Metal-olefin complex, schematic

Metal-olefin complex, schematic, 115 Metal oxidation state, supported metal... 

None of the theories proposed before 1951 to explain the nature of the bonding in metal-olefin complexes was entirely satisfactory (35). Chatt (S3) suggested that, in addition to the ordinary coordinate bond, some sort of bond involving the filled d-orbitals of the metal atom was essential for coordination of the olefin, but such a bond was difficult to formulate until Dewar (64) described it in terms of molecular orbitals. The structure which he proposed for the silver-olefin complexes, and that subsequently proposed for the platinum-olefin complexes by Chatt and Duncanson (35) are shown schematically in structures (I) and (II). The type bond, which has also been called a ji-bond (64, 4), is formed by the overlap of the filled bonding... 

Fig. 13. Schematic representation of a metal-olefin complex showing a -donation and d-jt back-donation.

Metathesis reactions do not, presumably, proceed via the formation of cyclobutane rings as intermediates, since cyclobutane is not reactive under these conditions and no cyclobutane has been found as reaction product. A metal-carbene complex is presumably formed, as shown schematically for the metathesis of acylic olefins ... 

In contrast to the increase in reactivity of hydrocarbons with nucleophiles after coordination to electron-accepting metal centers, an increase in reactivity of unsaturated hydrocarbons with electrophiles is observed upon coordination to particularly electron-ridi metal centers. This contrasting reactivity is shown schematically in Figure 11.3. Olefin complexes of very electron-rich metal centers are best described as metallacyclopropane complexes, as noted in Chapters 1 and 2. As such, the olefin ligands in these complexes contain a large degree of M-C cr-bond character and react with electrophiles. Reactions of electrophiles with coordinated ligands are described in Chapter 12. 

The existence of a Pd° >Mo" dative bond elongates the quadruple Mo2 bond to some extent. This complex has two Pd° centers that can react with a variety of olefinic compounds, shown schematically in Figure 111 such reactions give Mo2Pd2(pyphos)4(L)2 [L = acrylonitrile, fumar-onitrile, tetracyanoethylene] in moderate yields. Unlike the previous compounds which have a strictly trans arrangement of the P-donor atoms, the last compound has a cis arrangement and thus the four metals are not linear.959... 

From the energetics point of view, the epoxidation act should occur more easily (with a lower activation energy) in the coordination sphere of the metal when the cleavage of one bond is simultaneously compensated by the formation of another bond. For example, Gould proposed the following (schematic) mechanism for olefin epoxidation on molybdenum complexes [240] ... 

If a catalytic cycle composed of several elementary processes is promoted on an isolated single site, we could make distinctions about the function of the active sites. For example, some metal complexes which are active for the isomerization reaction of olefins via alkyl intermediates are not effective catalysts for the hydrogenation reaction, and such differences in catalytic ability of the metal complexes is explained by the numbers of coordinatively unsaturated sites which are available for the reactions as described schematically in Scheme 7. 

Figure 3 Schematic representation of the metal catalyzed formation of R- and S enantiomers starting from a non-chiral olefin and using a metal complex containing a chiral bidentate ligand (L-L ) the rate constants kj, kg, refer to the reagent-promoted transf ormation of the diastereoisomer into the enantiomer.

Figure la shows a schematic representation of the Dewar-Chatt-Duncanson (DCD) model. The pivotal idea is that the olefin serves as a donor and an acceptor at the same time. There is ligand metal donation and metal -> ligand back-donation. The former interaction involves a donor orbital of the ligand which has n symmetry in the free ligand but cr symmetry in the complex. The metal acceptor orbital is mainly the d 2 orbital of the metal. Quantum chemical calculations have shown that the valence s orbital of the metal is less important as an acceptor orbital than the d 2 orbital. The metal ligand back-donation takes place via a d( r) orbital of the metal and the n orbital of the olefin. 

Fig. 4. a) Coordination of an olefin (ethylene) to a transition metal (titanium), schematic representation of the relevant orbitals in the x—y plane of an octahedral complex (empty orbitals are shaded) ... 

Many complexes between olefins and transition metals are now known. Some are stable compounds others are thought to exist only as reaction intermediates. In Fig. 20 is a schematic picture... 

Although many transition metal complexes catalyze olefin isomerization, early studies have shown that the presence of silane markedly affects such reactions that occur concurrently with hydrosilylation. Double-bond migration, which is a characteristic feature of most coordination catalytic reactions, can be illustrated schematically in (Scheme 6) as a side reaction occurring during hydrosilylation (3,10,47). 

---