Putting the Bite on Metals– Chelation

The classic simple ligand is ammonia, since it offers but one lone pair of electrons, and thus cannot form more than one coordinate covalent bond (Figure 2.2). A water molecule has two lone pairs of electrons on the oxygen, yet also usually forms one coordinate covalent bond. If one looks at the arrangement of lone pairs, this is hardly surprising; once one coordinate bond is formed, the remaining lone pair points in the wrong direction to allow it to become attached to the same metal ion– only through attachment to a different metal could this lone pair achieve coordination (a situation for the ligand called bridging). We shall return to examine whether this can actually happen for a water molecule later.

Figure 2.2

Figure 2.2 coordinated free H coordinated M' H bridging Free and coordinated ammonia and water molecules. The second lone pair of water is oriented in a direction prohibiting its interaction with the same metal centre as the first. However, it does have the potential, in principle, to use this lone pair to bind to a second metal centre in a bridging coordination mode. (Other groups bound to the metals are left off to simplify the views.)

Let’s try to make it a bit easier for two lone pairs to interact with a single metal ion by putting them onto different atoms, and examine the result. We’ll start with two ammonia residues linked by a single carbon atom– not a particularly chemically stable entity, but one that will suffice for illustrative purposes. Either the lone pair on the first N atom or the lone pair on the second N atom could form a single bond to a metal ion initially. While the second amine group is free to rotate about the resulting fixed M N C assembly, if the second lone pair is oriented in the same plane, it is now pointing more in the direction of the metal that was the case with the second lone pair on the water molecule. If the existing covalent bonds are somewhat deformed, coordination of both lone pairs to the same metal may be achieved (Figure 2.3). Another and more stable example is the carboxylate group (R COO−), which can coordinate in at least three ways– to one metal through one oxygen, bridging to two metals with each bound to one oxygen, or bound to one metal via both oxygen atoms (Figure 2.4). Note that the ring of atoms which includes the metal and donor atoms formed in both Figures 2.3 and 2.4 is identical in size, but differs in the type of donor atoms. Where the one ligand employs two different donors to attach to the same metal, we have a situation called chelation–achelate ring has been formed. The name derives from the concept of a lobster using both claws to get a better grip on its prey, put forward by Morgan and Drew in a research paper in 1920; not a bad analogy, given that chelates usually form much stronger complexes than an equivalent pair of simple monodentate ligands. A chelate ring is defined formally as the cyclic system that includes the two donor atoms, the metal ion, and the part of the ligand framework joining the two coordinated donors. The size of the chelate ring is then obtained by simply counting up the number of atoms linked covalently

Figure 2.3

Diaminomethane, a molecule with two amine groups. Once the first is coordinated, the second lone pair from the other amine can be oriented in a direction more appropriate for bonding than is the case for two lone pairs on a single atom, with limited bond angle distortion permitting both to coordinate, illustrated at right, in a chelated coordination mode.

Figure 2.4

Metal ion binding options for a carboxylate group, featuring various monodentate and didentate coordination modes. in the continuous ring, starting and ending at the metal, and including it. For example the chelated carboxylate in Figure 2.4 involves the sequence M→O→C→O→, with the last Oreturning us to the M, so four atoms are involved in the continuous ring, meaning it is a four-membered chelate ring. For the diaminomethane of Figure 2.3, like the carboxylate discussed above, the chelate ring is a four-membered ring, as it involves four atoms (including the metal) linked together in aring byfourcovalent bonds, twoofwhicharecoordinatebonds.Justinthewaythatring structures of a certain range of sizes in organic compounds are inherently stable, chelation leads to enhanced stability in metal complexes for chelate rings of certain sizes. If, instead of diaminomethane, the much more chemically stable diaminoethane (for mally named ethane-1,2-diamine, but also called ethylenediamine or often simply ‘en’) is employed, chelation leads to a five-membered chelate ring. For this to happen, first one nitrogen must form a bond to the metal, then the remaining lone pair must be rotated to an appropriate orientation and the nitrogen approach the metal so as to lead to effective binding and hence chelation. The anchoring of the first nitrogen to the metal means the second one cannot be too far away in any orientation, facilitating its eventual coordination (Figure 2.5). Looking along the C-C bond of diaminoethane, the two amines must adopt a cis dis position for chelation; in the trans disposition (shown at centre left in Figure 2.6), only bridging to two separate metals can result. With a flexible ligand like this, rotation about the C-C bond readily permits change from one conformation to another in the free ligand

Figure 2.5

The stepwise process for chelation of diaminoethane. This features initial monodentate formation, rearrangement and orientation of the second lone pair, and its subsequent binding to form the chelate ring.

