Chatt-Dewar-Duncanson

Features (2) and (3) are explicable in terms of the Dewar-Chatt-Duncanson model for bonding in alkene complexes (Figure 3.63), which involves... 

In low oxidation states, transition metals possess filled or partly filled d shells. The Dewar-Chatt-Duncanson model envisages some of that electron density in (local) d (e.g. d., d y) orbitals being donated into the empty n orbitals of the carbon monoxide ... 

Surprisingly, in contrast to the reaction of the Si=P bond with mesityl azide, the reaction of 15a with diphenyldiazomethane resulted in the formation of the [2+l]-cycloadduct 35 (Scheme S).38 The bonding situation in 35 (Fig. 11) may be described in terms of a 7r-complex, by employing the Dewar-Chatt-Duncanson model, in which the Si=P bond acts as v donor and acceptor at the same time (Scheme 9). The corresponding [2+31-cycloaddition product 36 was generated only on thermal activation of 35. 

Scheme 9. Description of 35 as 77-complex (Dewar-Chatt-Duncanson model) (a) Si=P bond as ir-donor (b) Si=P bond as 77-acceptor.

Fig. 3. Schematic bonding pictures for transition metal atoms (M) binding to C2H4 via the Dewar-Chatt-Duncanson model.

C) Synergistic H2 coordination. When the metal has both donor and acceptor orbitals, interaction with the cthh and ohh orbitals of H2 can act synergistically (cooperatively) in the Dewar-Chatt-Duncanson mode. Such self-reinforcing charge flow enables a significantly stronger interaction than does simple dative coordination of H2. 

The bonding between these two fragments can be understood using the Dewar-Chatt-Duncanson model of donation and backdonation [79, 80], The frontier orbitals responsible for these interactions between 15 and 16-R are drawn to scale in Figure 15. 

In accord with the Dewar-Chatt-Duncanson model, we find that the dominant interaction is donation from the C C n bond into the rhodium LUMO. This interaction is enhanced when the double bond lies in the rhodium-diphosphine plane and with electron donating substituents which raise the energy of 7ito more closey match the LUMO Charge Decomposition Analysis (CDA) [81] shows that the amount of donation is... 

B. Dewar-Chatt-Duncanson Scheme - A Simple Model for Electron-Deficient Three-... 

The Dewar-Chatt-Duncanson model of the binding of an olefin in a transition metal complex involves two types of interactions. Transfer of electron density from the relatively high-lying olefinic ic-orbital to the metal (cf. 20) represents a Lewis acid Lewis base interaction (a-bonding). A metal-olefin jr-bond due to interaction... 

The course of modern organometallic chemistry has been greatly influenced by three simple generalizations the Dewar-Chatt-Duncanson synergic bonding model for metal-olefin complexes (40, 72) Pauling s electroneutrality principle (174), and the 18-electron or inert gas rule (202). In this section the impact of recent theoretical calculations on these important generalizations will be evaluated. 

The electronic structure of the surface chemical bond is discussed in depth in the present chapter for a number of example systems taken from the five categories of bonding types (i) atomic radical, (ii) diatomics with unsaturated -systems (Blyholder model), (iii) unsaturated hydrocarbons (Dewar-Chatt-Duncanson model), (iv) lone pair interactions, and (v) saturated hydrocarbons (physisorption). 

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