Of square-planar substitutions

An Appraisal of Square-Planar Substitution Reactions R. J. Cross... 

The kinetics of square planar substitution reactions are complex, as shown in Figure 17.6. The experimentally determined rate law has two terms, as shown in Equation (17.39). 

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 ... 

There is also evidence supporting a as effect on the rate of square planar substitution reactions, although it is not quite as dramatic. However, steric factors can be very important in the c/s position. Steric congestion enhances the rate of a dissociative substitution reaction and slows the rate of an associative one. Because the bulky ligands are closer to each other in the c/s position than in the trans position. 

The substitution reactions (23), where the chelate ligands are cyclo-octa-1,5-diene (cod), ethylenediamine (en), or iV,iV -dimethylethylenediamine, offer two further illustrative examples of square-planar substitution reactions, where reversible solvolysis of the substrate complex has to be considered. In these cases also, the observed rate constants depend on the concentration of the leaving ligand (X=C1 ), so the more complete rate law (2) applies. 

Transition metal square-planar complexes generally contain eight d electrons and are almost always diamagnetic. This includes complexes of Pt, Pd % Au, Rh, and Ir. While such complexes can imdergo other reactions such as redox processes, we shall focus on substitution reactions. Good reviews of square-planar substitution reactions are available. The following is a summary of some of these substitution processes, wifli emphasis on those involved with polymer formation. These substitution reactions are the most widely studied of the transition metal square-planar complex reactions. 

The effect of the central metal ion on the rates of square-planar substitutions can be very strong. Hence, the tendency to undergo substitution is in the same order as the tendency to form pentacoordinate complexes Ni > Pd" Pt". Thus, the substitution rates of chlorine by pyridine in EtOH depend on the metal, as follows ... 

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