Mechanistic studies were carried out to gain insights in the reaction mechanism (Scheme 3). Upon addition of PPh3 only traces of E/Z-stilbene were obtained. This is attributed to coordination of PPh3 to the manganese center blocking a vacant side of the catalyst. This again indicates that an inner-sphere mechanism is operating in this system. Furthermore, deuterium labelling experiments were performed. If 1a was treated with KBH4 in MeOH-d1, a deuterium content of 81% was observed in E-stilbene. This amount of deuterium incorporation did not change significantly when MeOH-d4 was used instead. As expected, up to 97% deuterium content was incorporated when 1a was treated with NaBD4 in MeOH-d4. In contrast, only a negligible amount of merely 9% deuterium was incorporated when NaBD4 was used in combination with MeOH. When Z-stilbene (1c) was treated with NaBD4 in MeOH-d4 only a deuterium content of 57% was observed in the isomerized product. These observations clearly show that the acidic proton of MeOH is almost exclusively incorporated into the substrate presumably via fast proton exchange with hydride intermediates shown the catalytic cycle (vide infra). Table . Mn1 catalyzed semihydrogenation of terminal alkynes a a Reaction conditions: alkyne (0.4 mmol, 1 equiv), Mn1 (2 mol%), KBH4 (0.3 equiv), MeOH (3.5 mL, 150 equiv), 90°C, 20 h, yield corresponds to alkene (b), values in brackets correspond to yield of saturated alkane (c) as determined GC-MS using dodecane as internal standard. The activation of Mn1 by dihydrogen has been reported recently. Substitution of the weakly-bonded n-butanal by 1a leads to formation of the hydride species A featuring an η 2 -bound acetylene ligand (free energy values are referred to this complex). Insertion of the acetylene into the Mn-H bond affords the coordinatively unsaturated vinyl complex B which is stabilized by an agostic C-H bond. This process is exergonic by -23.8 kcal/mol with a very small barrier of 0.6 kcal/mol. Addition of dihydrogen leads via the van der Waals adduct C to the formation of intermediate D. This step is endergonic by 9.9 kcal/mol with an overall barrier of 18.6 kcal/mol being the highest of the entire cycle. Heterolytic cleavage of dihydrogen results in the formation of the hydride complex E featuring a C-H bound Z-stilbene in an exergonic step (ΔG = -9.4 kcal/mol) with a negligible barrier of 1.8 kcal/mol. The reaction proceeds with reorientation of the olefin from the C-H σ-complex in E to the π-coordinated complex in H. This process proceeds via intermediates F and G which are equivalent van der Waals pairs of the Z-stilbene and the metallic fragment. It involves olefin clockwise rotation of the by about 90 o and recoordination of the olefin. The overall process has a negligible barrier of 1.2 kcal/mol and is favorable with ΔG = -7.1 kcal/mol, reflecting the stronger coordination of the η 2 -olefin in H compared to the C-H σ-bound olefin in complex E. In the next step of the reaction the hydride ligand in H migrates to the adjacent olefin C-atom resulting in an alkyl complex stabilized by a C-H agostic interaction in intermediate I. This is a facile step with a barrier of 4.6 kcal/mol and a free energy balance of ΔG = -6.1 kcal/mol. In I, a switch between the C-H agostic interaction (Z to E isomerization) yields intermediate J (ΔG ‡ = 2.9 kcal/mol and ΔG = -0.4 kcal/mol) and finally β-hydrogen elimination to afford the hydride E-stilbene intermediate [Mn(dippe)(CO)2(H)(η 2 -CH(Ph)=CHPh)] (K) with a barrier of 9.2 kcal/mol in an endergonic final step (ΔG = 8.9 kcal/mol).