Polydentate ligands, chelate effect

Chelation with di- and trivalent metal ions showed even more drastic permeability changes. The experiment was performed by alternating addition of and ETA (ethylenediamine-tetraacetic acid disodium salt), an effective polydentate ligand for Cu. The result is shown in Fig. 25. It is seen that the alternating addition of Cu ... 

The Chelate Effect and Polydentate Ligands 147 Table 8-1. Stability constants for some nickel(ii) complexes of ammonia and 1,2-diaminoethane. 

Whatever theoretical explanation of the chelate effect is adopted, there is no doubt as to the extra stability conferred by polydentate ligands capable of forming one or more chelate rings. This is of especial importance in titrimetry (see Section 10.6) and, as we shall see shortly, bidentate ligands play a significant role in the formation of precipitates of value in gravimetry. 

The effects considered above may be expressed rather more dramatically upon moving from a monodentate to a polydentate ligand. If a polydentate ligand binds to a single metal atom by two or more donor atoms, the resultant complex is known as a chelate (Greek chelos, claw). Such chelated complexes are kinetically and thermodynamically more stable, with respect to ligand displacement, than comparable compounds with an equal number of equivalent monodentate ligands. What are the steric consequences of chelation ... 

The design of polydentate ligands containing imines has exercised many minds over many years, and imine formation is probably one of the commonest reactions in the synthetic co-ordination chemist s arsenal. Once again, the chelate effect plays an important role in stabilising the co-ordinated products and the majority of imine ligands contain other donor atoms that are also co-ordinated to the metal centre. The above brief discussion of imine formation will have shown that the formation of the imine from amine and carbonyl may be an intra- or intermolecular process. In many cases, the detailed mechanism of the imine formation reaction is not fully understood. In particular, it is not always clear whether the nucleophile is metal-co-ordinated amine or amide. Some intramolecular imine formation reactions at cobalt(m) are known to proceed through amido intermediates. A particularly useful intermediate (5.24) in metal-directed amino acid chemistry is... 

Usually, there is a compensation effect, that is AHd —TAS11 so that AGd 0. Experimentally, AS1 is positive (especially for macrocyclic ligands), and so is very often A Hr (which means that usually, the Ln-L bonds are weaker than the Ln-OH2 ones) so that complexation reactions in water are entropy driven and, moreover, a linear relationship between AH and A. S 1 holds for the Ln(III) series of cations. One has, however, to be cautious when this approach is applied to polydentate ligands. The thermodynamic parameters may also reflect other factors such as the formation of stable 5-membered chelate rings. When another solvent is considered, the solvation enthalpy is much smaller than in water and the above considerations may no more hold. 

Theoretical and experimental studies on penta-coordinate silicon derivatives demonstrate that their existence is determined by a combination of factors electronegativity of the substituents and steric interactions between substituents. One should also emphasize the role and significance of polydentate ligand (chelate effect), the size and number of chelate rings involving the silicon atom, and strain reduction for five-membered ring system, wich can stabilize unusual structures, as well as to the role of medium effect. This is consistent with the results of NMR spectroscopic studies. 

Central to the stability and chemistry of complexes formed by mixed donor ligands are two key concepts of coordination chemistry. The first is the chelate effect, which applies to all polydentate ligands, and reflects the increase in stability of a type of complex as monodentate donor molecnles are replaced by polydentates with donors linked by chelate rings. The hard-soft acid-base theory is particnlarly relevant to mixed donor ligands where donors of distinctly different character may bind to a central metal ion. The like prefers like concept means hard nonpolarizable donor atoms (N and O, for example) bond preferentially to hard nonpolarizable metal... 

The thermodynamic chelate effect, which causes polydentate complexes to be thermodynamically more stable than their monodentate counterparts, was described in Section 10-1-1. The difference in the attachment and dissociation of the second (and third or higher numbered) point of attachment for the ligand is also observed kineticaUy. 

For a polydentate ligand with n complexing groups, this chelate effect adds (n — 1) log 55.5 to the logarithm of the stability constant, relative to the stability constant for a complex with n uniden-tate ligands of chemically similar type. 

The chelate effect is amplified in the case of polydentate ligands that form several rings with a single metal atom. The extreme of this form of stabilization is found with hexadentate ligands such as ethylenediaminetetraacetate (edta), ( CXDCCHjljNCHjCHjNfCHjCOO-), which has six ligating atoms. 

Danish workers have synthesized several polydentate ligands derived from (44a) that illustrate the sensitivity of the spin transition to small ligand modifications and indicate that the chelate effect may be as important as conjugation in the ligand. Thus, with (44j n = 2,3 and R = = H) low-spin... 

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 ... 

Although the donor atom is nitrogen in both instances, [Ni(en)3] has a formation constant that is more than 10 times larger than that of [Ni(NH3)g]. This trend of generally larger formation constants for bidentate and polydentate ligands, known as the chelate effect, is examined in the A Closer Look essay on page 977. 

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

Figure 3. Chelate effect and size complementarity in linear polydentate ligands.

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