Square-planar substitution reactions group

As Chemyaev articulated in his 1926 paper, the rates of square planar substitution reactions strongly depend on the ligand that is trans to the leaving group. The observed trans effect series is as follows ... 

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. 

We believe that this intermediate is a five coordinated nickel complex. This is in agreement with the general finding that substitution reactions of ds square planar complexes take place via a five-coordinated intermediate (24, 25). During the formation of the nitrile there is a reductive elimination in which the oxidation state of nickel is reduced to zero. This process is obviously favored by electron-releasing substituents on the aryl group. This is exactly what has been observed. 

Since H20 is known to be a good leaving group in nucleophilic substitutions at square planar platinum(II)49, the final step in reaction (37) would be expected to be very rapid with MeOH as the leaving group. 

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