Video narration
How can an alkene bind a metal while its C-C bond gets weaker? In a molecular-orbital interaction diagram, follow electron flow. In ethylene, its filled π bond donates into an empty metal orbital. That π donation anchors the alkene. But the interaction runs both ways. Metal d electrons flow back into ethylene's empty π* orbital: π back-donation. They are partners in one bond. The first interaction secures contact; the second changes the alkene itself. Now follow the consequence: π* is antibonding with respect to C-C. Adding electron density there lowers C-C bond order, lengthening the bond and making it more susceptible to reaction. The Dewar-Chatt-Duncanson model names this paired donation-and-back-donation picture. The degree of back-donation and bond weakening depends on the metal and its complex. So the diagram's answer: π donation binds ethylene; back-donation into π* weakens C-C, priming it for reaction.