Greed is Good– Polydentate Ligands

 The Simple Chelate

The concept of a chelate ring has been introduced earlier in this chapter. The simplest chelates consist of just two potential donors attached to a (usually, but not necessarily) carbon chain in positions that permit both donors to attach to the same metal ion through bonding at two adjacent sites around that central metal. There is no compulsion for such a ligand to chelate, but the fact that they usually do is an indication that such arrangements are especially favourable, or the complex assembly is more stable as a result of chelation. Consider an ethane-1,2-diamine molecule binding to a metal ion. The free ligand is flexible, and rotation about C-C and C-N bonds is facile. This allows it to move around to adopt shapes that suit it best for coordination and chelation. When a molecule of the free ligand and an aquated metal ion react, the most stable product is a chelated species; this overall reaction is represented by the solid arrow in Figure 2.18. This representation is not an indication of how the process occurs, however, and it is very unlikely that the two amines bind to the metal ion at once in a concerted process because the organization of the molecules required to permit this to occur is too great and the probability of it happening too low. Rather, the process of chelation occurs through a stepwise process. The sequence of steps

Figure 2.18 The process of chelation. A single concerted step (solid arrow) from reactants to products is not favoured but rather a stepwise process (hollow arrows) is involved.

represented by the pathwayofhollowarrowsinFigure2.18isaplausiblemechanismforthe reaction. Along the way, an intermediate compound presumed to form is the monodentate ethane-1.2-diamine complex (albeit short-lived, because it reacts on rapidly). This is a viable entity, as this mode of coordination has been seen in some isolable complexes where only one accessible binding site exists around a metal ion due to other ligands already being firmly and essentially irreversibly bound in other sites. The step-by-step process of chelation described above is believed to be the usual way all polydentate ligands go about coordinating to metal ions in general– ‘knitting’ themselves onto the metal ion ‘stitch by stitch’ (or more correctly undergoing stepwise coordination). It also provides a way whereby compounds can achieve only partial coordination of a potential donor set. when there are insufficient coordination sites for the full suite of potential donor groups available on a ligand. Although most of the chelate ligands you will encounter have a carbon backbone and heteroatoms like nitrogen, oxygen or sulfur as donors, this is not essential. This can best be illustrated by comparing the two pairs of chelated systems in Figure 2.19 which have identical ring sizes and shapes yet one of each pair contains no carbon atoms at all.

Figure 2.19

Simple chelate rings with common donor atoms but different chain atoms.

There are a range of ligand systems without carbon chains known, but they need no special consideration. They can be treated at an elementary level just like the more common carbon-backboned ligands. Herein it is this latter and most common type upon which we shall concentrate.

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اخبار العتبة العباسية المقدسة

متحف الكفيل يباشر صيانة المجموعة الرابعة من مقتنيات العتبة العسكرية المقدسة

ضمن برنامجها التبليغي اليومي.. شعبة الخطابة تنظم مجلسًا للوعظ والإرشاد

قسم الشؤون الفكرية يواصل إقامة مسابقة النور الزكي الرمضانية في ذي قار

المجمَع العلمي يواصل إقامة 18 ختمة قرآنية رمضانية في الديوانية

المزيد

الآخبار الصحية

دراسة: تناول القهوة يوميا يخفض خطر الإصابة بالاضطرابات النفسية

عرض شائع في الفم قد يكون مرتبطا بخطر الإصابة بأمراض خطيرة

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

"ناسا" تكشف موعد إعادة إطلاق رحلتها المأهولة إلى القمر

علماء الجيولوجيا يفكون لغز الصدع العملاق تحت الماء في المحيط الأطلسي

المزيد

مواضيع ذات صلة

Meeting More Metals - Bridging Ligands

More Teeth Stronger Bite - Polydentates

Monodentate Ligands– The Simple Type Basic Binders

Ligands with More Bite– Denticity

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