A New Face - Oxidation-Reduction

Oxidation-reduction (electron transfer) reactions are important in chemistry and biology. When a chemical oxidation of A by B occurs B itself is reduced - an electron transfer process has occurred. For such chemical processes, there is always a partnership between an oxidant (which is reduced in carrying out its task) and a reductant (which is oxidized in the reaction); thus we frequently talk of oxidation-reduction, or (for ease of use) redox, reactions for what are essentially electron transfer processes. Of course, an electron can be

Figure 5.18

A reaction involving electron transfer alone. The Fe(II) centre is oxidized to Fe(III) and the Ir(IV) centre is reduced to Ir(III), without any change in the ligand set for either complex.

supplied or accepted electrochemically via an electrode rather than through using a chemical reducing or oxidizing agent, and this is another way to initiate change in the oxidation state of complexes. Here we shall concentrate mainly on chemically based systems, where an oxidant and reductant of appropriate potentials need to be combined. A reaction will be favourable if E" > 0 with E being the difference between the standard potentials for the two half-reactions (one for the oxidation part, one for the reduction) that are combined to form the overall reaction.

There are two processes that are important in inorganic redox reactions. The key process is of course electron transfer. However, some reactions also involve atom transfer whereby a component of one reacting molecule is transferred to another during the reaction. It is not essential for both to occur, and many reactions are purely electron transfer reactions. A classical example of a pure electron transfer alone is the reaction shown in Figure 5.18.

This reaction of the two octahedral complexes occurs without any change in the coordination spheres, or ligand sets, of either metal. However, if you inspect the two metal centres, you will note that the iron complex (the reductant) is oxidized from Fe(II) to Fe(III) and at the same time the iridium complex (the oxidant) is reduced from Ir(IV) to Ir(III) - an electron has been transferred from one metal to the other. This reaction can be conveniently followed, since the colour of each species changes as it is converted from one oxidation state to another.

Atom transfer alone can also occur, although it is a more difficult concept to come to grips with. Think of it as an atom moving with its normal complement of valence electrons, and no others from one central atom to another. Oxidative addition reactions in organometallic chemistry can be considered as a form of atom transfer. A typical example of this type is given in Equation 5.53.

[IrCl(CO)(PR3)₂] + Cl₂ → [IrCl₃(CO)(PR₃)₂]          (5.53)

Some reactions occur with both electron transfer and atom transfer. A simple example of this class is the reaction of two octahedral complexes shown in Figure 5.19.

In the above example an electron is transferred from the Cr(II) to the Co(III) and a chloride ion is transferred from the cobalt to the chromium ion as well. The Cr(III) product is inert, allowing it to be isolated and identified, thereby defining the presence of the chloride ion in its coordination sphere. Colour changes in this reaction allow the reaction to be readily monitored spectrophotometrically.

Figure 5.19

An example of electron transfer with associated atom transfer. Clearly the different processes of electron transfer alone and electron transfer along with atom transfer, illustrated above, need not, and most likely do not, occur by a common mechanism - in fact, there are two general mechanisms for electron transfer involving metal complexes. These are called the outer sphere mechanism and the inner sphere mechanism and we shall examine each in turn to learn about their characteristics. Both of course involve electron movement from one metal to another a process involving 'tunnelling'. This is a quantum-mechanical process associated with the wave nature of electrons that permits passage through an energy barrier that would otherwise be too high to allow the process to proceed; an understanding of this process is, fortunately, not essential to obtaining an understanding of the mechanisms at a basic level.

1 The Outer Sphere Electron Transfer Mechanism

The key to the outer sphere mechanism is that electron transfer from reductant to oxidant occurs with the coordination shells (or spheres) of each reactant staying intact throughout. Since the coordination (or inner) sphere, that is the set of bound ligands, is not changed during the reactions it appears that the key to the process lies beyond these, in the outer sphere around the reactants. We have seen an example earlier involving two different metal centres. Another classical example of pure electron transfer alone, involving two oxidation states of the one metal ion, is Equation (5.54).

