Meeting More Metals - Bridging Ligands

We have identified ammonia (NH₃) as the classical example of a monodentate ligand, with only a single lone pair. However, as already mentioned if ammonia is stripped of a proton to form the anion NH₂ it now offers two lone pairs. Although accessible in aqueous solution only at high pH this ion is a strong base and is known for its capacity to bind

Figure 2.15

Ammonia coordinates as a monodentate ligand to one metal. When a proton is removed it exhibits the capacity to attach the resultant additional lone pair to a second metal ion in a bridging mode.

efficiently to two metal ions in a bridging mode. This is an example of how the creation of more lone pairs associated with forming a new anion can expand access to more metal ions, through these ligands making additional attachments to other metal ions at the same time - the process of bridging. This is illustrated for ammonia and its anion in Figure 2.15. The water molecule (OH₂) is able to lose protons sequentially to form hydroxide (OH) and oxide (O₂). As the protons are stripped off, the number of lone pairs increases, with the hydroxide offering three and oxide offering four. Just having more than one lone pair is no guarantee of more bonds forming. Binding as a monodentate ligand to a single metal occurs for all of OH₂ OH- and O₂- in all cases involving an oxygen donor atom. However, these ligands can, in principle, expand their linkages by making additional bridging attachments to other metal ions. We see this with the anions, but rarely with water itself. It is useful to examine why neutral H₂O doesn't bridge between two metal ions. When a coordinate covalent bond is formed to a positively charged metal ion, there is a strong tendency for electron density to 'shift' towards the metal ion; we can visualize this as a diminution in the size of the remaining lone pair making it less accessible and attractive to a second metal ion. If a proton is removed from the coordinated water, creating a second free lone pair, there is a significant increase in electron density on the oxygen and so attractiveness for a second metal cation to bind and form a second coordinate covalent bond is increased. The negative charge on the HO ligand acts to balance the positive charges on the two metal ions forced to be located closely together (Figure 2.16). Bridging groups are represented by the Greek letter μ (mu) as a prefix in formulae (see Appendix 1).

Figure 2.16

Deprotonation of water enhances the prospect of bridging between two metal ions by increasing electron density on the O atom. Successive deprotonation can permit multiple bridging to metal ions resulting in small metal-oxide clusters.

Figure 2.17

Modes through which bridging between two different metals can arise, illustrated for simple N-donor ligands.

An extension of this process is illustrated in the lower part of Figure 2.16 (ignoring nonbonding or 'spectator' lone pairs) starting from water bound in its usual form as a monodentate ligand to one metal. As each proton is successively removed, the capacity to attach the resultant lone pair to a second and then even, with the next deprotonation, to a third metal ion arises. The O donor is acting as a monodentate ligand to more than one metal at once. This is well known behaviour. What isn't shown (for simplicity) are the other bonds to the metals that can lead to large clusters in some cases such as the box-like framework shown at bottom right in Figure 2.16 nor those additional ligands not involved in bridging. Thus ligands can accommodate two metal ions in two distinct ways: by using two different donor atoms on the one ligand to bind to two separate metal ions; or by using two different lone pairs on the same donor group to bind to two separate metal ions (Figure 2.17).

The former leads to greater distance between the like-charged metal centres and is thus likely to be a less demanding process. The latter brings the metal ions much closer together but can only occur, of course, with donor groups that have the potential to provide at least two lone pairs, and that are usually anionic so as to help two like-charged metal centres approach each other closely. Systems where several metal ions are bound in close proximity employ the same basic concepts discussed here.

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المزيد

الآخبار الصحية

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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

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

Making Attachments– Coordination

The Road Ahead

Natural or Made-to-Measure Complexes

Metals in Contrived Environments

Metals in the Natural World