Cationic metal-hydrocarbyl species

Catalytic activity in olefin polymerization is related to the presence of cationic metal-hydrocarbyl species [90], which can be obtained by (i) using oxide supports that have high Br0nsted and Lewis acidity, (ii) the addition of a co-catalyst to a neutral supported species or (iii) modification of the surface with Lewis acid cocatalysts prior to grafting of the metal-hydrocarbyl species (Scheme 11.8a-c) [91-97]. 

Bis-phenoxo hydrocarbyl five-coordinated titanium complexes (Scheme 21) have been synthesized and spectroscopically characterized. The molecular structure determined by X-ray diffraction shows the metal to have a distorted trigonal-bipyramidal coordination geometry. Cationic four-coordinate derivatives can be generated by reaction of the methyl derivatives with B(C6F5)3. These cationic species undergo stoichiometric insertions of ethylene and propylene into the Ti-C bond.67... 

Simpler p-halide eliminations occur from late transition metal catalysts for olefin polymerization (Equations 10.25 and 10.26). Reactions of the cationic palladium-alkyl complexes occur in a similar fashion to the reactions of the cationic group 4 complexes, despite the softer nature of these species. In this case, propylene and the metal chloride are formed. Even a neutral nickel-hydrocarbyl complex (the salicaldimine complex in Equation 10.26) undergoes reactions with vinyl chloride that involve insertion followed by P-chloride elimination. 

This result is of considerable significance and suggests that, particularly on supports activated at relatively high temperatures (perhaps >500°C), transfer of a hydrocarbyl group from electropositive transition metal centres to exposed support cations must be considered. Certainly these reactions have their counterparts in solution organometallic chemistry. Ignoring them may lead to erroneous conclusions based upon the sole expectation of protonolysis of M-C bonds as the surface reaction. A further point is the clear distinction between the major species formed on dehydroxylated and partially dehydroxylated surfaces (Fig. 9). 

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