Square planar complexes substitution

Kinetically labile and inert complexes Dissociation, association and interchange Activation parameters Substitution in square planar complexes Substitution in octahedral complexes Racemization of octahedral complexes Electron-transfer processes... 

The reductions are effected in nature by ferredoxin (p. 1102). This behaviour can be reproduced surprisingly well by simpler, model compounds. Some of the best known of these are obtained by the addition of axial groups to the square-planar complexes of Co with Schiff bases, or substituted glyoximes (giving cobaloximes) as illustrated in Fig. 26.7. The reduced Co species of these, along with vitamin... 

Square planar complexes of palladium(II) and platinum(II) readily undergo ligand substitution reactions. Those of palladium have been studied less but appear to behave similarly to platinum complexes, though around five orders of magnitude faster (ascribable to the relative weakness of the bonds to palladium). 

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

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. 

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... 

The study of rapid, intermolecular ligand exchange between square-planar complexes trans-Ir(CO)L2X (X = C1 or Me, L - PPh3, P(p-tolyl)3, or PMePh2) by variable-temperature 31P NMR spectroscopy indicates that the reaction proceeds through dissociation of phosphine from the metal center and a subsequent associative substitution with other complexes 559,560 Ligand exchange between square-planar Ir and Pt complexes is slow. 

The [Ni(NCS)f,]4 ion is almost perfectly octahedral, with Ni—N distances of around 209.5 pm and N—Ni—N angles around 89.5°. The Ni—N—C and N—C—S entities are practically linear.438,439 In [Ni(NCS)2L2] where L is a R-substituted pyridine, stereochemistry and spin state depend on the type and positions of R.431 While for 2-Me- and 2-Et-pyridine square planar complexes are observed, other pyridins lead to coordination polymers with pseudo-octahedral Ni11 due to N,S-bridging thiocyanate. Ni11 thiocyanato complexes have been studied quite intensively as hosts for inclusion compounds.440"442... 

The majority of square planar complexes are those that contain d8 metal ions, of which the most common examples are Ni2+, Pd2+, and Pt2+, although some complexes containing Au3+ have also been studied. As a general trend, the rate of substitution in these complexes is... 

I FIGURE 20.8 Substitution in a square planar complex in which the solvent participates in a second-order step. 

One of the fascinating characteristics of substitution in square planar complexes is illustrated by the following equations ... 

The second-order redox reaction, giving rise to the rate constant k2, is accompanied also by loss of the iron(II) complex by hydrolysis, which leads to the /tj term. The latter can be more accurately measured in the absence of Tl(III). The kinetics of substitution of many square-planar complexes conform to behavior (c), see Sec. 4.6. It is important to note that an intercept might be accurately defined and conclusive only if low concentrations of B are used. In the base catalyzed conversion... 

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... 

Indirect substitution of the type indicated in (4.1) and (4.2) appears to be the method much preferred by octahedral complexes, while direct substitution is more relevant with square-planar complexes. This situation could perhaps be predicted in view of the more crowded conditions with octahedral than with planar complexes. For other geometries both routes are used. 

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

Fig. 4.9 Simplified reaction profiles for various situations in the associative mechanism for substitution in square planar complexes, focusing attention on the replacement M-X-l-Y —> M-Y + X(4.93).

As with substitution in octahedral complexes, in chelation in square planar complexes the formation of the first bond is usually rate-determining (see (4.101)). 

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