Free energy diagrams (523 K) of proton transfer (species 1 to 5a, Fig. ), β-hydride elimination (species 4 to 6b), and ethylene-assisted β-hydride transfer (species 4 to 7c) pathways on the 4CN Ga site are shown in Figure . The 4CN Ga site has a different free energy landscape for site activation than does the 3CN site. Before the migratory insertion of ethylene, the physisorption step is greatly endothermic (step 2 to 3, +1.2 eV), which can be explained by a strong steric hindrance due to the nearby siloxane framework. Although a lower intrinsic activation barrier of ethylene insertion is found (step 3 to ‡ 2 , +1.2 eV), the transition state is high in free energy if Ga-vinyl is used as a reference (3CN: 1.2 eV, 4CN: 2.5 eV). Similarly, the positive free energy change of ethylene physisorption leads to a significant unfavourability of ethylene-assisted β-hydride transfer (step 4 to 5c, +1.3 eV), which is, in contrast, a viable pathway on 3CN Ga site.