Substitution at Square-Planar Complexes

A systematic study of the effects of solvent on the leaving group in substitution reactions at palladium(II) has been carried out. Rate and equilibrium constants were reported for reactions (1) and (2) in methanol. 

Rate and equilibrium constants for reactions (4) (L is tertiary phosphine or arsine am is a heterocyclic nitrogen base) in methanol have been 

The kinetics of reaction (5), where RR SO represents a set of 11 sulfoxides, allowed a comparison to be made of the effects of the substituent 

Kinetics of cyanide exchange at [M(CN)4] (M = Ni, Pd, Pt, and Au) have been followed by NMR spectroscopy, using enriched samples in D2O solvent. Table 5.1 gives the rate constants and activation parameters. Only the second-order dependence could be detected, leading to a simple rate law in equation (6). The exceptionally high value for the nickel complex (too high to be measured by NMR spectroscopy) was assigned to the known stability of the five-coordinate species [Ni(CN)5f which may be similar to the intermediate or transition state for this associative process. 

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Figs. 11 and 12 show typical mo diagrams for square planar and octahedral complexes. Inspection reveals that the metal orbital (z is the axial direction) in a square planar complex is involved in the n bonding system and available for a bonding in the transition state. This is a feature shared by nucleophilic substitution at square planar complexes with the spectacularly associative nucleophilic aromatic substitutions. The octahedral complexes discussed in this chapter... 

Gold(m).—There has for some time been discussion as to whether the A 2[Nu] term in the two-term rate law characteristic of substitution at square-planar complexes,... 

Rates of substitution at square-planar complexes are usually sensitive to variation of the nature of the solvent. Both the ki and values for the normal rate law for substitution at such complexes. 

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... 

Nickel(ii).—A number of variables were studied for substitution at square-planar bis-dithiolatonickel(ii) complexes. These variables included the reactivity of the incoming nucleophile, leaving-group effects, the trans effect, and the denticity of the incoming group. The rate law here is simple... 

Pearson and Sweigart have reported one of the first kinetic studies involving substitution at square-planar nickel(n), in Ni"-dithiolate complexes. They discuss nucleophilic reactivity, leaving group, and trans effects, and report that the reactions follow an associative pathway, as expected. The solvent path is not important and, although for Pt" the formation of a five-co-ordinate intermediate may be rate-determining, for Ni" this step is fast compared with subsequent steps. The formation constant of the five-co-ordinate intermediate does, however, markedly affect the overall rates, and it seems that the trans effect for nickel(ii) operates through the stability of this intermediate. 

PIatinum(ii).—General. Activation volumes are often useful guides to the diagnosis of mechanism, frequently permitting distinction between associative and dissociative mechanisms and sometimes permitting more subtle distinctions, as between /a and/) or 7a and A alternatives. Reactions of a series of complexes [Pt(dien)X]+ (X = Cl, Br, I, or N3) with a range of incoming nucleophiles Nu (Nu = OH, I", Ng-, NO2 , SCN, or py) follow the usual rate law (1) characteristic of substitution at square-planar species. Activation volumes have been determined, in aqueous solution, both... 

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. 

Table 4.12 Activation Parameters for Substitution in Some Square-Planar Complexes in Water at 25 °C Refs. 149-152.

Because substitution chemistry at square-planar palladium is dominated by associative processes [48], coordination of the alkene in 22.2 would undoubtedly initially generate penta-coordinate intermediate 22.6. Complex 22.6 could then either evolve to square-planar complex 22.5 by a series of pseudorotations and eventual expulsion of the halide ligand or undergo... 

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