Substitution reactions of square planar

Solvent paths and dissociate intermediates in substitution reactions of square planar complexes. R. J. Mureinik, Coord. Chem. Rev., 1978, 25,1-30 (133). 

Application of the principle of microscopic reversibility can be used to eliminate a mechanism suggested at one time for the nucleophilic substitution reactions of square-planar platinum(II) complexes. For the sake of specificity, we take PtCl - as a typical... 

As already mentioned, complexes of chromium(iii), cobalt(iii), rhodium(iii) and iridium(iii) are particularly inert, with substitution reactions often taking many hours or days under relatively forcing conditions. The majority of kinetic studies on the reactions of transition-metal complexes have been performed on complexes of these metal ions. This is for two reasons. Firstly, the rates of reactions are comparable to those in organic chemistry, and the techniques which have been developed for the investigation of such reactions are readily available and appropriate. The time scales of minutes to days are compatible with relatively slow spectroscopic techniques. The second reason is associated with the kinetic inertness of the products. If the products are non-labile, valuable stereochemical information about the course of the substitution reaction may be obtained. Much is known about the stereochemistry of ligand substitution reactions of cobalt(iii) complexes, from which certain inferences about the nature of the intermediates or transition states involved may be drawn. This is also the case for substitution reactions of square-planar complexes of platinum(ii), where study has led to the development of rules to predict the stereochemical course of reactions at this centre. 

Rate constants for the substitution reactions of square-planar dithio-phosphates and dithiocarbonate complexes of Ni(II), Pd(II), and Pt(II), with ethylenediamine and cyanide ion as nucleophiles, have been measured in methanol. The results were compared with those obtained in previous investigations, and interpreted in terms of the stabilities of 5-coordinate species that are formed prior to substitution (377). 

The kinetics and mechanism of ligand substitution reactions of square-planar platinum(II) dimethyl sulfoxide complexes have been exhaustively studied (173), and these workers conclude that the cis and trans influences and the trans effects of Me2SO and ethylene are similar in magnitude whereas the cis effect of Me2SO is about 100 times as large as that of ethylene. The results for reaction (5), where the stability constants, Kt, are reported to be 1.5 x 108 (L = S-Me2SO) and 4.5 x 108 (L = ethylene) corroborate this analogy (213). 

Cquare planar complexes are generally of the low-spin d8 type. This includes the four-coordinated complexes of Ni (II), Pd(II), Pt(II), Au(III), Rh(I) and Ir(I). The best known and most extensively studied are the compounds of Pt(II). The kinetics and mechanisms of substitution reactions of these systems have been investigated in considerable detail. Studies on complexes of the other metal ions are rather limited, but the results obtained suggest that their reaction mechanism is similar to that of the Pt(II) systems. This paper briefly surveys some of the available information, and presents the current view on the mechanism of substitution reactions of square planar complexes. 

Figure 2. Bimolecular displacement mechanism for substitution reactions of square planar complexes. ka is the rate constant for the solvent path and ky is the rate constant for the direct reagent path.

Reagent Reactivity. One of the most interesting aspects of substitution reactions of square planar complexes is that the reaction rates depend on the nature of the reagent. This permits a thorough investigation of the factors responsible for reagent reactivity towards these substrates. Note that this has not been possible for the reactions of most six-coordinated metal complexes, since their rates do not depend on the reagent. 

One of the earliest studies of the kinetics of substitution reactions of square planar complexes is that of the Cl exchange of [AuClJ- (22). A two-term rate law was found for the exchange rate and it was suggested that this may prove to be general behavior for square complexes. 

The significant changes imposed on the dithioaromatic ligands and complexes upon sulfur addition are illustrated in the structure of the Ni(p-/-PrPhDtaXp-(-PrPhDtaS) complex (Fig. 48) (Table XXII), determined by Fackler et al. (233, 257). The same workers explored the sulfur addition and abstraction reaction in depth (232) (see also Section IV). The rates and mechanisms of substitution reactions of square planar nickel(II) 1,1 -dithiolate complexes (502) is discussed in Section IV. 

Substitution at Square Planar Pt(II) and Pd(II) Studies on the substitution reactions of square planar Pt(II) and Pd(II) complexes are abundant. Part of the reason for this was the finding that [czj-Pt(NH3)2Cl2], cisplatin, possesses antitumor... 

The evidence cited previously shows clearly that there is a trans influence in square planar complexes and that the magnitude depends on the nature of the ligand. In fact, the trans directing influence is a factor of major importance in the structures and substitution reactions of square planar complexes. It is now necessary to provide an explanation of how this phenomenon is caused and how it is related to the nature of the ligands. 

The substitution reactions of square-planar complexes of the type [M(terpy)X] + ... 

Ligand substitution reactions of square-planar complexes most often occur by associative reaction sequences. An example of square-planar organometallic complexes that will illustrate this reactivity is trani -Ir(Cl)(CO)(PPh3)2 (frequently referred to as Vaska s Complex), see Vaska s Complex). This complex undergoes rapid ligand substitution with CO, PR3, and... 

Much of what is currently known about substitution reactions of square planar complexes came from a lar e number of careful studies executed in the I960s and I970S.3 You should not conclude, however, that details of the mechanisms of these I eactions are of historical intei est only. Work in this area continues unabated as studies focus on chelation, steric effects, biological i eactions. and homogeneous catalysts. For example, the mechanism for the Wacker process (Chapter 15), which utilizes squai e planar [PdCl ] as a homogeneous catalyst for the industrial conversion of ethylene to acetaldehyde, is still a subject of investigation. The overall reaction for the process is ... 

There is now much kinetic data on substitution reactions of square-planar complexes all of which are explained in terms of a bimolecular (8 2) displacement mechanism. For reactions such as... 

This requires that a plot of versus [Y] be linear with an intercept of ky for the reagent-independent path and a slope of /c2 for the reagent path. Plots of this type are common for substitution reactions of square-planar complexes. 8uch a plot is shown in Figure 2 for the reaction trans-[PtCl2(py)2] (py = pyridine) with a variety of different reagents. 

Figure 1 General Sf 2 mechanisms of ligand substitution reactions of square-planar metal complexes, such as platinum n) compounds, where S is solvent and Y is entering nucleophile...

Since substitution reactions of square-planar complexes in most cases tend to follow an associative mechanism, a large number of complexes were synthesized in efforts to determine whether a changeover in mechanism is induced. The increase in steric hindrance caused by introducing alkyl substituents on the three N donor atoms of diethylenetriamine (dien) in going from dien to Mesdien to Etsdien, caused a decrease in the aquation rate constant for [Pd(Rsdien)Cl]+ of six orders of magnitude, but not a changeover in mechanism. The reported activation... 

Substitution Reactions of Square-Planar Complexes 459 TABLE 12.13 Rate Constants for Leaving Groups... 

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