Of square-planar complexes

In the cis isomer, the two CH3 groups (or the two H atoms) are as close to one another as possible. In the trans isomer, the two identical groups are farther apart. The two forms exist because there is no free rotation about the carbon-to-carbon double bond. The situation is analogous to that with cis-trans isomers of square planar complexes (Chapter 15). In both cases, the difference in geometry is responsible for isomerism the atoms are bonded to each other in the same way. 

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. 

C20-0004. Platinum forms a large collection of square planar complexes. Draw ball-and-stick models similar to those in Example for the cis and trans Isomers of [Pt (NH3)2 CI2 ]. 

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... 

G. K. Anderson and R. J. Cross, Chem. Soc. Revs. 9, 185 (1980) for a comprehensive and incisive review of isomerization mechanisms of square-planar complexes. 

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 Ni—N bond distances, N-N bite distances, and N-M-N bite angles of Ni(II) macrocyclic complexes depend on the coordination number of the metal ion and the type of macrocycle. These structural parameters influence the electronic spectra and the electrochemical data. In general, Ni—N bond distances of square-planar complexes are shorter than those of the octahedral complexes because of the absence of electrons in dx2 . Furthermore, as the Ni—N bond distance in-... 

Spectroscopic data for selected examples of square planar complexes are shown in Table 36. 

The 2-carboxylic acid derivative had normal magnetic behaviour (ju = 1.81 BM at 295 K). A diffuse reflectance spectrum was characteristic of square planar complexes and an IR spectrum showed the presence of covalently bound carboxylate groups. 

The Aig - A2u and Aig - E transitions are allowed, with the Aig Eu transition expected to have considerably greater intensity. Thus a three-band charge transfer spectrum, with an intensity order Ajg - E, > Aig - Aju > Alg - Biu, should he typical of square planar complexes with case 2 type ligands. 

In addition to photosubstitution and photoelimination reactions, in the cases of some Ni(II) complexes, photoexcitation of square-planar complexes Ni(TP) and formation of the photoassociative ligand-field (LF) excited state 3Blg can lead to photoaddition reactions yielding hexacoordinate complexes Ni(TP)L2 [65, 66, 75-77], Such processes differ from the second step of photosubstitutions since an excited complex participates in them and the addition is conditioned by the electronic structure of the complex in its excited state (see Table 3). 

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