This reaction of the octahedral complex with 2,2'-bipyridyl chelates occurs rapidly (k = 5 x 10 M1 s1), but there appears to be nothing happening at first glance. However, if you inspect the two differentiated Os complex ions (differentiated above by one being underlined), you will note that one is Os(III) on the left and Os(II) on the right, and vice versa - an electron has been transferred from one complex ion to another, although there is no colour change in the reaction as the products equate with the reactants exactly. The only way this reaction can be conveniently followed, and indeed shown to occur, is by using an optically active form of the complex in one of the two oxidation states in the initial mixture since its optical properties change as it is converted from its original oxidation state to the other, even though the overall percentage of Os(II) and Os(III) complex remains constant. Another example involves the Ru³ aq/Ru²⁺ aq system, where the self-exchange process may be distinguished through having different oxygen isotopes present in the two aquated

There is no scrambling of the isotopically-labelled water between metal ions; all water molecules remains completely with the one metal centre throughout the reaction, because the rate at which water is exchanged with bulk water is very slow compared with the rate at which the electron transfer reaction occurs. The reaction is observed to be first order with respect to both reactants, consistent with a bimolecular encounter process.

There is a sequence of elementary steps that must occur for an outer sphere reaction to proceed. Firstly, the oxidant and reductant complexes must come together sufficiently close and in an arrangement in space appropriate for electron transfer to proceed, through collision and rotational orientation processes. Because electron transfer over long distances is a high energy process, close approach of reactants to short separation distances is important; the upper limit to this distance between metal centres for an effective reaction is uncertain, but appears to be~1000 pm, although examples of exchange over longer distances are known. To achieve optimal contact, the complexes usually shed some of their outer sphere layer of tightly bound solvent molecules so as to make a close and specifically oriented approach appropriate for orbitals involved in the electron transfer to interact sufficiently, sometimes termed 'forming an encounter assembly' or 'precursor assembly'.

Next, this activated assembly (sometimes called an ion pair') undergoes extremely rapid electron transfer, with subsequent adjustment of the successor complexes to their new oxidation states (sometimes termed 'relaxation'). Since metal-ligand bond lengths vary for many metal ions in different oxidation states, the size of the ions may change as a result of electron transfer, which becomes a key part of the adjustment process. For example Co(III)-N distances are typically 195 pm, whereas Co(II)-N distances are near 210 pm. For cobalt complexes of N-donor ligands, there is consequently a significant change in complex ion size as a result of electron transfer associated with the shrinking or expanding first coordination sphere tied to these bond length variations. For some ions, however, the bond distance change with metal oxidation state alteration is small; ruthenium is such an ion with Ru (III)-N distances of 210 pm and Ru (II)-N distances of 214 pm. Low-spin iron cyanide complexes are another system displaying small changes in bond distances, as exemplified by Fe(III)-CN distances of 192 pm and Fe (II)-CN distances of 195 pm.

A significant change in ion size will perturb the solvent molecules in the immediate outer sphere of the complex ions and solvent reorganization processes that occur will add to the free energy of activation. This is shown by comparing reactions involving Ru(III) and Ru(II) where substantially smaller changes occur than for Co(III) and Co (II). Because there is little expansion or contraction of the complex ions, electron transfer reactions between Ru (III)/Ru (II) compounds will involve less energy-demanding solvation sheath rearrangement effects than for Co (III)/Co (II) and reactions should be faster; this is observed experimentally. For simple systems, rates can vary significantly even allowing for different ligand sets; for example [Co (NH₃)613+/+ (10-6 M^-1 s¹ in water at 298 K), [Ru (OH₂)613+2+ (45 M^-1 s^-1) and [Fe (CN) 3-4 (700 M¹ s¹). This is consistent with an observation that electron exchange is more rapid wherever there is no movement of atoms (no stretching or compression of metal-ligand bonds) in reactants and products (called the Frank-Condon principle).

Finally, following electron transfer, there is no need for the complex ions often of the same charge, to remain in close contact and they relax and move apart. This process of the

Figure 5.20

Electron transfer processes octahedral Ru (top) and Co (bottom) complexes.

successor assembly breaking up to form separated ions is a form of dissociation, but in this case of whole complex ions.

If we return to ligand field theory recall that the d electrons for an octahedral complex lie in the tag and e* levels also designated as π and σ* respectively to equate with the character of bonding in which they participate in a complex exhibiting both σ-donor and π-donor/acceptor bonding character. For electron transfer, a metal d electron needs to move from a location in a π or σ orbital on one metal ion to a π or σ* orbital on the other metal ion. Generally, it will be more favourable for an electron to move between orbitals of the same symmetry (or from like-to-like orbitals); that is → or σ *→σ* transitions are energetically favoured. Further the character of π and σ orbitals differ and so the electron transfer process will be affected by the nature of the donor and acceptor orbitals. Models predict that the d orbitals are more 'exposed' than do orbitals, thus more able to interact with orbitals on a different metal complex, and as a result it is anticipated that → electron transfer should occur faster than σ*→σ* electron transfer. Let's examine this for a Ru (III)/Ru (II) and a Co (III)/Co (II) couple.

