Ligands with More Bite– Denticity

So far we have restricted our discussion to ligands that attach to a metal at one or two coordination sites. While these are very common, they by no means represent the limit. It is possible to have ligands that can provide sufficient donor groups in the one molecule to satisfy the coordination sphere of a metal ion fully. Indeed, they may have sufficient to bind several metal ions. Denticity is the way we define the number of coordinated donors of a ligand; denticity simply refers to the number of donor groups of any one ligand molecule that are bound to the metal ion, which need not be all of the donors offered by a ligand. Where coordination is by just one donor group, we are dealing with monodentate ligands. When two donors of a ligand are bound, it is a didentate ligand; three bound yields a tridentate, and four bound is called a tetradentate ligand. The terminology is outlined in more depth in Appendix One. Denticity represents the way we regularly discuss ligand binding in spoken form; it provides brevity at least, as we can then say ‘a didentate ligand’ rather than ‘a ligand bound through two donor groups’. It is important to recognize that denticity and the number of potential donor groups of a ligand are not always the same thing. We can see this with a very simple example the amino acid glycine which can bind to a metal via only the carboxylate group, only the amine group or as a chelate through both at once. The first two modes are examples of

Figure 2.12

Possible coordination modes for the amino acid ligand glycine.

the molecule acting as a monodentate ligand, the last, of the molecule acting as a didentate chelate ligand (Figure 2.12). The groups that bind to a metal are influenced also by the shape of the ligand and their location on the ligand framework. Some further examples of ligands of different denticity are given in Figure 2.13. To retain our focus on the concept at this stage, the examples involve coordination to a simple flat square shape that we shall meet again later; of course, other shapes are able to accommodate ligands as well. Note that some ligands undergo

Figure 2.13

Examples of ligands of different denticity. Several undergo deprotonation prior to coordination as indicated by the binding of an anionic form. X-Groups inserted in other available metal coordination sites identify those sites not used by the target ligand.

deprotonation to form an anion in achieving complexation, consistent with the anion being a better donor for the cationic metal than a neutral ligand on purely electrostatic grounds. Not all donors groups can readily do this, as it depends on the acidity of the group. Further, some ligands contain groups with more than one heteroatom in principle capable of acting as a donor atom; examples in the figure above are carboxylate(-COO−) and amido (-N− CO-). Coordination of both donors to the one metal ion may be forbidden by the shape of the ligand molecule, or else occurs under only special synthetic conditions. We shall explore polydentate ligands in more detail in Section 2.3.

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المزيد

الآخبار الصحية

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نصائح عامة لاستخدام سماعات الرأس بأمان

تأثير اللياقة البدنية على الدماغ

المزيد

الآخبار التكنلوجية

ديدان الأرض تكشف تلوث التربة بالمعادن الثقيلة

خبير أرصاد جوية: ظاهرة النينيو القادمة قد تكون الأقوى خلال 10 سنوات

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المزيد

مواضيع ذات صلة

Meeting More Metals - Bridging Ligands

More Teeth Stronger Bite - Polydentates

Greed is Good– Polydentate Ligands The Simple Chelate

Monodentate Ligands– The Simple Type Basic Binders

Different Tribes– Donor Group Variation

Do I Look Big on That?– Chelate Ring Size

Putting the Bite on Metals– Chelation

Making Attachments– Coordination

The Road Ahead

Natural or Made-to-Measure Complexes

Metals in Contrived Environments

Metals in the Natural World