A New Body - Stereochemical Change

As outlined at the commencement of Section 5.3, there are other types of reactions apart from substitution reactions. One of these of key importance involves what we can describe as a change in the stereochemistry. This may involve a change in gross shape of the molecule, such as transition from square planar to tetrahedral, or, more often observed, a change in the relative position or three-dimensional arrangement of ligands in a particularly-shaped complex.

Change in Overall Shape of the Complex , A complex prepared and isolated usually exists in a thermodynamically stable form. As such, there seems likely to be little reason for the complex to undergo a change in gross shape or stereochemistry, since this would involve energy and not usually lead to a more stable complex. For a complex that exists in a particular geometry to undergo a change in its overall shape without any change in the ligand set, there must be some change in its environment. This may involve a change of temperature or else dissolution in a different solvent that may favour a different shape. An example of this type of behaviour is transition from square planar to tetrahedral shape, for which the activation barrier can be low. This has been exemplified earlier (Figure 4.12). Another example involves the reaction below

This system displays a relatively small activation barrier of ~45 kJ mol1, with a rate constant of ~10 s-1 at 298 K.

Molecules that exhibit stereochemical non-rigidity are said to display fluxional character. All molecules undergo vibrations about an equilibrium position that does not alter their average spatial location; for a limited number however rearrangement that changes the configuration can occur. One of the commonest ligands ammonia is a simple example of the concept (Equation 5.49) since as a pyramidal molecule it can invert.

This has a low energy barrier (~25 kJ mol1) and occurs rapidly (~2 x 1010 s1). However, for pyramidal PR3 compounds, the activation barrier has climbed to over 100 kJ mol1, so inversion is very slow, sufficiently so to even allow enantiomer separation in the case of chiral phosphines. Coordination of the lone pair freezes the configuration. Examples of fluxional complexes tend to be found mostly but not exclusively, amidst organometallic compounds. Spectroscopic methods, particularly NMR spectroscopy, assist in defining the fluxional behaviour.

Change in the Mode of Ligand Coordination - Linkage Isomerization

Whereas many ligands offer a single lone pair on only one atom or at least a single donor, and are thus are able to coordinate at one site in only one manner, a range of ambivalent ligands were introduced in Chapter 4 that offer two or more potential donors carrying lone pairs of electrons - put simply, they have a choice of donors by which they may be attached to a particular site. While isolated species are usually the thermodynamically stable form it may be possible to prepare a complex where the ligand is bound in its less stable form. For example, a route to the less stable [Co (NH3) s (SCN)2+ (S-coordinated form of thiocyanate) has been devised. When dissolved in solution, this complex is seen to spontaneously undergo ligand donor group exchange, forming the thermodynamically more stable [Co (NH3) s (NCS)]2+ (N-coordinated form). This process is termed linkage isomerism, the name referring to a change in the way the ligand is linked to the metal

centre.

The process can be represented in a general form as Equation 5.50:

where there is no change to the coordination sphere except for that involving the binding mode of the ambivalent ligand. Because the two alternate donors are usually distinctly different, a distinctive change in the colour of the complex occurs in this reaction. Another example of an ambivalent donor is the nitrite ion (NO2) which can coordinate via either the N or an O atom. Coordination modes for nitrito (O-bound; nitrito-0) and nitro (N-bound; nitrito-N) linkage isomers are shown in Equation (5.51) along with the intermediate that may be involved in the process of linkage isomerization.

Because isotopic labelling studies have shown that the reaction above occurs without the coordinated anion departing the coordination sphere, the mechanism of isomerization for the nitrito-nitro reaction has been proposed to involve a symmetrical intermediate where both oxygen atoms and the nitrogen atom are disposed equidistant from the metal ion in the transition state. Relaxation from this activated state occurs by capture of an oxygen atom (which means no effective reaction occurs, apart from possibly oxygen scrambling') or capture of the nitrogen alone (leading to the more stable isomer).

