Making Attachments– Coordination

Only in the gas phase is a ligand likely to meet a ‘naked’ metal or its ion, and this is a highly contrived situation. In the liquid or solid state, where we overwhelmingly meet coordination complexes, a potential ligand will normally be confronted by a metal already carrying a set of ligands. In many cases in solution these other ligands are solvent molecules themselves but no less legitimate as ligands simply because they can serve two roles, as ligand or solvent. Binding of a new ligand in what is termed the inner coordination sphere of the metal ion usually requires that it replace an existing ligand, a process termed substitution.

 The strong drive towards complexation of nominally ‘naked’ metal ions is readily observed. For example if a simple hydrated salt like copper (II) sulfate (CuSO₄·5H₂O) is dried in a vacuum oven to the point where no attached water molecules are present a colourless anhydrous salt CuSO₄ is obtained. If this is dissolved in water, a pale blue solution is immediately formed as the metal ion is hydrated, a process which simply involves a set of water molecules rapidly binding to the metal ion as ligands. An energy change is associated with this process– the heat of hydration. If the solvent is removed by evaporation and the residual solid gently dried, a blue solid is recovered. This is the hydrated, or complexed, salt that has the formulation CuSO₄·5H₂O. That the water molecules are tightly bound to the copper ion can be shown by simply measuring weight change as temperature is slowly raised. What is observed is that all water is not removed simply by heating to 100 ◦C but is eventually removed fully only following heating to over 200 ◦C for an extended period, with recovery after that stage of the anhydrous species. Application of an array of advanced experimental methods allows us to observe the species in solution also; not only can we observe the presence of separate cations and anions but the size, shape and environment of the ions can be elucidated. This confirms that the copper ion exists with a well-defined sheath of water molecules, the inner coordination sphere, which are in effect simple ligands, each water molecule attached to the central metal through a coordinate covalent bond via an oxygen lone pair. When this entity is ionic, as is the case for copper(II), this complex is surrounded by a partially ordered outer (or secondary) coordination sphere where water molecules are hydrogen-bonded to the inner-sphere ligated water molecules; a third and subsequent sheath surrounds the second layer the process continuing until the layers become indistinguishable from the bulk water. The various layers moving outwards from the centre undergo successively decreasing compression as a result, in the simplest view, of the progressively diminishing electrostatic influence of the metal ion. It is also important to think about the lifetime of a particular complex ion. For an aquated metal ion in pure water, there is but one ligand type available. However, it is not correct to assume that, once formed, a complex ion inevitably remains with the same set of water molecules for ever. In solution, it is possible (indeed usual) for water molecules in the outer coordination sphere to change places with water molecules in the inner coordination sphere. Obviously, this is a difficult process to observe, since there has been no real change in the metal environment when one water molecule replaces another– a little like taking a cold can out of a refrigerator and replacing it with another warm one of the same kind, so that no one can tell unless they pick up the warm can. At the molecular level, one can adopt the cold/warm can concept to probe what is called ligand exchange, by adding water with a different oxygen isotope present and following its uptake into the coordination sphere. The facility of this water exchange process varies significantly with the type and oxidation state of the metal ion. Moreover, the rate of exchange varies not only with metal but with ligand– to the point where longevity of a particular complex can indeed be extreme, or the coordination sphere is for all intents and purposes fixed.

اخر الاخبار

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

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

Do I Look Big on That?– Chelate Ring Size

Putting the Bite on Metals– Chelation

The Road Ahead

Natural or Made-to-Measure Complexes

Metals in Contrived Environments

Metals in the Natural World