Chelate effect kinetics

In the case of inert cobalt(m) complexes it is possible to isolate the chelated products of the reaction. Let us return to the hydrolysis of the complex cations [Co(en)2(H2NCH2C02R)Cl]2+ (3.1), which contain a monodentate iV-bonded amino acid ester, that we encountered in Fig. 3-8. The chelate effect would be expected to favour the conversion of this to the chelated didentate AO-bonded ligand. However, the cobalt(iu) centre is kinetically inert and the chloride ligand is non-labile. When silver(i)... 

Hydration enthalpy Stability or formation constant Overall and stepwise stability constants Chelate effect Macrocyclic effect Preorganization Equilibrium template effect Kinetic template effect Self-assembly... 

Biologically active platinum complexes have now been under investigation for nearly two decades. The large data base on structure-activity relationships has revealed a number of principles as well as raised new questions. Mechanistically, the aquation of the compounds and their ability to cause intrastrand cross-links in defined regions of DNA appear to be the chemical events most closely associated with antitumour activity. The reaction kinetics of the compounds in aqueous systems which may be influenced by chelate effects, steric hindrance of bulky ligands or metal oxidation state have been studied for... 

Hence, dissociation of tiron occurs in a concerted fashion when the porphyrin is incorporated into the first coordination sphere. Obviously, if proton transfer from H2P has been established, the series of substitutions occurring at MA are rather fast steps dominated by kinetic chelate effects. 

The chelate effect (Section 10.1.1) causes polydentate complexes to be thermodynamically more stable than their monodentate counterparts. Substitution for a chelated ligand is generally a slower reaction than that for a similar monodentate ligand. Explanations for this effect center on two factors. First, the AH associated with removal of the first bound atom is larger than for a related monodentate ligand. If this atom does separate from the metal center, its kinetic barrier for subsequent reattachment is lower than for a related monodentate ligand since the former remains in close proximity to the metal center. Consider the general scheme below ... 

The first dissociation (1) is expected to be slower than a similar dissociation of ammonia, because the ethylenediamine ligand must bend and rotate to move the free amine away from the metal. The reverse reaction associated with the first dissocation is fast. Indeed, the uncoordinated nitrogen is held near the metal by the rest of the ligand, making reattachment more likely. This kinetic chelate effect dramatically reduces aquation reaction rates. 

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