Change in the Relative Position of Ligands - Geometrical Isomerization

Ligand rearrangement of the type mentioned in the previous section involves a change in the way a particular ligand is bound. Another process focussed on ligand arrangement involves changing the way a set of ligands are assembled around a central metal. We have already established in Chapter 4 that ligands can in some cases organize themselves in a number of ways to form geometric isomers. Apart from separating and observing geometric isomers it became apparent early on that interconversion between geometric isomers was possible. Many of these interconversions are accompanied by a colour change, while analysis shows clearly that the set of ligands originally present have been maintained in the reaction. Two key conclusions were made: the way ligands are arranged around a central metal ion influences physical properties such as colour; and there must be a sufficiently low energy barrier to reaching the activated state to permit the observed interconversions.

In effect, there are two types of isomerization reactions (Figure 5.15), and we shall examine each in turn. However, it is appropriate initially to identify the two and the differences. In one form called optical isomerization change in optical properties alone

Figure5.15 , Optical and geometrical isomerization represent the two classes of reaction where change of relative position of ligands is involved.

(such as the direction of rotation of plane polarized light interacting with a solution of the complex) occurs. Because no other physical change is observed, this type of reaction is not readily 'seen', as it requires the use of devices that measure specific optical properties. In the other and more familiar form, geometrical isomerization, change in most physical properties (such as colour) occurs, so this type is readily 'seen', often even by the naked eye. The reason for the stark difference between observable behaviour lies in the way the set of donors in the coordination sphere is affected. In optical isomerization, one optical isomer is converted into the other; this occurs without any change in the positional arrangement of the set of donors themselves, and thus has no influence on most physical properties. In geometrical isomerization, although the set of donors remains unchanged, the actual physical arrangement of donors in space around the central metal changes. This is accompanied by a change in molecular symmetry, with subsequent effects on physical properties.

The mechanism by which one geometric isomer converts to another can employ the type of intermediates used in substitution reactions, discussed earlier. The clear difference here is that no new introduced ligand can replace one already bound; any additional molecule present in the coordination sphere in the activated state must be non-competitive as a ligand, allowing the original coordination sphere to remain intact. While both A and D mechanisms can account for isomerization reactions represented in Figure 5.15 another prospect is for the molecule to undergo a bond angle distortion and twisting (without any bond-breaking or participation of any entering group) that takes it to a more symmetrical trigonal pyramidal geometry in the activated state from which it can revert to the original isomer or continue adjusting to form the other isomer (Figure 5.16).

Figure 5.16

Mechanisms for geometrical isomerization: dissociative associative and distortion/twist mechanisms.

While geometric isomerization without any ligand substitution processes interfering is well known, it is not unusual to observe substitution reactions also occurring along with geometrical isomerization. An example is given in Equation (5.52).

trans-[CoCl2(en)₂] +OH₂ → cis-[CoCl(en)₂(OH₂)]₂+CI ̄

Introduction of a different ligand alters the relative stabilities of cis- and trans- isomers, so there may be a driving force for isomerization, leading to this occurring concomitant with substitution. In circumstances where two geometric isomers are energetically similar, both may exist in solution once thermodynamic equilibrium is reached, with the amount of each form present defined by the relative stability of the isomers.

Change in the Relative Position of Ligands - Optical Isomerization

Optical isomerism is a hidden but constant component of coordination chemistry. It is a constant because molecules that have a non-superimposable mirror image are very common, whereas it is hidden since the physical properties of optical isomers are identical. It is necessary to separate the A and A optical isomers by resolution before their hidden optical properties can be exposed, and then it requires special instrumentation to record those properties. If this is the case, one might ask why the interest in optical isomerism anyway? One answer lies in biological systems, where optical isomers do behave differently as a result of the way they match with a specific optically active (chiral) environment. This is merely an extension of the way optical isomers are separated in chemistry; while the A and A form of a complex behave identically as isolated species, when partnered with another chiral entity A', the {A-A'} and {A A') assemblies are no longer equivalent in their properties, allowing their separation. Presented with a naturally chiral protein, the A and A forms of a complex may likewise interact differently. Natural coordination complexes may exist as one particular optical isomer, as exemplified in Chapter 8. Another observation is that one of the spectroscopic methods for reporting optical activity, circular dichroism, is important in providing information on the splitting of energy levels in complexes under reduced symmetry, and also is very sensitive to the surrounding environment of a complex, providing a method for probing outer-sphere interactions of complexes. Apart from these aspects, it is simply important to understand the full breadth of the chemistry of complexes, of which chirality is an important component.

