Polydentate ligands thermodynamic stability

Under these conditions, the formation rate constant, k, can be estimated from the product of the outer sphere stability constant, Kos, and the water loss rate constant, h2o, (equation (28) Table 2). The outer sphere stability constant can be estimated from the free energy of electrostatic interaction between M(H20)q+ and L and the ionic strength of the medium [5,164,172,173]. Consequently, Kos does not depend on the chemical nature of the ligand. A similar mechanism will also apply to a coordination complex with polydentate ligands, if the rate-limiting step is the formation of the first metal-ligand bond [5]. Values for the dissociation rate constants, k, are usually estimated from the thermodynamic equilibrium constant, using calculated values of kf ... 

The reaction control should be emphasized amongst the conditions of reactions of competitive complex formation [19,23], It is necessary to take into account that it is possible to determine, and frequently predict, the direction of the electrophilic attack to the donor center of di- and polyfunctional donors (ligands) only in the case when the thermodynamically stable products are formed under conditions of kinetic control. Thus, the thermodynamic stability of complexes is discussed, when the bond between the metal and di- and polydentate ligands is localized in the place of primary attack on one of any of the donor centers by the electrophilic reagent, without further change of coordination mode in the reaction of complex formation. 

This term refers to the greater thermodynamic stability of a complex with a cyclic polydentate ligand when compared to the complex formed by a comparable noncy-clic ligand. A representative comparison would be between the following pair ... 

For a given metal ion, the thermodynamic stability of a chelated complex involving didentate or polydentate ligands is greater than that of a complex containing a corresponding number of comparable monodentate ligands. This is called the chelate ejfect. 

Polydentate ligands containing carboxylate groups form complexes with Ga and In " which exhibit both thermodynamic and kinetic stability. Some equilibrium constants for the formation of such complexes are given in Table 18 and show an increase in thermodynamic stability with increasing numbers of carboxylate groups in the polyfunctional amine acetate series. Among... 

The fact that the equilibrium for the reaction favors the chelate Co(en)3 means that the chelate has thermodynamic stability. A similar argument could be made for any equilibrium involving the replacement of monodentate ligands by polydentate ligands. Reaction tends to favor the chelate. 

In an example that shows the strong donor character of NHCs, the tripodal polydentate NHC ligand in 11.26 stabilizes Fe(V) as an organo-metallic nitride. Not all potentially chelating bis-NHC ligands in fact form chelates, however, even when the chelate would be thermodynamically favored. Each NHC often binds to a separate metal in a 2 1 complex as kinetic product because, unlike M-PR3, M-NHC bond formation is not reversible, so the error cannot be remedied by reversible dissociation (Eq. 11.42). 

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