Figure 2.6

Freedom to rotate about the C-C bond in diaminoethane permits cis or trans isomers, capable of chelation and bridging respectively (top). For rigid diaminobenzene (bottom), rearrangement is not possible, and the two isomers shown have exclusive, different coordinating functions as didentate ligands. (Figure 2.6); this will not be possible with rigid ligands like diaminobenzene, where the 1,4 (para or trans) isomerandthe1,2- (ortho or cis) isomer are distinctly different molecules, the former able only to bridge whereas the latter may chelate (although both are called didentate ligands (di = two) since they each bind both of their two nitrogen donors). The chelate ring formed with 1,2-diaminobenzene is flat, because of the dominating influence of the flat, rigid aromatic ring. However, the ring with diaminoethane is not flat, since each N and C centre in the ring is seeking to retain its normal tetrahedral shape. Looking into the ring with the N-M-N plane perpendicular to the plane of the paper, the shape of the ring is clearer; one C is up above this plane, the other down– the ring is said to be puckered (Figure 2.7). If planarity of the carbon joined to the donor atom is enforced, such as is the case for the planar sp2-hybridized carbon in a carboxylate, planarity

Figure 2.7

Chelate ring conformations in chelated diaminoethane (ethane-1,2-diamine, en), designated as σ and λ . Views looking into the N M N plane (centre; H atoms bound to C atoms disposed roughly in the plane, H_eq, and perpendicular to the plane, H_ax, also included) and along the C-C bond (sides; H atoms removed for clarity) are shown. Ready interconversion between the two conformations (which are mirror images) is possible, as only a small energy barrier exists between them.

in the chelate ring arises. The glycine anion (H₂N-CH₂-COO−), with one tetrahedral and one trigonal planar carbon, forms a five-membered chelate ring with less puckering than diaminoethane, whereas the oxalate dianion (−OOC-COO−), with two trigonal planar carbons, is completely flat in its chelated form. For the puckered diaminoethane, there are some further observations to make. The chelate ring is more rigid than the freely-rotating unbound ligand, so that the protons on each carbon are nonequivalent, as one points essentially vertically (axial, H_ax), the other sideways approximately parallel to the N-M-N plane (equatorial, Heq). Nevertheless, it is sufficiently flexible that it can invert– one carbon moving upwards while the other moves downward to yield the other form. These two forms are examples of two different conformations; one is called, the other, by convention; they are mirror images of each other. Any chelate ring that is not flat may have such conformers. A vast array of didentate chelates exist, so that this one type alone can be daunting because of the variety. However, there are a number of essentially classical and popular examples, many of which tend to form flat chelate rings rather than puckered ones as a result of the shape of the donor group or enforced planarity of the whole assembly due to conjugation. A selection of common ligands appears in Figure 2.8, along with ‘trivial’ or abbreviated names often used to identify these molecules as ligands. One aspect of the set of examples is that the chain of atoms linking the donor atoms can vary– they do not all lead to the same chelate rings size; however, it is notable that a four-atom chain leading to f ive-memberedchelate rings are most common. This aspect is addressed in the next section.

Figure 2.8

some common didentate chelating ligands. Common abbreviations used for the ligands are given to the right of each line drawing.

Figure 2.9

Chelate ring formation may not be ideal in terms of the ‘fit’ of the ligand to the metal. Potential mismatch resolution involves in large part (but not exclusively) adjustment in the metal–donor distances and the angles around the metal.

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

Making Attachments– Coordination

The Road Ahead

Natural or Made-to-Measure Complexes

Metals in Contrived Environments

Metals in the Natural World