It has been known for decades that an optically pure complex (100% of either the ▲ or the A form) can undergo a process called racemization, whereby it returns to an optically inactive racemic (50:50 A:A) mixture of its two optical isomers. For this to occur, it is assumed that it must proceed through a symmetrical transition state, from which there is no particular preference regarding whether it reverts to the original or the opposite optical isomer, thus providing a process that will lead to a racemate. For most complexes, it is possible to devise an appropriate transition state by invoking traditional D or A mechanisms. In addition, the twist mechanism developed above for geometrical isomerism can be usefully applied to optical isomerism; unlike the other two mechanisms, this involves no bond-breaking or bond-making but merely bond angle distortion in moving to and from the activated state. These mechanisms are illustrated in Figure 5.17 for one of the classical examples of optical activity in coordination complexes, octahedral compounds with three didentate chelate ligands.

All three mechanisms involve a transition intermediate of higher energy and lower stability than the two optical isomers and so there is an activation barrier to overcome.

Figure 5.17 Mechanisms for isomerization of an octahedral complex with three didentate chelates proceeding through a symmetrical transition state.

For some complexes, the isomerization is very slow (as for the inert cobalt(III) complex [Co(en)₃]³⁺) whereas for others the process is sufficiently fast (as for the nickel (II) analogue [Ni(en)₃]³⁺) that the complex cannot be easily resolved into its optical forms through conventional crystallization methods. The isomerization reaction can be readily followed  by observing the loss of optical rotation at a selected wavelength over time; this is usually a simple first order exponential decay process.

اخر الاخبار

اخبار العتبة العباسية المقدسة

قسم رعاية الصحن الشريف ينشر لافتات الحزن بذكرى هدم قبور أئمة البقيع (عليهم السلام)

قسم صناعة الشبابيك يباشر صناعة كتيبة قرآنية لمرقد الإمامين الكاظمين (عليهما السلام)

قسم الشؤون الفكرية يقدم ورشة عن البوابة العراقية للمعرفة في الجامعة المستنصرية

سماحة السيد الصافي يتشرف بزيارة مرقد الإمامين العسكريين (عليهما السلام) في سامراء

المزيد

الآخبار الصحية

اكتشاف هام قد يمهّد لعلاج جديد للسمنة

تغييرات بسيطة في نمط الحياة تقلل خطر أمراض القلب بنسبة كبيرة

ليس اللب فقط!.. قشرة وبذور المانغو تخفي فوائد صحية مذهلة

زيادة استهلاك الكالسيوم ومنتجات الألبان يحمينا من مشكلات صحية خطيرة

المزيد

الآخبار التكنلوجية

العلماء الروس يكتشفون طريقة تحسن فرص نجاح زراعة الأسنان

لحظة زمنية خاطفة في الدماغ تتحكم في التعلم والحركة

القمر لم يعد مجرد محطة.. خطوة جديدة تغيّر مستقبل البشرية

العلماء يكتشفون "نوعا جديد تماما" من الكواكب

المزيد

مواضيع ذات صلة

Block f : Inner Transition Metals (Lanthanoids and Actinoids)

Block d: Transition Metals

Block p: Main Group Metals

= Block s : Alkali and Alkaline Earth Metals

A New Face - Oxidation-Reduction

Lability and Inertness in Octahedral Complexes

Kinetic Rate Constants

Thermodynamic and Kinetic Stability

Undergoing Change– Kinetic Stability

Overall Stability Constants

Factors Influencing Stability of Metal Complexes

Bedded Down– The rmodynamic Stability