allowed high conversions with moderate to good selectivity. The practical applicability of the protocol was demonstrated in the gram-scale synthesis of E-stilbene. Mechanistic investigations including DFT calculations and deuterium labelling studies were undertaken to provide a reasonable reaction mechanism showing that initially formed Z-isomer undergoes fast isomerization to the afford the thermodynamically more stable E-isomer.