For Ru (III)/Ru (II) we have a process operating as defined in Figure 5.20, with a vacant position in the π orbital of a Ru (III) ion able to accommodate an electron transferred from the π orbital of a Ru (II) ion - a favourable π→π process. Moreover the bond lengths for similar donor groups in Ru (III) and Ru (II) complexes are very similar (typically within 5 pm), so there is little solvent rearrangement required. As a result of efficient electron transfer between π orbitals along with small bond length changes following electron transfer the electron transfer reaction rate is predicted to be fast in this example. For Co (III)/Co(II) we have a process operating (Figure 5.20) with vacant positions only in the σ orbitals of a low-spin Co (III) ion able to accommodate an electron transferred from the σ* orbital of a Co (II) ion; a less favourable σ*→σ* process operates.

Moreover, the resultant electron arrangement of this electron transfer shown in the lower centre of Figure 5.20 is an unfavourable excited state for both the Co (II) and Co (III) ions; both need to undergo electron rearrangement (a change in spin multiplicity) to restore their normal ground state arrangements. In addition, the bond lengths for similar donor groups in Co(III) and Co(II) complexes differ significantly (typically by 20 pm or more) so there is substantial solvent rearrangement required. As a result of a o→σ* electron transfer process and significant bond length changes following electron transfer, the electron transfer reaction rate is predicted to be slow in this example. Experimental support of the above arguments comes from the observation that outer sphere electron transfer reactions of Ru systems are commonly at least 10-fold faster than for equivalent Co systems.

The reactions of cobalt (III) in mixed-metal outer sphere electron transfer reactions are usually slow also, as a result of the influence of effects discussed above. The reaction (5.56) of the hexaamminecobalt (III) ion with the hexa aqua chromium (II) complex

occurs slowly with a rate constant of ~10-3 M1 s1. Decomposition of the initial Co (II) ammine complex product in the aqueous acid reaction conditions to release ammonium ion and form the Co2+aq ion is characteristic of the outcome for Co (III) systems upon forming a labile Co (II) product.

However, the similar reaction (5.57) is substantially faster (a rate constant of ~

M-1 s-1).

This is despite the fact that the sole change in this reaction compared with that above it, is the replacement of one ammonia ligand by one chloride ligand in the precursor Co (III) complex. Such a substantial change of ∼10⁹-fold for such an apparently minor change is unprecedented if the same mechanism is operating. The obvious conclusion is that a different mechanism occurs in this example. It was observations like this that pointed to an alternative mechanism, the so-called inner sphere mechanism.

The Inner Sphere Electron Transfer Mechanism

The key to the inner sphere mechanism is that electron transfer from reductant to oxidant occurs across a bridging group, which is a ligand shared between the reductant and the oxidant, and in the coordination sphere of both during the activation step. This bridging group has one obvious requirement - it must be able to bind to two metals simultaneously. which means that it must have two lone pairs on one or several donors orientated in such a way as to accommodate this shared arrangement. Of course there are many examples of stable compounds where ligands bridge between two metal centres, in dimer and higher oligomer complexes, so invoking such a short-lived intermediate in a reaction profile does not introduce a large leap away from conventional chemistry. The chloride ion is an example of a single-atom ligand capable of bridging, being replete with lone pairs of electrons; many examples of M-Cl-M coordination exist. Another type commonly met is an ambivalent ligand with two different donor atoms available, such as thiocyanate (SCN), which offers both a N and a S donor, and is able to form M-SCN-M linkages.

The basic process can be exemplified by the reaction of the cobalt (III) complex [Co (NH₃) s(NCS)]²⁺ (the oxidant) with the chromium (II) complex [Cr (OH₂)₆]²⁺ (the re- ductant), where thiocyanate acts as the bridging ligand. The overall reaction in aqueous acidic solution is shown in Equation (5.58).

