Isomer Preferences

Polydentate ligands present particular problems, as a range of geometric isomers, often with enantiomers, may exist in principle. This is illustrated in Figure 4.34 for a simple system where a symmetrical tetradentate is bound. This can occur in three ways, two of which

Figure 4.33

Selected examples of ways in which dissymmetry is introduced into octahedral complexes, with case numbers equating with those in the text immediately above. Enantiomers are shown for two cases only [(1) and (2)]. A process by which conversion in case (2) from one enantiomer to the other (optical isomerization) may occur is illustrated, based on twisting of one octahedral face by 120◦ while keeping the other fixed. The two isomers must twist in opposite directions to avoid breaking the chelate chains, to generate a symmetrical intermediate, from which it may continue twisting to yield the opposite enantiomer. The ‘left-twisting’ isomer is called and the ‘right-twisting’ one the symbols for the two enantiomers or optical isomers.

lead to dissymmetric complexes. What should be appreciated is that these three isomers are not equal in thermodynamic stability meaning that the percentage of each formed is not based purely on a statistical ratio, but related to their relative stabilities. This means, in effect, that in some cases only one geometric isomer could be observed experimentally, if it is significantly more stable than others, or that at least only several of a set of options

Figure 4.34

The three geometric isomers (diastereomers) for M(AAAA)B2 complexes one of which is of trans geometry with respect to the two simple ligand B (centre) and two of which are of cis geometry (cis-at left cis-at right) and in those cases have enantiomers, also drawn.

Figure 4.35 Left: One of the geometric isomers of a M(AAAA)B2 complex is transformed into two options when a mixed donor ligand replaces the common donor ligand in forming a M(AAAC)B2 complex. Consider the two arising from replacement of two different terminal donors, so the product has the introduced C group either opposite a B ligand (left) or opposite an A ligand (right). In reality, of course, the mixed donor ligand is preformed and coordinates as a single entity. Right: One of the geometric isomers of a M(AAAA)B2 complex is also transformed into two options when two different monodentate ligands replace the common monodentate ligands in forming a M(AAAA)BC complex. Consider the two arising from replacement of two different monodentate donors, so the product has the introduced C monodentate either opposite a central A group of the tetradentate (left) or opposite a terminal A group (right).

may occur experimentally. The isomers usually display different total strain energy, and it is where there are large energy differences that there is observed one thermodynamically stable form or at most a limited number of forms. Thus only some, and not all, will be found experimentally, as some may be too strained relative to others to exist. If we introduce mixed donors into the simple tetradentate AAAA of Figure 4.34, for example, to form AAAC, the number of isomers will increase. For example, now there are two different forms of the isomer shown at the left in Figure 4.34, as illustrated in Figure 4.35. The two new isomers have the same spatial arrangement of the chelate chains, but the unique terminal groups (A and C) are located differently. An alternative reaction that increases geometric isomers is where the two common monodentate donor groups are replaced by two different monodentate donor groups (Figure 4.35, right); there are again different outcomes depending on which of two groups is replaced. There is adequate nomenclature to deal with naming these difference diastereomers, but this is not required to comprehend the outcomes when pictorial representations are available; some aspects of naming complexes appears in Appendix A. These two simple examples suffice to illustrate the array of geometric isomers that can result when polydentate and mixed donor ligands bind to a central metal ion.

Geometric isomers are energetically inequivalent because of a suite of effects that con tribute additively to strain in each complex. Sources of strain in complexes are: bond length deformation (enforced contraction or extension); valence bond angle deformation (enforced opening/closing up); torsion angle deformations (for chelate rings); out-of-plane deformations (for unsaturated groups); nonbonded interactions; electrostatic interactions (of charged groups); and hydrogen bonding interactions. Models that allow estimation of the strain energies in isomers exist, providing prediction of and/or interpretation of experimental behaviour.

اخر الاخبار

اخبار العتبة العباسية المقدسة

أكاديمية التطوير الإداري تقيم ورشة عن منهجية الإشراف الأكاديمي الفعّال على بحوث الطلبة

قسم الشؤونِ الخدمية يوفر ثلاثة آلاف صندوق أمانات لزائري مرقد أبي الفضل العباس (عليه السلام)

خلال عيد الفطر المبارك.. قسم الشؤون الدينية يضاعف جهوده للإجابة عن الاستفتاءات الشرعية للزائرين

قسم الشؤون الفكرية يقيم ندوةً ثقافية حول فوائد تشريع العيد في ذي قار

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

Bedded Down– The rmodynamic Stability

Host–Guest Molecular Assemblies

Encapsulation Compounds

Compounds of Polydentate Ligands

Sophisticated Shapes

Constitutional (Structural) Isomerism

Isomerism– Real 3D Effects

Chameleon Complexes

Moulding a Relationship - Ligand Influences

Metallic Genetics– Metal Ion Influences

Higher Coordination Numbers (ML7 to ML9)

Six Coordination (ML6)