Structure-Function Relationships for the Ligands

A Hammett plot for the oxidative addition of CH3I (closed circles) and benzyl chloride (open circles) to Vaska s complex [traus-IrCl(CO)(p-X-PPh3)2]. Ugo, R., Pasini, A., Fusi, A., and Cenini, S. A Kinetic Investigation of Some Electronic and Steric Factors in Oxidative Addition Reactions to Vaska s Complex.  

Aliphatic R groups are not the only ones that undergo oxidative addition. Vinyl and phenyl groups work also Eqs. 12.25 and 12.26 give examples. Sn2 mechanisms cannot be operative, because nucleophilic substitution does not occur on sp hybridized carbons. With alkenes, the stereochemistry of the double bond is almost always retained. This rules out radical intermediates because a vinyl radical would undergo rapid inversion, thereby scrambling the stereochemistry of the alkene in the product. 

Therefore, the mechanism must be concerted. However, in contrast to the concerted oxidative addition of H2, the addition of vinyl and phenyl C-X bonds occurs more rapidly with complexes than with d complexes. The reactions are still second order, and the normal order for leaving group reactivity is foimd. 

The best way to view the mechanism of these reactions is to postulate prior formation of a metal-olefin tt complex, and to view the olefin complex with the metal as a metallacy-clopropane. From this vantage point, it is a simple matter of electron pushing to expel the leaving group and retain the stereochemistry (see the examples in the margin). 

Before analyzing the next organometallic reaction type, it should be noted that oxidative addition is the most common manner that a bonds are cleaved in organic compounds, lead- 

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