Ligand substitution reactions inert octahedral complexes

Octahedral complexes of Cr (d ), and of the low-spin d species Co , Rh , Ii, and Pt, are numerous and well studied. They are kinetically inert, that is, they undergo ligand-substitution reactions relatively slowly, and the bulk of kinetic and mechanistic stndies on transition element complexes have been performed... 

Because of the inertness of Co(III) and Cr(III) complexes, their substitution reactions were the first among those of octahedral complexes to be extensively studied. Most evidence supports the fd mechanism for substitution in Co(fll) complexes. First, there is little dependence of reaction rates on the nature of the incoming ligand, if bond making were of significant importance, the opposite would be expected. Data are presented in Table 13.4 for the anation reaction of penta-ammineaquacobaltdll) ... 

We have already touched on some aspects of inorganic reaction mechanisms kinetically inert metal centres such as Co(III) (Section 21.10) and organometallic reaction types (Section 23.7). Now, we discuss in more detail the mechanisms of ligand substitution and electron-transfer reactions in coordination complexes for the substitution reactions, we confine our attention to square planar and octahedral complexes, for which kinetic data are plentiful. 

Although Ru(III) ammine complexes are known to be very inert low-spin d species which only very slowly undergo substitution reactions, their ability to rapidly and efiectively bind nitric oxide seems to be a rather unusual behavior (92). Common characteristics of the Ru(III) nitrosyl complexes, formally Ru NO, studied to date are their octahedral stereochemistry and the presence of an extremely stable Ru—NO mode (93). A broad array of available kinetic and electrochemical data dealing with the formation of Ru(III) nitrosyls clearly shows that the mechanism of unusual fast coordination of nitric oxide to the Ru(III) ammine center cannot be accounted for in terms of a classical ligand substitution process. In this context, the fundamental kinetics of the fast reactions between [Ru (NH3)5X] pC = Cl, ... 

The overall substitution reaction of a homoleptic complex is represented in Figure 5.1. To discuss mechanisms, however, it is helpful to consider a more specific situation. Take, for example, an octahedral coordination compound containing a metal bound to five inert ligands (L) and one labile ligand (X) that is about to be replaced by an incoming ligand (Y).The overall equation for this reaction is... 

This chapter deals with substitution reactions, including aquation, base hydrolysis, formation, and ligand exchange and replacement, and isomerization of inert metal complexes in which the metal has a co-ordination number of five or more. In fact the great majority of the references reported are concerned with octahedral complexes references to complexes of other coordination numbers have been collected together at the end of this chapter (Section 10). 

In contrast to the facile reduction of aqueous V(III) (—0.26 V versus NHE) [23, 24], coordination of anionic polydentate ligands decreases the reduction potential dramatically. The reduction of the seven-coordinate capped-octahedral [23] [V(EDTA)(H20)] complex = —1.440 V versus Cp2Fe/H20) has been studied extensively [25,26]. The redox reaction shows moderately slow electron-transfer kinetics, but is independent of pH in the range from 5.0 to 9.0, with no follow-up reactions, a feature that reflects the substitutional inertness of both oxidation states. In the presence of nitrate ion, reduction of [V(EDTA) (H20)] results in electrocatalytic regeneration of this V(III) complex. The mechanism was found to consist of two second-order pathways - a major pathway due to oxidation of V(II) by nitrate, and a minor pathway which is second order in nitrate. This mechanism is different from the comproportionation observed during... 

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