Dewar–Chatt–Duncanson model

The Dewar–Chatt–Duncanson model is a model in organometallic chemistry that explains the chemical bonding in transition metal alkene complexes. The model is named after Michael J. S. Dewar,^[1] Joseph Chatt and L. A. Duncanson.^[2][3] The Dewar–Chatt–Duncanson model describes the binding of a transition metal to the C=C bond.^[4][5][6]

Orbital interactions in a metal-ethylene complex. On the left, a filled pi-orbital on C₂H₄ overlaps with an empty d-orbital on the metal. On the right, an empty pi-antibonding orbital on C₂H₄ overlaps with a filled d-orbital on the metal

The alkene donates electron density into a π-acid metal d-orbital from a σ-symmetry bonding orbital between the carbon atoms. The metal donates electrons back from a (different) filled d-orbital into the empty π^* antibonding orbital. Both of these effects tend to reduce the carbon-carbon bond order, leading to an elongated C−C distance and a lowering of its vibrational frequency.

In Zeise's salt K[PtCl₃(C₂H₄)]^.H₂O the C−C bond length has increased to 134 picometres from 133 pm for ethylene. According to the DCD models, this small change in the C-C bond indicates that there is little back-donation from the Pt(II) center. In the nickel(0) compound Ni(C₂H₄)(PPh₃)₂ the value is 143 pm. Ni(0) is expected to be a more powerful pi-donor, resulting in greater donation into the pi antibonding level of the alkene.

The orbital interactions of alkyne-metal complexes, with A & B being interactions between the parallel π orbitals with the d orbital, and C & D being the interactions between the perpendicular π orbital and the d orbitals. Note that B & D are backbonding interactions.

The interaction also causes carbon atoms to "rehybridise" from sp² towards sp³, which is indicated by the bending of the hydrogen atoms on the ethylene back away from the metal.^[7] In silico calculations show that 75% of the binding energy is derived from the forward donation and 25% from backdonation.^[8] This model is a specific manifestation of the more general π backbonding model.

Like alkenes, alkynes adopt a similar bonding interaction, as shown in the image on the right. Not all alkyne-metal complexes utilize all four of these interactions for bonding (due to reasons like unviable d orbitals).^{[citation needed]}