The new Mn(I) borohydride complex Mn4 was prepared in 79% yield by treatment of Mn1 with NH3BH3 at 50 o C for 3.5 h. It was fully characterized by 1 H, 13 C{ 1 H} and 31 P{ 1 H} NMR and IR spectroscopy, and high-resolution mass spectrometry. The catalytic performance of Mn4 and the known Mn(I) complexes Mn1-Mn3 were then investigated for the isomerization of 1-allyl-4-flurobenzene as model substrate in THF as solvent. Selected optimization experiments are depicted in Table . Although Mn1 effectively catalyzes a variety of hydrofunctionalizations it turned out to be catalytically inactive for isomerization purposes. Likewise, the hydride complex Mn2 was found to be inactive for the desired transformation. With Mn3 bearing the σ-B-H bound HBpin ligand full conversion to the internal alkene was achieved at 60 °C. However, lowering the temperature to 25 °C resulted in no catalytic activity. Mn4 afforded high conversion and yield with very good selectivity towards the E-isomer even at room temperature within 10 h. Notably, no additives were required to activate either Mn3 or Mn4. In other solvents such as CH2Cl2 and toluene Mn4 was considerably less active and lower yields were achieved, whereas in DMSO, acetone, and MeOH no reaction took place at all (Table , entries 8-12). Finally, in the absence of catalyst no reaction took place. Mn4 (2.5) MeOH 25 -a Reaction conditions: 1-Allyl-4-fluorobenzene (0.5 mmol), [Mn] and 1,4-dioxane as internal standard (0.13 mmol) in the respective solvent (0.5 mL) at 60 °C for 10 h. Conversion and E/Z ratio were determined by 1 H NMR spectroscopy.