Ligand exchange thermodynamic stability

If the metals bound in complexes exchange with biological ligands, the dissociation kinetics of these complexes, the ligand-exchange kinetics and the association kinetics with the biological ligands must be considered. Simple dissociation kinetics of complexes are related to their thermodynamic stability constants by the relationship ... 

Probably the most common attachment reaction in a late transition metal catalyzed reaction is transmetalation. This reaction, depicted in Figure 1-6, is the reversible exchange of covalently bonded ligands between two metal centres. The placement of the equilibrium is usually determined by the difference between the thermodynamic stability of the sacrificed and the formed bonds. From the practical point of view the placement of the equilibrium is less interesting as long as it is able to provide enough of the transmetalated complex for the follow up reaction. 

The trans effect illustrates the importance of studying the mechanisms of complex substitution reactions. Before continuing with a discussion of mechanisms, the distinction between the thermodynamic terms stable and unstable and the kinetic terms labile and inert should be clarified. Consider the following cyano complexes [Ni(CN)4]2-, [Mn(CN)6]3-, and [Cr(CN)6]3-. All of these complexes are extremely stable from a thermodynamic point of view is yet kinetically they are quite different. If the rate of exchange of radiocarbon labeled cyanide is measured, we find that despite the thermodynamic stability, one of these complexes exchanges cyanide ligands very rapidly (is labile), a second is moderately labile, and only [Cr(CN)6]3 can be considered to be inert ... 

Ligand exchange has proved to be very successful in the separation of several enantiomers. Davankov and Rogozhin (41) used chiral copper complexes bonded to silica. The enantiomeric separation is based essentially on the formation of diastereomeric mixed complexes with different thermodynamic stabilities. It is generally accepted that chiral discrimination proceeds via the substitution of one ligand in the coordination sphere of the metal ion. Ligand exchange technique is especially effective for the enantiomeric resolution of aminoacids, aminoacids derivatives, and hydroxy acids (42). 

The chelate ligand in dionato complex 3 is planar and it is particularly stabilized by 71-delocalization. In addition to this thermodynamic stability, the iron center has 17 valence electrons in an octahedron, hence its coordination sphere is kinetically labile. By ligand exchange, the acceptor 41a is coordinated at a vacant site to form species 44 (Scheme 8.18). The function of the center metal is not only to hold the acceptor in proximity to the donor. Additionally, the acceptor is activated by Lewis acidity of the center metal. Subsequently, the nucleophilic carbon atom of the dionato ligand is... 

The solvation of transition metal ions bound to the surfaces of nanocrystals clearly relates to the thermodynamics of their interaction with the surface. It is interesting to note that Mn2+ solvation from CdSe nanocrystal surfaces appeared to be complete after a Py ligand-exchange procedure that took 24h (47), whereas Co2+ on the surfaces of CdS nanocrystals requires weeks to be solvated by Py (68), and Co2+ on the surfaces of ZnS nanocrystals was not solvated by Py to any measurable extent (91). The thermodynamic variations thus depend sensitively on the geometries of the surface-binding sites offered to the dopants. For example, the S S separations of CdS surfaces are apparently too large to stabilize Co2+ ions to the same extent as those of ZnS. As discussed in Section II.C, the capacity a surface has to stabilize bound dopants is intimately related to... 

Despite the kinetic lability of the Ln-X-cr-bonds (even the thermodynamically very stable Ln-OR bond is subject to rapid ligand exchange reactions [49]) organolanthanide compounds are thermally very robust over a wide range of temperature (Fig. 5) [114, 116, 139, 144-151]. Thermal stability is not only favorable in catalytic transformations at elevated temperatures [47], for the support of volatile molecular precursors is of fundamental importance in chemical vapor deposition techniques the sublimation behavior is a criterion of thermal stability and suitability for these processes (Fig. 5). 

Stability constants are not always the best predictive tool for measuring the ease and the extent of chemical reactions involving complexes nor their stability with time, because their kinetic behavior can often be even more crucial. For example, when ligand exchange reactions of ML (e.g., [FeEDTA]) with other metal ions (e.g., Zn2+ or Ca2+) are ki-netically slow, they do not significantly influence ligand speciation. Another typical example of the thermodynamics vs kinetics competition is the fact that the degradability of some metal complexes (e.g., metal-NTA) is related to their kinetic lability, rather than to their thermodynamic stability constants. Kinetic rather than thermodynamic data are then used to classify metal complexes as labile, quasi-labile, slowly labile, and inert (or stable). See Section 3.2.6. 

While acychc polyaminocarboxylate (DTPA, ethylenedi-aminetetraacetic acid (EDTA)) complexes of copper(n) have high thermodynamic stabihty, these complexes are kinetically labile to ligand exchange. Cu(ll) has been found to have much greater kinetic stability (and consequently greater serum stability) with macrocyclic chelates, such as TETA and DOTA,... 

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