Dewar-Chatt-Duncanson model

The Dewar-Chatt-Duncanson model is a model in organometallic chemistry which explains the type of chemical bonding between an alkene and a metal (pi-complex) in certain organometallic compounds. The model is named after Michael J. S. Dewar ^[1] , Joseph Chatt and L. A. Duncanson ^[2] ^[3].

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

In Zeise's salt K[PtCl₃(C₂H₄)]^.H₂O the C-C bond length has increased to 134 picometer from 133 pm for ethylene. In the nickel compound Ni(CH₂CH₂)(PPh₃)₂ the value is 143 pm.

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 pi backbonding model.

References

^ Dewar, M. Bull. Soc. Chim. Fr. 1951, 1 8 , C79
^ Olefin co-ordination compounds. Part III. Infra-red spectra and structure: attempted preparation of acetylene complexes J. Chatt and L. A. Duncanson, J. Chem. Soc., 1953, 2939 doi:10.1039/JR9530002939
^ Directing effects in inorganic substitution reactions. Part I. A hypothesis to explain the trans-effect J. Chatt, L. A. Duncanson, L. M. Venanzi, J. Chem. Soc., 1955, 4456-4460 doi:10.1039/JR9550004456
^ Miessler, Gary L.; Donald A. Tarr (2004). Inorganic Chemistry. Upper Saddle River, New Jersey: Pearson Education, Inc. Pearson Prentice Hall. ISBN 0-13-035471-6. .
^ Herrmann/Brauer: Synthetic Methods of Organometallic and Inorganic Chemistry Georg Thieme, Stuttgart, 1996