To get better insight into the nature of TS-3a an analysis of the key localized molecular orbitals (LMOs) along the intrinsic reaction coordinate (IRC) was carried out. LMOs were calculated following the Pipek-Mezey criterion and a procedure described by Vidossich and Lledóss was used to generate centroids of these LMOs (CLMOs). The CLMOs were used to follow the bond rearrangements around TS-3a (Figure ). For the sake of clarity, depiction of the overall process was separated in a pre-and post-TS stage. From INT-3a to TS-3a (pre-TS), migration of the β-C of the bound alkene goes along with an electron transfer from the Fe-C bond to Al while the electrons from the Al-C are shifted towards the α-C and get delocalized through participation of the arene π-system (vide infra). In the post-TS stage, the most significant rearrangement of the electronic structure occurs during the C-H bond cleavage. Electrons from the C-H σ-orbital are shifted towards Fe forming the Fe-H bond. This process is consistent with a hydride transfer and distinct from the reaction of 1 with pyridines and CH3CN 17 which were found to proceed via a proton transfer (reductive deprotonation). Simultaneously, the delocalized lone pair at the α-C is shifted back towards the β-C to re-establish the π-system of the C=C double bond in the vinyl ligand. The proposed mechanism is supported by deuterium labelling studies. A comparison of the reaction rates of 1 with styrene and styrene-d8 at 323 K from two separate experiments resulted in a kinetic isotope effect (KIE) of 2.64 ± 0.04. The overall KIE is likely affected by an additional equilibrium isotope effect caused by the reversible alkene binding which, however, appeared to be relatively small at room temperature (EIE = 1.06 at 298 K). This contrasts the ortho C-H activation of pyridine for which an unusually large kH/kD value of 14.0 ± 0.2 at 298 K was obtained likely caused by a quantum tunneling and thus diagnostic for a proton transfer reaction. 15 Table . Substituent effect on the activation energies for INT-3 to INT-4 in the reaction of styrenes with 1.