The original inert Co(III) complex with N-bound thiocyanate has been reduced to a labile Co(II) complex, which has dissociated into free protonated ligands and the Co(II) aqua ion. Not only is the labile Cr(II) centre oxidized to an inert Cr(III) complex, but an S-bonded thiocyanate group has been introduced into its coordination sphere. The only source of this ligand is the original Co(III) complex. While this anion may have been captured from solution following dissociation of the reduced Co(II) species, the observation that it is bonded entirely by the S-donor group suggests that it may have been transferred in a 'handshake' operation where it is shared between the cobalt and chromium centres; an intermediate of the type {[(H₃N),Co-NCS-Cr(OH₂)s]+) is presumed to form, where one water group in the labile Cr(II) complex has been displaced and replaced by the thiocyanate S atom, as the N atom is already attached to the Co centre. The overall mechanism that has been proposed is shown in Figure 5.21, for a general bridging ligand represented as X-Y.

Figure 5.21

Electron transfer between Co(III) and Cr(II) complexes by the inner sphere mechanism, involving bridge formation in the intermediate. Transfer of the bridging group from the oxidant to the reductant is not required, but is characteristic of the mechanism where it does occur. Key experiments by Nobel laureate Henry Taube and co-workers, involving complexes where chloride ion acts as a bridging ligand, cemented the mechanism. One reaction probed was the classical reaction mentioned earlier (Equation 5.59):

which is similar to the thiocyanate-bridged reaction discussed above, but with chloride ion as the potential bridging group. The study made use of chloride ion introduced as an enriched isotope, rather than as the natural isotopic mixture; by following the fate of the isotope in the reaction, mechanistic information could readily be gleaned. Using isotopic (or 'labelled') chloride ion (Cl), it was found that the original cobalt-bound halide transferred completely from precursor to the chromium product. If the labelled chloride was on the cobalt initially. it was transferred fully to the chromium product, even in the presence of added natural- isotope chloride ion in solution; conversely, using unlabelled cobalt-bound chloride and labelled free chloride in solution, labelled chloride is not introduced into the chromium product. These experiments effectively require a 'tight' interaction between oxidant and reductant, best viewed as one where an intermediate that shares the transferring ligand exists, that is {[(H3N)s Co-Cl-Cr(OH₂)s]₄₊}, which means an inner sphere mechanism. The process of bridge formation can sometimes lead to a marked acceleration of the electron transfer process compared with outer sphere reactions of related compounds. In general, the key requirements for an inner sphere mechanism to operate can be summarized as follows:

(a) one reactant must possess at least one ligand capable of binding simultaneously to two metal ions in a bridging arrangement (this reactant is often the oxidant); and (b) at least one ligand of one reactant must be capable of being replaced by a bridging ligand in a facile substitution process (this replaced ligand in many examples is found on the reductant).

The latter requirement means a relatively labile site must be available, and often involves a coordinated water group. Note that atom transfer is not a requirement in this mechanism. However, it can occur, and its occurrence is usually good evidence for the mechanism. For example, when atom transfer occurs with an ambidentate ligand (like SCN-), the donor preferred and bound by the precursor metal ion is often not the one attached to the stable product, as described earlier, this helps define the mechanism. Nevertheless, the bridging ligand may remain with its parent metal ion.

The type of bridging ligand can affect the observed electron transfer rate markedly. Its role is to bring the metal ions together, and mediate the transfer through itself. For the reaction discussed above but extended to employ a range of halide ions (Equation 5.60). the rate increases with increasing size of the halide ion in the ratio for F: Cl: Br: I of 2:6 : 14:30.

This is assigned to increasing polarizability with increasing halide ion size. The higher- charged Co(III) attracts more halide electron density towards its side of the bridging unit, depleting the side near the Cr(II) from where the electron departs and thus facilitating attraction of the transferring electron to the bridging ligand.

Figure 5.22

The proposed intermediate in the reaction of [Cr(OH)²⁺ and [Co(NH), (OOCR)]²⁺; the size of the R-group impacts on the reaction rate.

The effect of altering the bridging group can be seen more starkly in the reaction of Cr²⁺ aq with the carboxylate complex [(NH3), Co(OOCR)]²⁺, where it arises from a different cause. Attack by the Cr(II) ion at the carboxylate carbonyl of the Co(III) complex leads to the proposed intermediate in Figure 5.22, and rate constants clearly vary with the size of the R group, as a result of increasing steric bulk of the R-group limiting facile bridge formation; that is, there is a clear steric effect on the electron transfer rate.

