Octahedral complexes kinetic stability

The kinetically-stabilized complexes of the cage ligands normally yield redox reagents free of the exchange problems often associated with simple complexes. Indeed, the redox chemistry of the complexes shows a number of unusual features for example, saturated cages of the type mentioned in Chapter 3 are able to stabilize rare (monomeric) octahedral Rh(n) species (d7 electronic configuration) (Harrowfield etal., 1983). In a further study, radiolytical or electrochemical reduction of the Pt(iv) complexes of particular cages has been demonstrated to yield transient complexes of platinum in the unusual 3+ oxidation state (Boucher et al., 1983). 

Substitution Reactions in Square Planar Complexes 538 Thermodynamic and Kinetic Stability 547 Kinetics of Octahedral Substitution 548 Mechanisms of Redox Reactions 557... 

In summary, some coordination compounds are kinetically inert, whereas others turn out to be labile. Furthermore, this lability seems to be unrelated to the thermodynamic stability of the compound. Now, being a veteran chemistry student trained to ask critical questions, you are about to ask How can we tell which complexes will be inert and which will be labile As you might suspect, this is indeed a crucial question. It turns out that complexes of thefirst-row transition metal ions, with the exception of and Cr , are generally labile, whereas most second- and third-row transition metal ions are inert. But how, you ask, do we explain such a statement Why, for example, should the rates of reactions involving Co " and Cr be different from those involving other first-row transition metal atoms and cations What is it about these particular cations that makes them so inert To start to answer such queries, we now turn to a discussion of some of the most extensively studied reactions of coordination compounds, those involving the substitution of octahedral complexes. 

The d metal ions, such as Pt(II), Pd(II) and Ni(II), often fonai square planar complexes. The square planar complexes of Pt(II) are of particular interest in kinetic studies due to their high stability, ease of synthesis and moderate rates of reaction that enable the monitoring of the reaction. The area of discussion in these complexes is restricted only to the substitution reactions. As compared to the octahedral complexes, the crowding around the metal ion is less in square planar complexes. This is one of the important reasons that most of the substitution reactions in these complexes follow the SN (associative mechanism). 

Octahedral metal complexes of d , Cr(III) and low spin d , Co(III), have particularly high kinetic stability to ligand replacement or exchange. The activation energy for ligand substitution in such complexes has been shown to be considerably greater than in comparable complexes in which the metal possesses other d° configurations. This kinetic stability is associated with the half filled or filled t2g environments, in which the... 

These data provide some qualitative guides to the kinetic behavior of octahedral species. If the change in the d-electron stabilization energy (i.e., the CFAE) is negative for a particular mechanism, the reaction is favored and the complex should be relatively labile—i.e., the substitution process should occur easily. Conversely, if the CFAE is positive, the reaction is disfavored and the complex should be relatively kinetically inert. 

It has already been stated that chromium complexes of tridentate metallizable azo compounds occupy their position as the single most important class of metal complex dyestuffs because of their high stability. It should be noted, however, that in this context the term stability is not used in the thermodynamic sense but relates to the kinetic inertness of the complexes.25 Octahedral chromium(III) complexes have a tP electronic configuration and the ligand field stabilization energy associated with this is high.26 Ligand replacement reactions involve either a dissociative... 

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