Alkene complexes bonding

Palladium-carbon Ti-bonded alkene complexes characteristics, 8, 328... 

The bonding in monometal alkyne complexes is usually interpreted in terms of the Dewar-Chatt-Duncanson model (293), since the alkyne molecule has a pair of n and n molecular orbitals which lie in the plane of the metal and the two carbon atoms. These two orbitals are denoted n and n, and are analogous to those in jr-bonded alkene complexes (394). There is also a pair of n and n molecular orbitals which lie perpendicular to the metal-carbon plane, denoted nL and n . These orbitals are illustrated in Fig. 14. Both sets of n and n orbitals have the correct symmetry to interact with metal d orbitals. The interaction... 

The alkene adsorbs to the catalyst surface, using its 7r bond to complex to the metal atoms. 

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

Alkene complexes Alkynyl complexes Ammine complexes Aqueous chemistry Arsine complexes Auranofin Auride ion Aurophilicity Binary compounds Bond lengths acetylacetonate complex alkyls and aryls ammine complexes carboxylates cyanide complexes dialkyl sulphide complexes dithiocarbamates to gold... 

Alkene complexes Ammine complexes Aqueous chemistry Arsine complexes Binary compounds Bipyridyl complexes Bond lengths acetylacetonate alkene complexes alkyl and aryl complexes ammine complexes aqua ion... 

Figure 6-13. Synergic back-bonding in a platinum alkene complex. In (a), the interaction of a (filled) platinum 5d orbital with the tf molecular orbital of the alkene is shown, whilst in (b), the interaction of a dsp hybrid orbital with the n molecular orbital of the alkene is shown. Note that the two interactions result in electron density moving in opposite directions.

Carbonylation reactions have been observed using both Pd(II)-alkene complexes and CT-bonded Pd(II) species formed by oxidative addition. Under reductive conditions, the double bond can be hydrocarbonylated, resulting in the formation of a carboxylic acid or ester.238 In nucleophilic solvents, the intermediate formed by solvopalladation is intercepted by carbonylation and addition of nucleophilic solvent. In both types of reactions, regioisomeric products are possible. 

Note that although it may be possible for metal atoms to form a cr-complex with an alkane prior to insertion into a C-H bond, such complexes would be bound by only a few kcal/mol (comparable to dispersion forces), while metal-alkene 7r-complexes are typically bound by 20-60 kcal/mol. 

Four kinds of adsorbed species are postulated to result from the adsorption of alkenes on noble metals, Jt-com plcxcd double bonds, Jt-complexed jt-allyl species, mono-O-bonded alkanes, and di-o-bonded alkanes. Distinguishing among these is difficult and has been the subject of much speculation. For example, alkenes may adsorb either by Jt-complexing or by di-C-adsorption, and 7t-allyl species are thought to occur readily on Pd but not so readily on Pt.55... 

As a typical case, olefin-metal complexation is described first. Alkene complexes of d° transition metals or ions have no d-electron available for the 7i-back donation, and thus their metal-alkene bonding is too weak for them to be isolated and characterized. One exception is CpfYCH2CH2C(CH3)2CH=CH2 (1), in which an intramolecular bonding interaction between a terminal olefinic moiety and a metal center is observed. However, this complex is thermally unstable above — 50 °C [11]. The MO calculation proves the presence of the weak metal-alkene bonding during the propagation step of the olefin polymerization [12,13]. 

As shown in Table 1, a remarkable variety of alkene complexes bearing metal centers in a low oxidation state have been isolated and their structures have been determined by X-ray analysis. All the C-C bond distances in olefins coordinated to early transition metals at low oxidation states are more or less elongated compared to free ethylene. These structural data, together with those from NMR studies [14], indicate a major contribution of the metallacyclo-propane structure (2), a fact which is also supported by calculation studies [15]. In the case of ethylene bridging two metal centers such as [ Cp2ZrX 2(iu-f/-C2H4)] (3), the M-C bond could be characterized as a er-bond and there is a little contribution from the / -ethylene canonical structure [16-18]. 

The 7r-back donation stabilizes the alkene-metal 7c-bonding and therefore this is the reason why alkene complexes of the low-valent early transition metals so far isolated did not catalyze any polymerization. Some of them catalyze the oligomerization of olefins via metallocyclic mechanism [25,30,37-39]. For example, a zirconium-alkyl complex, CpZrn(CH2CH3)(7/4-butadiene)(dmpe) (dmpe = l,2-bis(dimethylphosphino)ethane) (24), catalyzed the selective dimerization of ethylene to 1-butene (Scheme I) [37, 38]. 

In this case, the bond between the carbon atoms is about the same as it is for a C-C single bond. Moreover, unlike the anion of Zeise s salt, the carbon atoms are in the plane formed by platinum and the other ligands. Clearly, this represents a significant difference from the usual alkene complexes. In essence, a three-membered C-P-C ring is formed. It appears in this case that the ability of the... 

The rearrangement of platinacyclobutanes to alkene complexes or ylide complexes is shown to involve an initial 1,3-hydride shift (a-elimina-tion), which may be preceded by skeletal isomerization. This isomerization can be used as a model for the bond shift mechanism of isomerization of alkanes by platinum metal, while the a-elimination also suggests a possible new mechanism for alkene polymerisation. New platinacyclobutanes with -CH2 0SC>2Me substituents undergo solvolysis with ring expansion to platinacyclopentane derivatives, the first examples of metallacyclobutane to metallacyclopentane ring expansion. The mechanism, which may also involve preliminary skeletal isomerization, has been elucidated by use of isotopic labelling and kinetic studies. 

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