Dewar-Chatt-Duncanson model

How can a molecule like ethylene, which has no lone pair to offer, possibly bind to a metal? Its electrons are all tied up in bonds. The Dewar-Chatt-Duncanson model answers this with the same two-way-handshake idea that explains carbon monoxide: the alkene uses the electrons of its pi bond as the gift, and the metal gives some back. It is the standard picture for how alkenes, and by extension alkynes, sit on a metal.

Picture an ethylene molecule lying sideways across the metal, its carbon-carbon axis parallel to a metal orbital. In the first part of the bond, the filled pi orbital of the alkene, sitting between the two carbons, donates its electron pair into an empty metal orbital — a sigma-type donation from the pi bond. In the second part, a filled metal d orbital of the right symmetry overlaps with the empty pi-star antibonding orbital of the alkene and pushes electron density back into it — the pi back-donation. The two flows are synergic, just as with CO. The back-donation has a structural fingerprint you can see by X-ray: filling the antibonding orbital weakens and lengthens the carbon-carbon bond, and the two carbons bend their other substituents back away from the metal, the alkene losing some of its flat sp2 character. In the extreme, with very strong back-donation, the picture shades into a three-membered metallacyclopropane ring.

This model matters because alkene coordination is the first step of an enormous amount of catalysis: hydrogenation, polymerization, hydroformylation and more all begin by an alkene binding this way before it inserts or is attacked. The model also explains reactivity: heavy back-donation makes the bound alkene electron-rich and resistant to nucleophiles, while weak back-donation leaves it electron-poor and primed for nucleophilic attack, which is the basis of palladium-catalysed oxidations. Like the CO picture, it is a molecular-orbital model, an honest and predictive one, but a model of where electrons go rather than a literal photograph.