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]

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. In the nickel compound Ni(C₂H₄)(PPh₃)₂ the value is 143 pm.

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.^[4] In silico calculations show that 75% of the binding energy is derived from the forward donation and 25% from backdonation.^[5] This model is a specific manifestation of the more general π backbonding model.

Similar to alkenes, alkynes adopt a similar bonding interaction, as shown in the image on the right. It should be noted, however, that not all alkyne-metal complexes utilize all four of these interactions for bonding (due to reasons like unviable d orbitals).

Main group elements can also form π-complexes with alkenes and alkynes. The β-diketiminato aluminum(I) complex Al{HC(CMeNAr)₂} (Ar = 2,6-diisopropylphenyl), which bears an Al-based sp^x lone pair, reacts with alkenes and alkynes to give alumina^(III)cyclopropanes and alumina^(III)cyclopropenes in a process analogous to the formation of π-complexes by transition metals.^[6][7] However, in most cases, the backbonding interaction is absent in these complexes due to the lack of energetically accessible filled orbitals for backdonation, resulting in π-complexes that dissociate readily and are therefore more challenging to observe or isolate.^[8][9]