Many-Armed Monsters - Introducing Ligand Shape

High denticity ligands need not be simply composed of linear chains like the examples in Table 2.1. The shape of the ligand has an important role to play in the manner of coordination and the strength of the complex, but there is no particular shape that is forbidden to a ligand. Molecules may be linear, branched podal cyclic polycyclic, helical, as well as aliphatic or aromatic or mixtures of components of these - the over-arching requirement is simply a set of donor groups with lone pairs of electrons. In some cases, they may be dinucleating or polynucleating that is capable of binding to more than one metal at the same time. Some fairly simple examples and their family name appear in Figure 2.20. We shall deal more with shape later, since ligand shape or topology can have an important influence on coordination, but it may be useful to introduce some concepts now.

Let's consider two different types of hexadentate ligands: a linear polyamine and a branched polyaminoacid. To coordinate a metal ion and use all six donors, the linear molecule must wrap around the metal ion like a ribbon, whereas the branched molecule wraps up the metal ion more like a hand with fingers (Figure 2.21). On a macroscopic scale, the latter is like holding a ball in the palm of your hand with the fingers wrapped tightly around it - a nice secure way of holding it. On the molecular scale, branching can likewise lead to a more 'secure' complex, which makes the branched polyaminoacid (called EDTA) represented in its anionic form in Figure 2.21 such an effective, strong ligand for binding most metal ions.

The shape of the ligand places demands on how it can coordinate to a metal ion. For a many-branched ligand, where the branches or 'fingers' are terminated with donor groups capable of binding to a metal ion the ligand can achieve a firm grip by employing these 'fingers'. Looking at the way such ligands wrap around a metal ion you may also get an idea of how they can be removed - by detaching each 'finger' in turn, then taking the metal ion out of the 'palm' of the ligand. Initial coordination is in effect the reverse of this process - step by step binding to eventually achieve the final complex. Of course, because the metal ion is not 'naked' in the first place but carries a suite of usually simple ligands when it encounters the polydentate, each step of attachment of a donor of the polydentate involves departure of a simple ligand; it is a substitution process.

Figure 2.20

Examples of polydentate ligands of selected different basic shapes.

Figure 2.21

The process of complexation of polydentate ligands involves an aspect of ‘wrapping’ around the metal ions usually with gross rearrangement of the ligand shape.

اخر الاخبار

اخبار العتبة العباسية المقدسة

ضمن جهوده المتواصلة.. قسم الشؤون الخدمية ينفذ حملة غسل وتنظيف لمحيط العتبة العباسية المقدسة

قسم الشؤون الفكرية يختتم مسابقة نجم الأئمة (عليهم السلام) بنسختها الخامسة في بغداد

المجمع العلمي يواصل الختمة القرآنية الرمضانية المرتلة حفظًا في مقام الإمام المهدي (عجَّل الله فرجه)

الحلقة 26 من جائزة العميد القرآنية تشهد مشاركة 5 قراء من الفتية ضمن المرحلة النهائية

المزيد

الآخبار الصحية

كوب قبل النوم.. مشروب طبيعي يحارب الأرق

دون أدوية.. طريقة طبيعية لخفض ضغط الدم بفعالية مذهلة

المعكرونة ليست عدوك.. كيف تصبح جزءا من وجبة صحية متوازنة؟

دراسة: تناول القهوة يوميا يخفض خطر الإصابة بالاضطرابات النفسية

المزيد

الآخبار التكنلوجية

علماء روس يخططون لبناء منشأة للكشف عن المادة المظلمة

ثاني أكسيد الكربون ليس تهديدا للمناخ فقط.. بل لصحة الإنسان أيضا!

ديدان الأرض تكشف تلوث التربة بالمعادن الثقيلة

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

المزيد

مواضيع ذات صلة

?Preferences– Do You Like What I Like

Coupling– Polymetallic Complexes

A Separate Race - Organometallic Species

Supersized - Binding to Macromolecules

Vive la Difference– Mixed-metal Complexation

Polynucleating Species - Molecular Bigamists When One is Not Enough

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