Leaving group effect substitution reactions

Unfortunately, there is a lack of data on leaving group effects in mechanism SE2(co-ord), but the analysis on p. 161 indicates that leaving groups of the type SnR3 accelerate reaction in the sense SnMe3 < Sn( -alkyl)3 < SnPr 3. Thus for alkyls of type RMX (and R M), the sequence of reactivity in substitution by mechanism SE2(co-ord) should approach that of sequence (4). 

Lanthanide shift reagents silver complexes, 806 Leaving group effect platinum complexes substitution reactions, 494 Leucine aminopeptidase zinc, 1005 Ligases zinc, 1002... 

Leaving group effects on the ratio of C- to O-alkylation can be correlated by reference to the hard-soft-acid-base (HSAB) rationale. Of the two nucleophilic sites in an enolate ion, oxygen is harder than carbon. Nucleophilic substitution reactions of the Sn2 type proceed best when the nucleophile and leaving group are either both hard or both soft. Consequently, ethyl iodide, with the very soft leaving... 

While the trans effect is the major factor governing square-planar substitution reactions other factors contribute. These factors are the electronic effect of cis ligands (generally relatively unimportant), leaving-group effects, effect of the entering ligand, and solvent effects. 

Another example of thiol chck reaction is the rapid and efficient substitution of leaving group bearing substrates by thiols, a soft nucleophile [58]. These reactions proceed better in the presence of mild organic bases such as trialkylamines. The halide salts formed during this displacement reactions can be removed easily as precipitates in a very simple and effective manner. 

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