Square-planar substitution reactions nucleophilic ligand

For an associative mechanism, one would expect that the rate of substitution would show a strong dependence on the nucleophile, as the nucleophile directly participates in the RDS. This is indeed the case for square planar substitution reactions, as illustrated by the data in Table 17.12, which show that the rates of reaction vary by five orders of magnitude with the nature of the incoming ligand Y. 

There are several pathways by which one ligand may replace another in a square planar complex, including nucleophilic substitution, electrophilic substitution, and oxidative addition followed by reductive elimination. The first two of these are probably familiar from courses in organic chemistry. Oxidative addition and reductive elimination reactions will be covered in detail in Chapter 15. All three of these classes have been effectively illustrated by Cross for reactions of PtMeCItPMe-Ph),.-... 

A completely empirical LFER can also be constructed with recourse only to kinetic data. This has been the case in the setting up of a scale of nucleophilic power for ligands substituting in square-planar complexes based on the Swain-Scott approach. The second-order rate constants Ay for reactions in MeOH of nucleophiles Y with tra 5-Pt(py)2Cl2, chosen as the standard substrate... 

This type of map can be used to discuss the different types of nucleophilic displacement reaction. Using the simplified version shown in Fig. 2 we have already seen that SN1 reactions, for instance the solvolysis of triarylmethyl halides, go through the separated ions in the top right-hand corner (Swain et al., 1953 Ritchie, 1971). At the opposite extreme, nucleophilic substitution at centres where the number of ligands can be increased may proceed over the bottom left-hand corner of the diagram. Examples are acyl transfer reactions and substitution at tetrahedral phosphorus centres (Alder et al., 1971) as well as substitution at square planar transition metal compounds (Wilkins, 1974). The nucleophilic reactions studied by Ritchie (1976), for which the rate... 

Another impetus to mechanistic studies arose from the recognition that compounds of these d ions were those on the energy borderline between stable 18-electron and 16-electron molecules (1) and that the reactions involving transitions between these states are those encountered in catalytic cycles based on these compounds. Nucleophilic ligand substitution, involving association of an entering nucleophile with a square-planar compound, is just one example of the easy 16- 18- 16... 

A main objective of this work is to develop the relationship between the many reaction pathways leading to ligand substitution at square-planar molecules. Concentrating on more recent results to illustrate the processes under discussion, we examine in detail the evidence for operation of the less common and sometimes controversial routes such as dissociative ligand exchange (6). It cannot be stressed too much, however, that the field is still dominated by associative reactions, so to maintain a balance, as well as to provide the now necessary comparative evidence, we also cover the essential features of nucleophilic ligand replacements. 

As stated by Chatt and co-workers, their early speculation on the role of trans-n bonding groups in ligand substitution of platinum(ii) complexes was based on the assumption that the reactions proceed by an 8 2 mechanism. However, at the time (1955) most of the observations reported on such reactions were qualitative and little had been done to use detailed kinetic studies in attempts to elucidate the mechanism of ligand substitution. Since the valence bond theory in use then assigned dsp hybridisation to the square-planar plati-num(ii) complexes, coordination chemists believed an entering nucleophile would readily attack the low energy vacant p orbital on the metal and substitution would take place by an 8 2 mechanism. Furthermore, a coordination... 

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...

The reader is referred elsewhere for discussions of substitution reactions with macrocyclic ligands (2-6), and in micelles (36) like the effects of bound ligands, these reactions are of importance in reactions of biological interest. Complex-formation reactions involving a change of covalency, such as reactions of square-planar nickel (ll) complexes with nucleophiles, are also omitted (37). All these reactions offer interesting applications of fast-reaction methods. 

Several other recent reviews contain material relevant to this section. An article by Blandamer and Burgess on the thermodynamics, kinetics, and mechanisms of solvation, solvolysis, and substitution in nonaqueous solvents contains a contribution on the controversial dissociative mechanism for isomerization of square-planar molecules. This is outlined in Section 5.5. A review of ligand substitution reactions at low-valency transition-metal centers contains sections on five-coordinate metal carbonyl complexes and on ML4 complexes (mainly tetrahedral configurations with L being a tertiary phosphine), as well as on acid- and base-catalyzed reactions. A review by Constable " surveying the reactions of nucleophiles with complexes of chelating heterocyclic imines contains a sizable section on square-planar palladium and platinum derivatives. Most discussion centers on [Pt(bipy)2] and [Pt(phen)2] (bipy = 2,2 -bipyridine phen = 1,10-phenanthroline). The metal center, ligand, or both are susceptible to nucleophilic attack and the mechanisms involved are critically assessed. 

Prompted by earlier results which indicated that the rate law for substitution of Cl in square-planar rran -[Pt(PEt3)2(R)Cl] (R = phenyl, jp-tolyl, or mesityl) complexes included an associative as well as the normal dissociative path only in the case of substitution by strong biphilic ligands (e.g. CN, SeCN ), Ricevuto et al. have re-examined the reaction with weakly nucleophilic pyridine in methanol ... 

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