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, Joseph Chatt and L. A. Duncanson. The Dewar–Chatt–Duncanson model describes the binding of a transition metal to the C=C bond.
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 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^.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 center. In the nickel compound Ni₂ the value is 143 pm. Ni is expected to be a more powerful pi-donor, resulting in greater donation into the pi antibonding level of the alkene.
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. In silico calculations show that 75% of the binding energy is derived from the forward donation and 25% from backdonation. 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.