Polydentate binding

Humus can form stable complexes such as chelates with polyvalent cations. SOM is capable of strong polydentate binding to transition metals in a chelate [17,19,45, 65-67]. The complexation of metal ions by SOM is extremely important in affecting the retention and mobility of metal contaminants in solid phases and waters [45]. Several different types of SOM/humus-metal reactions can occur (Fig. 11), and include reactions between DOC-metal ions, complexation reactions between SOM-metal ions, and bottom sediments-metal ions. The functional groups of SOM (Fig. 10) have different affinities for metal ions as shown below ... 

As already mentioned, vacant POMs can be considered as inorganic ligands featuring rigid polydentate binding sites. They can thus coordinate redox-active transition metals, such as Ru, Fe, Co, Mn and Cu, leading to the formation of TMSPs.4 44... 

If the ligand already contains a Greek prefix (as in ethylenediamine) or if it is polydentate (able to attach at more than one binding site), then the following prefixes are used instead ... 

Some ligands are polydentate ( many toothed ) and can occupy more than one binding site simultaneously. Each end of the two-toothed (that is, bidentate)... 

Which of the following ligands can be polydentate If the ligand can be polydentate, give the maximum number of places on the ligand that can bind simultaneously to a single metal center, (a) Chloride ion (b) cyanide ion ... 

A monodentate ligand is one that binds to the central metal atom using only one donor atom (lone pair). A monodentate ligand only occupies one position in the coordination sphere. A polydentate ligand is one that binds to the central metal atom using two or more donor atoms and occupies two or more positions in the coordination sphere of the complex. 

Metallodendrimers can be constructed via binding of groups with suitable donor atoms (e.g., polydentate ligands) on either the periphery or the core of the dendrimer and the subsequent complexation/coordination of these ligands to an appropriate metal salt. Ultimately, this binding can involve the formation of a direct a bond linkage (i.e., a M-C bond). This chapter describes various... 

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). 

EDTA is a common polydentate ligand. In EDTA, the hydrogen atoms are easily removed in solution to produce anionic EDTA4. In its anionic form EDTA has six binding atoms, two nitrogen and four oxygen as depicted in Figure 39.1. 

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. 

In the presence of anions such as phosphate and oxalate, the relaxivity of [Gd(Tex)]2+ is considerably reduced revealing that these anions compete with water for binding to the Gd3+ ion [167,171]. Most likely, texaphyrin complexes self-associate due to strong van der Waals interactions. UV-VIS studies suggest that the aggregates dissociate upon interaction with polyuronides (pectate, alginate), which probably act as polydentate polycarboxylate ligands for the [Gd(Tex)]2+ complexes [173]. 

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