Dewar–Chatt–Duncanson model

The Dewar–Chatt–Duncanson model is a model in <a href="/facts/Organometallic_chemistry/bhqFz2P0">organometallic chemistry</a> that explains the <a href="/facts/Chemical_bond/ArNUMnIB">chemical bonding</a> in <a href="/facts/Transition_metal_alkene_complex/QMNU6cno">transition metal alkene complexes</a>. The model is named after <a href="/facts/Michael_J._S._Dewar/mTUEsYvf">Michael J. S. Dewar</a>, <a href="/facts/Joseph_Chatt/KFGvTSBC">Joseph Chatt</a> and L. A. Duncanson.
The <a href="/facts/Alkene/RtYcS6kU">alkene</a> donates <a href="/facts/Electron_density/L5yDsk6j">electron density</a> into a π-acid metal <a href="/facts/Atomic_orbital/tKhgl6mL">d-orbital</a> from a σ-symmetry <a href="/facts/Bonding_orbital/1cpledcB">bonding orbital</a> between the carbon atoms. The metal donates electrons back from a (different) filled d-orbital into the empty π* <a href="/facts/Antibonding_orbital/9lhLNyt3">antibonding orbital</a>. 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 <a href="/facts/Zeise%27s_salt/DWyXJEh0">Zeise's salt</a> K[<a href="/facts/Platinum/qJC6lQf7">Pt</a>Cl3(C2H4)].H2O the C−C <a href="/facts/Bond_length/rHHIBH6q">bond length</a> has increased to 134 <a href="/facts/Picometre/Cb54mVBX">picometres</a> from 133 pm for <a href="/facts/Ethylene/QMNBYLYn">ethylene</a>. In the <a href="/facts/Nickel/KwMCHYRP">nickel</a> compound Ni(C2H4)(PPh3)2 the value is 143 pm.

The interaction also causes carbon atoms to "rehybridise" from <a href="/facts/Sp2_hybridization/Fl7sz8Jm">sp2</a> towards <a href="/facts/Sp3_bond/Fl7sz8Jm">sp3</a>, which is indicated by the bending of the hydrogen atoms on the ethylene back away from the metal. <a href="/facts/In_silico/g2Cz0ejg">In silico</a> 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 <a href="/facts/Pi_backbonding/un6buomN">π backbonding</a> model.
Similar to 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).
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 spx 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. 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.

Dewar–Chatt–Duncanson model open-in-new

Dewar–Chatt–Duncanson model