Some Polydentate Ligands

Incidentally, the formation of complexes with derivatives from the last four groups of derivatives listed here is accompanied by the expulsion of one or several protons. This point is developed in Chap. 25, which is devoted to the superimposition of acid-base and complexation phenomena. 

Crown ether ligands deserve particular mention, although so far they have not been used much in analysis. They are quasi-planar macrocyclic compounds with 

Numerous other crown ether derivatives are described. They differ from one another in the size of the macrocycle and the number of oxygen atoms. These derivatives sequestrate metallic ions more or less selectively in proportion to the size of the cavity existing inside the cycle. Electrostatic interactions between the sequestrated metallic ion and the oxygen atoms of the cycle also play a part in the formation of this kind of complexes. This is why crown ether ligands, although recently discovered, can be considered classical ligands. 

Some natural products are crown ether derivatives. They can sequestrate alkali and alkaline earth ions. These polypeptides include valinomycin, monactin, and mo-nensin. They play an important biological role They bind K+ in preference to Na ions, except for monensin, where the tendency is reversed. They also contribute to the maintenance of the different concentrations of these ions inside and outside the cells and, consequently, to that of the potential differences across cell membranes. Beyond these biological aspects, valinomycin is the basis for an ion-selective electrode for K+. 

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How Does the Electrochemical Ligand Parameter Influence Real Versus Possible Hapticity of Some Polydentate Ligand ... 

Table 2.8 Selection of complexes formed by some polydentate ligands...

Table 1-4. Some typical polydentate ligands and their complexes.

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

For such a mechanism, the overall second-order formation rate constant is given by the product of the first-order constant ktx and the equilibrium constant Kos. The characteristic solvent exchange rates are thus often useful for estimating the rates of formation of complexes of simple monodentate ligands but, as mentioned already, in some cases the situation for macrocyclic and other polydentate ligands is not so straightforward. 

Multiple metal-metal bonds have since been found in Re and many other transition metal complexes [41]. The structure of the unsupported Re2 unit (2) is shown in Chart 1. Bonds to the ligands in the equator around each metal may be staggered or eclipsed depending on electronic structure, and solvent or unidentate ligands may bind weakly at the axial positions. In some cases, polydentate ligands may bridge the metal-metal-bonded unit (3). 

Other types of polydentate ligands are those with different donor atoms that can be of the type P,C or N,C or S,S,C,C or P,N, and so on, [172-175]. Many gold(I) complexes with these heterofunctional ligands have been prepared. Figure 1.25 shows some examples. 

Finally, polydentate ligands can affect the geometry of a complex merely as a result of their own steric requirements. For example, we find some tetradentate ligands such as tris(2-dimethylaminoethyl)amine, [Me6tren = ((CHj NCHjCH N], form only five-coordinate complexes (Fig. 12.10), apparently because the polydentate ligand cannot span a four-coordinate tetrahedral or square planar complex and cannot conform ( fold ) to fit a portion of an octahedral coordination sphere. 

Polydentate ligands do not necessarily need to bind a metal at all possible locations. In some cases, only one site is bound to the metal and the other one or more are left dangling. Such complexes are rare, and it has been necessary to develop special synthetic methods to prepare them. Explain in terms of thermodynamic arguments why polydentate ligands will almost always form chelating complexes rather than leave dangling arms. 

Angelici and his coworkers have overcome some of the practical problems arising from the low formation constants of simple monoamino esters, by incorporating the ester moiety into a larger polydentate ligand. A typical example is the use of ethyl glycinate-N,N-diacetic acid (12) which hydrolyzes to give nitrilotriacetic acid (13). 

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... 

The last of the important concepts that we will consider is self-assembly. Most chemists have, at some time in their careers, wondered why molecules cannot just make themselves. The process by which molecules build themselves is termed self-assembly and is a feature of many supramolecular systems. If the molecular components possess the correct complementary molecular recognition features and their interaction is thermodynamically favourable then simply mixing them could result in the specific and spontaneous self-assembly of the desired aggregate. This assumes that there is no significant kinetic barrier to the assembly process. The recognition features within the components may be any of the intermolecular bonding processes mentioned above, but we will be concerned with interactions between transition metal ions and polydentate ligands. 

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