The addition of pyridine (Table , entry 2) resulted in a significant drop in reactivity of Mn4. The isomerization of allylbenzene resulted in a yield of 77 %, while in presence of PMe3 no catalytic activity was observed (Table , entry 3). A deactivation by the strongly coordinating pyridine and PMe3 ligands is in line with the presence of a vacant coordination site during catalysis. Monitoring of the experiments involving pyridine and PMe3 by 31 P{ 1 H} NMR spectroscopy revealed the formation of new hydride species giving rise to signals at -4.52 (t, JHP = 54.8 Hz) and -9.00 (q, JHP = 49.3 Hz) ppm, respectively. The reaction mechanism was explored in detail by means of DFT calculations, 21 with allylbenzene taken as model substrate and Mn4 (A in the calculations as allylbenzene adduct) as pre-catalyst. The resulting free energy profiles are represented in Figures while Scheme 5 depicts a summary of the catalytic cycle. The free energy profile for the initiation process, where the active catalyst is formed, is depicted in Figure . The first step is the addition of allylbenzene in A to a non-coordinated B-H bond of the BH4 ligand thereby being hydroborated to form B containing a κ 2 -B,H-coordinated borane (B' is a rotamer of B). This first step requires a high barrier of 30 kcal/mol and is slightly exergonic. This is in line with in situ NMR spectroscopy during catalysis. The dominant species throughout the reaction was found to be Mn4 even upon full conversion. Dissociation of H2BCH2CH2CH2Ph affords the catalytically active species C ([Mn(dippe)(CO)2H]) in an endergonic step (ΔG = 15 kcal/mol) with a barrier of 17 kcal/mol (see also Scheme 5). Addition of another allylbenzene affords complex D bearing a η 2 -bound CH2=CHCH2Ph ligand. In the next step of the reaction the hydride migrates to the terminal olefin C-atom resulting in an alkyl complex E stabilized by a C-H agostic interaction with the terminal C-H bond (Figure ). This is a very facile step with a barrier of merely 5 kcal/mol and a free energy balance of ΔG = 1 kcal/mol. In the following steps, the agostic interaction is cleaved to yield F which is then further stabilized by forming an agostic interaction with the C-H bond adjacent to the phenyl substituent to give intermediate G. These processes are essentially thermoneutral and have a barrier of 11 kcal/mol. Complex G undergoes β-elimination to form complex H featuring the η 2 -coordinated product olefin (Figure ). The barrier is only 7 kcal/mol with a free energy balance of ΔG = 7 kcal/mol with respect to G. In the last step of the mechanism E-CH3CH=CHPh (2a) is liberated from H to yield the coordinatively unsaturated hydride complex J. This complex undergoes facile isomerization to afford the catalytically active species K thereby closing the catalytic cycle (see Scheme 5). It has to be noted that this intermediate is essentially identical to C but without the loosely bound borane H2BCH2CH2CH2Ph obtained in the initiating step (Figure ). Closing the cycle, from K back to D, with addition of a fresh allylbenzene molecule has a free energy balance of ΔG = -5 kcal/mol. In summary, initiation is an endergonic (ΔG = 8 kcal/mol) process with a barrier of 30 kcal/mol, from A to TSAB, while the catalytic cycle has a barrier of ΔG ‡ = 11 kcal/mol, from E to TSEF. Formation of the Z-isomer of the product was also addressed by the DFT mechanistic studies (see Supporting Information). The process involves a C-H agostic exchange in intermediate G, followed by βelimination and loss of the corresponding product Z-CH3CH=CHPh. The process has a barrier of 14 kcal/mol measured from G to the β-elimination transition state TSLM being thus less favorable than formation of the E isomer. This is in good agreement with the experimental observations.