It is well-known that many heterogeneous catalytic reactions are highly sensitive to the structure of active sites. The Cr 3+ -silica geometry in the Philips catalyst for ethylene polymerization is a useful example, wherein the reactivity of different Cr sites in terms of initiation, propagation, and termination energetics vary significantly with the local strain condition, causing a small percentage of the Cr sites to dominate the polymerization chemistry, Further, in recent works, an alternate C-A cycle with alkene-assisted β-hydride transfer has been explored on both Ni, for oligomerization , and Cr, for polymerization, and favorable energy landscapes are observed.