Phosphine ligands Electronic effects

In methanol, Cl and 7tH+ ions exhibit a retarding effect. The stabilities of nickel(o)-phosphine complexes have been assessed these seem to depend more on the size of the phosphine and electronic effects on bond strengths are of secondary importance. In the oxidative addition of aryl halides to nickel(o)-phosphine complexes the reaction appears to proceed via an initial slow dissociation step to give Ni(PR3)2 which then attacks the organic species. The mechanism of oxidative elimination of these nickel species thus contrasts with that for the platinum(o)-phosphines where the dissociation of the ligand is rapid and the rate-determining step is that involving the redox interaction. The oxidation of tetrahedral cobalt(i) complexes with carbon tetrachloride has been described ... 

More recently, reductive elimination of aryl ethers has been reported from complexes that lack the activating substituent on the palladium-bound aryl group (Equation (55)). These complexes contain sterically hindered phosphine ligands, and these results demonstrate how steric effects of the dative ligand can overcome the electronic constraints of the reaction.112,113 Reductive elimination of oxygen heterocycles upon oxidation of nickel oxametallacycles has also been reported, but yields of the organic product were lower than they were for oxidatively induced reductive eliminations of alkylamines from nickel(II) mentioned above 215-217... 

Figure 8 shows a plot of the CO stretching frequency vs. the trans/cis ratio. A good linear plot is obtained. As one can see, the electronic effect exerted by the phosphine ligands on trans/cis is real. Unfortunately, there is a limit as to how far one can take advantage of this effect to further increase the stereoselectivity. 

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