In sum, we have introduced an efficient protocol for the transposition of terminal alkenes catalyzed by the Mn(I) borohydride complex fac-[Mn(dippe)(CO)2(κ 2 -BH4)]. The reported procedure operates at room temperature with a catalyst loading of 2.5 mol% and no additives. High reactivities and excellent Eselectivities were achieved for allylbenzene derivatives. While maintaining good reactivity, the selectivity was slightly diminished for aliphatic systems. Increasing the steric bulk in these substrates reestablished high E-selectivity. The protocol was also applied for gram-scale synthesis with 4-allyl-1,1'-biphenyl. Preliminary studies also showed a temperature-dependent chain walking process leading to a sequential migration of the double bond. Deuterium studies were carried out to gain insights in the reaction mechanism. The influence of additives such as pyridine and PMe3 on the reactivity of fac-[Mn(dippe)(CO)2(κ 2 -BH4)] was investigated ultimately leading to the detection of hydride species. Both additives lead to deactivation by the strongly coordinating ligands being in line with the requirement of a vacant coordination site during catalysis. DFT calculations indeed disclosed a typical inner sphere mechanism with all reacting fragments coordinated to the metal. The reaction proceeds via a metal hydride mechanism involving hydride insertion of the terminal C=C bond of the olefin resulting in an alkyl complex. Consecutive β-elimination of internal C-H bond adjacent to the substituent yields the isomerization product. Notably, the formation of the E-isomer is favored both kinetically and thermodynamically.