Dewar-Chatt-Duncanson bonding scheme

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.

The description of the bonding of ethene to transition metals is known as the Dewar-Chatt-Duncanson model. This builds on the previous descriptions for CO and CH2 with the exception that we must now consider the frontier orbitals of the ligand to be molecular orbitals delocalized over the two (or more) donor atoms. The basic features are presented for ethene however, the bonding scheme applies in principle to the side-on coordination of any multiple bond to a transition metal. 

The latter fact and the energetically disfavored process via heterolytic bond cleavage of the SiPTe skeleton may rather imply a nonclassical conversion process in which both bonds to the Te atom (Si-Te, P-Te) were simultaneously cleaved and reformed after phosphorus inversion. Remarkably, the dissociation energy for the latter process takes only about 20 kcal mol [25]. This observation strongly supports the description of such small ring compounds as 7t-complexes in terms of the Dewar-Chatt-Duncanson model (see Scheme 5). 

As expected, there is generally a large dependence of the modes for bound H2 on both the metal center and coligands. Vibrational analysis of a M-T 2-H2 system is further complicated by the three-center, two-electron bonding. The bonding is of the Dewar-Chatt-Duncanson type present in metal-olefin complexes, where there is a strong M —> H2 G component, FBD, to the bonding in addition to electron donation, Ed, to the empty metal d-orbital from the H2 electron pair (Scheme 5.18). 

This bonding scheme is completely analogous to the Dewar-Chatt-Duncanson model for the interaction of an ethylene molecule with a metallic centre (Chapter 3, 3.4), the ncc and jr, MO being replaced here by the ohi and MO. 

The lobes of electron density outside the C-O vector thus offer cr-donor lone-pair character. Surprisingly, carbon monoxide does not form particularly stable complexes with BF3 or with main group metals such as potassium or magnesium. Yet transition-metal complexes with carbon monoxide are known by the thousand. In all cases, the CO ligands are bound to the metal through the carbon atom and the complexes are called carbonyls. Furthermore, the metals occur most usually in low formal oxidation states. Dewar, Chatt and Duncanson have described a bonding scheme for the metal - CO interaction that successfully accounts for the formation and properties of these transition-metal carbonyls. 

Fig. 4.6. Chatt-Dewar-Duncanson chemical bonding scheme of chemisorbed ethylene.

The interaction with the 5 a orbital is called donative, because this doubly occupied orbital can only donate electrons to the sohd the interaction with the empty 2 k orbitals is backdonative. It can only accept electrons. This chemical bonding scheme is basic to the Chatt-Duncanson-Dewar chemical bonding view of bonding of CO with a metal atom in carbonyl complestes [26, 27] or the related Blyholder view when applied to surfaces [28]. 

Carbon monoxide and ethylene are common substrates involved in homogeneous catalysis. The bonding of carbon monoxide to a transition metal has been depicted in Fig. 4.4. The bonding of alkenes to transition metals is described by the Chatt-Dewar-Duncanson scheme involving c donation by the filled it orbital of the alkene, and n back donation from the metal into the n orbital of the alkene (see Fig. 4.6). 

The orbital interaction scheme that describes bonding of ligands as CO, but also of other ligands, in terms of the sum of donative and backdonative interactions is called the Chatt-Dewar-Duncanson picture of the chemical bond. Historically it was first used to describe bonding of ethylene. As we will see later, bonding to surfaces (Section 4.4.1.2) is quite analogous and there it is called the Blyholder model. 

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