Do I Look Big on That?– Chelate Ring Size

As the size of the chain of atoms linking a pair of donor atoms grows, the size of the chelate ring that the molecule forms grows. This affects the ‘bite’ of the chelate which is the preferred separation of the donors in the chelate, with concomitant effects on the stability and strength of the assembly. Altering bond distances and angles in the organic framework of alig and involve a greater expenditure of energy than is the case with distances and angles around the metal, and so adjustments are often greater around the metal centre. Where a mismatch occurs in chelation, varying M-L bond length or L-M-L angles (or usually both in concert) is the dominant way adjustment is made to deal with the nonideal ‘fit’ of metal and potential chelate (Figure 2.9), although some limited adjustments in angles and distances within the organic ligand do occur. Nevertheless there is inherently no real upper limit on chelate ring size except that as the chains between donors get very long, the size of the ring is such that it imparts no special stability on the complex and thus no benefit is obtained– nor is it as easy for chelation to occur, since the second donor may be located well away from the anchored first donor and thus not in a preferred position for binding. Examples of three- to seven-chelate rings appear in Figure 2.10; note how the experimentally measured L M L angle changes with ring size. For simple didentate ligands of the type represented in Figure 2.10 you may sometimes meet a classification in terms of the number of atoms in the chain that separates the donor groups; thus one forming a four-membered ring is called a1.1-ligand, the five-membered ring a 1.2-ligand, and the six-membered ring a 1.3-ligand and so on; it is not heavily used, however, and we shall not employ it here. What we find is that there is a preferred chelate ring size; as the ring size rises there is a rise in stability of the assembled complex and then a fall as the ring continues to grow. This trend depends on a number of factors, such as what metal ion, what donor groups, and what ligand framework is involved. Nevertheless, for the common lighter metals (first row of the periodic d block) the trend is fairly consistent:

3-membered<4-membered<5-membered<6-membered >7-membered.

Overall the five-membered chelate ring is preferred. We can actually measure this trend experimentally such as for the series of O-donors in Figure 2.10 below. This is seen experimentally in terms of the stability of metal complexes (you can consider this as

Figure 2.10

Examples of from three- to seven -membered chelate rings. Note how ring size impacts on the L-M-L  intraligand angle shown in the lower set.

a measure of the desire to form a complex), as exemplified for oxalate, malonate and succinate with a range of divalent transition metal ions in Figure 2.11. In all cases above the trend 5-membered>6-membered >7-memberedringispreserved. The size of the metal ion and its preferred bond lengths also play a role– for example while the trend is consistent for the lighter metals, larger metal ions may prefer a different ring size. There is another trend exhibited here that we will return to later also– a consistent variation with metal, irrespective of ligand.

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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

Greed is Good– Polydentate Ligands The Simple Chelate

Monodentate Ligands– The Simple Type Basic Binders

Ligands with More Bite– Denticity

Different Tribes– Donor Group Variation

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