Model Chatt

In terms of the Dewar-Chatt model of bonding, for v metal complexation one double bond is effectively removed from the fullerene conjugation system due to extensive interaction between metal d orbitals and the fullerene HOMO and LUMO (7). The remaining 29 double bonds then behave almost identically to uncomplexed C60 with their IR, Raman, UV-vis, and 13C NMR spectra showing only slight perturbations, mainly as a result of diminution of symmetry effects. Nevertheless, it is important to state that the fullerene metal interaction is not confined purely to the former s HOMO and LUMO, and that other molecular orbitals are energetically suitable for interaction 89,90). The spectroscopic evidence cited for the preceding statement is as follows ... 

The strong forward donation-back donation of electrons (i.e., the Chatt model) between alkynes and ruthenium makes such a bond a very good ligand for Ru. Hence it is not surprising that reactions involving ruthenacyclopentadienes as intermediates, notably in the trimerization of alkynes to benzenes, occur readily. Intercepting the ruthenacyclopentadiene prior to its reaction with an additional alkyne, however, is rather rare. A unique juxtaposition of functionality occurs when a propargyl alcohol is the alkyne partner which allows such a diversion as shown in Scheme 1.3. 

The requirements for selectivity toward a particular product isomer in the dimerization of propylene have been investigated by using a square-planar nickel(II) complex with a chelating Schiff base and similar ligands and an alkyl-aluminum in the presence of a phosphinous atom containing additives [620]. Steric and electronic effects are separated. The observed electronic preference for the anti-Markovnicoff mode of reaction is discussed on the basis of Chatt model of olefin coordination, taking into account the assymmetrical nature of the propylene TT and IT molecular orbitals. 

Figure 4.1.30. Dewar-Chatt model of it-complexation (Safarik and Eldridge, 1998). Reprinted, with permission, from Ind. Eng. Chem. Res., 37(7), 2572-2581(1998), Figure 1. Copyri t (1998) American Chemical Society.

Dissociative substitution mechanism (Section 4.4) Dewar-Chatt model of M(C=C) bonding involving weak back donation (Section 5.1) Electron configuration (Section 1.4) a-Acceptor and ir-donor metal orbitals (see Section 1.4)... 

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

The chemistry of the disilenes (disilaethenes) has developed very rapidly since the discovery of stable compounds. It was an obvious challenge to explore also the possibility of a n-coordination of disilenes to transition metals. According to the Dewar-Chatt-Duncason bonding model, a high stability for a disilene complex should result. 

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.

A series of model studies initiated in the early 1960s by the groups of Chatt and Hidai demonstrated that dinitrogen could be bound and reduced to ammonia at a single metal centre by Mo and W complexes (Chatt et al., 1978 Hidai, 1999). However, although examples of virtually all the proposed intermediates in a Chatt cycle were isolated, no catalytic reduction of N2 to NH3 was ever achieved. Catalytic reduction of dinitrogen to ammonia... 

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