The mechanism of ligand replacements

Almost all substitutions of square-planar complexes follow the kinetic law d[ML3X] 

It should be stressed that neither process (a) nor kf in process (b) 

Negative entropies of activation (A S 0), negative activation volumes (A V 0) and the marked dependence of the substitution rate on the nature of entering and leaving ligands support the claim that the square-planar replacements are associative in nature. 

Generally speaking, the square planar substitutions proceed with retention of configuration ci5-complex yields a cw-product and tran -complex yields a trani -product. 

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Nickel(II) is one of the least reactive of the labile metal ions and the most amenable as regards kinetic investigation. Indeed, more than a hundred complex formations have been studied. Although some complexes require fast reaction techniques like temperature-jump relaxation, many systems have been studied adequately by more normal flow methods. The mechanisms of ligand replacement in Ni(II) complexes have been reviewed a number of times, notably by Wilkins [77]. 

Recent reviews have included a discussion by Wilkins of his extensive work on the mechanisms of ligand replacement in octahedral nickel(n) complexes. Hunt has reviewed the water-exchange studies in labile... 

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