Ligand Substitution on Labile Transition-Metal Ions

8 Ligand Substitution on Labile Transition-Metal Ions 

The common complexes of the M(II) and M(III) transition-metal ions of the first transition series are labile, except for Cr H) and low-spin Co(III), Fe(II) and Fe(III). These labile ions form a wide range of complexes of general chemical and biochemical importance. As a result, there have been many studies of the kinetics and equilibrium constants for reactions of the general form 

The majority of this work is in water, but there are an increasing number of studies in nonaqueous solvents. The results have been the subject of several reviews.  

The reactions of Ni(II) are slower than those of the other M(II) ions in this group, and this metal ion has been studied most extensively because the rates fall conveniently in the range of the stopped-flow technique. The substitution kinetic results on Ni(OHj)5 have been reviewed by Wilkins. Some representative results are given in Table 3.20. The first impression of these data is that there are significant entering group effects on the rate constant, so that the mechanism appears to be I,. However, 

The rate law for this system with a rapid pre-equilibrium has been developed previously. These studies are usually carri out with total [M] [L], in order to prevent formation of higher complexes such as M(0H2)4(L)2, therefore, the experimental pseudo-fust-order rate constant is given by 

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