The Kinetics of Replacement Involving Unidentate Ligands

Most studies have again been concerned with replacment of one unidentate ligand by another, and the rules and patterns of behavior that have evolved are based mainly on this simple type of substitution reaction. Consider the substitution scheme 

The rate law governing substitution in planar complexes usually consists of two terms, one first-order in the metal complex (M) alone and the other first-order in both M and the entering ligand Y  

The experiments are invariably carried out using excess Y and therefore with pseudo first-order conditions. The experimental first-order rate constant k is given by 

A plot of k vs [Y] will have an intercept A , and a slope ki- An example is shown in Fig. 4.8. For different nucleophiles reacting with the same complex, the value of A , is the same, whereas the value of Atj usually will be different. Often, k At2[Y] and it is then difficult to measure Atj accurately. Care has to be taken that a positive intercept in a plot of the type shown in Fig. 4.8 is not mistakenly assigned to A , in (4.96), when it may in fact represent the reverse rate constant of (4.93) (Sec. 1.5). 

As with solvolysis reactions of octahedral complexes, the rate-determining step may be solvolytic or dissociative in any case, it is independent of the concentration of Y  

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