Alkene binding to 1 was further investigated by ETS-NOCV calculations (see Figure ). Donation of electron density from the former Fe-Al bond into the π* orbital of the substrate (3a: Δρ1 = -420.7 kcal mol -1 ) accounts for >90 % of the total orbital stabilization energy (3a: ΔEorb = -421.0 kcal mol -1 ). NBO analysis identified a σ-bond between iron and the β-carbon of the substrate whereas bonding of Al to the α-carbon is defined as donor-acceptor interaction between a Ccentered lone pair and empty s/p orbitals at Al possessing a partial ionic character. This is underpinned by the NPA charges revealing that the Al-C bond is more polarized (3a: Al +1.80, C -0.88) in comparison to the Fe-C bond (3a: Fe -0.52, C -0.80).