To investigate the potential involvement of a radical mechanism we performed an experiment with TEMPO as a radical trap. The presence of TEMPO is presumed to inhibit an isomerization via radical pathway. However, the presence of the radical trap did not affect the isomerization of 1 (Table , entry 1). No products of a reaction between TEMPO and a substrate radical could be detected. Therefore, we deem the involvement of a radical mechanism unlikely. Further mechanistic investigations focused on the hydride functionality accompanied by an easily accessible coordination site. In order to test this assumption, pyridine and PMe3 were used as additives. a Reaction conditions: 1-Allyl-4-fluorobenzene (0.5 mmol), Mn4 (2.5 mol%), additive (12.5 mol%) and 1,4-dioxane as internal standard (0.13 mmol) in THF-d8 (0.5 mL) at 25 °C for 10 h. Yield and E/Z ratio were determined by 19 F and 1 H NMR spectroscopy.