Another obvious expectation of the inner sphere mechanism is that the electron is trans- ferred via the ligand; think of it like a ball rolling across a bridge. If this is the case, then for some time, albeit very short, the electron will reside on the bridging ligand, leading to a ligand radical. This has also been probed, using as a bridging ligand one that includes an aro- matic heterocyclic group. Such groups help stabilize ligand radicals, and the presence of the radical intermediate has then been detected by electron spin resonance spectroscopy, which is a highly sensitive method for detecting radicals. Thus, overall, experimental evidence for the inner sphere mechanism is strong.

The two mechanisms of electron transfer, or oxidation-reduction reactions, for metal complexes have some common aspects, with which we shall finish this section. These are the keys to electron transfer reactions. To occur efficiently, the molecular orbital in which the donated electron originates, and that to which it moves, should be of the same type. The most efficient outer sphere electron transfer requires transfer between π orbitals. Effective inner sphere electron transfer occurs with transfer between either both π or both σ* orbitals. The chemical activation process prior to electron transfer will be much greater if the above does not apply, meaning that under such circumstances reactions may not occur, or occur only very slowly. Further, structural deformation and/or electron configuration changes may be necessary (costing energy, and thus raising the activation barrier), and reactions under these conditions will, in general, be slower than those requiring little solvent reorganization and/or no electron configuration change. Overall, only where the activation barrier leading to the intermediate is sufficiently low will reactions proceed at measurable rates.

Electrochemical and Radiolytic Electron Transfer Reactions

It is useful to note that there are other methods for supplying electrons to complexes that will lead to a change in oxidation state. Whereas oxidation-reduction reactions are partnerships between two compounds, an alternative is to offer a direct source of or sink for electrons; this is achieved by electrodes in an electrochemical cell. At the appropriate potential, a   complex may be reduced by transfer of an electron from an electrode to the complex; oxidation can occur by transfer of an electron to an electrode. What is perhaps apparent is that, for a solid electrode and a solution of a complex, it is only near the surface of the electrode that the process can occur efficiently, so both the rate of transport to an electrode surface, as well as the rate at which an electron is transferred between solid surface and dissolved complex ion, are of relevance. In general, we can write a simple expression for the process (represented here for reduction) (Equation 5.61):

The oxidation and reduction potentials of complexes are usually probed by the experimental methods of voltammetry, which is a sensing rather than a complete conversion technique; the full reduction or oxidation of a complex is achieved by coulometry, using large surface area electrodes and stirring to enhance mass transport and thus speed up the process. These techniques and their application are beyond the scope of this textbook.

Yet another, and perhaps more exotic, method of supplying an electron in solution is to use radiolysis, where a very high energy source is directed into an aqueous solution to generate a range of products from reaction with the abundant water molecules, with the aquated electron (eq) and the hydroxyl radical (OH) being dominant species, and of particular importance. The former is a powerful one-electron reductant, the latter a powerful one- electron oxidant. The technique requires the use of high energy devices like a van der Graaf generator or synchrotron, so is not widely available. However, the powerful radical species formed can initiate definitive one-electron reduction or oxidation of complexes, either at the metal centre (direct reduction or oxidation) or at the ligand (with metal reduction or oxidation via a following intramolecular electron transfer possible). The former process is a form of outer sphere reduction, with eaq as the reductant here. The latter process is followed by electron transfer from the ligand radical to the metal ion, which is, in effect, 'half of the reaction in the electron transfer through a bridge in a chemical inner sphere process. When the high energy source is pulsed (the technique is called pulse radiolysis), a reaction in solution can be initiated rapidly and the outcome followed kinetically. Electron transfer reactions between two metal complexes can be initiated by pulse radiolysis also under certain circumstances. For example, if a relatively high concentration of Zn(II) and a low concentration of a Cr(III) complex are both present in solution, creation of eaq causes it almost exclusively to react rapidly with the Zn(II), because it is in such very large excess, to form the extremely rare and unstable Zn(I) ion. Then two reaction can occur with the highly reducing Zn(I), in competition: intermolecular electron transfer between Zn(I) and the Cr(III) complex; and disproportionation of Zn(I) to form equal amounts of Zn(II) and Zn(0). By examining colour change associated with the chromium centre, the intermolecular reaction can be examined. However, this technique is not one you are likely to meet often.

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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 Body - Stereochemical Change

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