Past exam of the chemistry B course of the University of Cambridge / 2023 / Paper 2 / B8 / a / i

The Dewar-Chatt-Duncanson model has two complementary interactions. The filled alkene $\pi$ orbital donates to an empty metal orbital, and an occupied metal $d_{\pi }$ orbital back-donates into the empty alkene ${\pi }^{\ast }$ orbital. The metal acceptor has the symmetry of the sum of the two ligand lobes facing it; the back-donor has opposite phases at the two carbon positions and matches the antisymmetric ${\pi }^{\ast }$ combination.Both channels strengthen metal–alkene attachment. Donation removes some electron density from the internal bonding orbital, while back-donation populates its antibonding partner, so coordination generally reduces C–C bond order. The schematic shows orbital phase signs, not positive and negative electric charges.