Inner-sphere reactions Scheme

The series of elementary steps which constitute the overall electron transfer mechanisms for outer-sphere and inner-sphere reactions are illustrated in Schemes 1 and 2. 

For an inner-sphere reaction there are necessarily more steps since both association and substitution must precede electron transfer. Intermediates like (H20)5CruClCoUI(NH3)54+ and (H20)5CrinClCoII(NH3)54 shown in Scheme 2 are often referred to as the precursor and successor complexes since they precede or follow the electron transfer step. 

In contrast to outer-sphere reactions, the simple observation that a reaction occurs by an inner-sphere mechanism necessarily introduces an element of structural definition. The relative dispositions of the oxidizing and reducing agents are immediately established and, except for structurally flexible bridging ligands such as NC5H4(CH2) C5H4N, the internuclear separation between redox sites can be inferred from known bond distances. Even so, bimolecular inner-sphere reactions necessarily occur by a sequence of elementary steps (Scheme 2) and the observed rate constant may include contributions from any of the series of steps. 

A principal mechanistic question for alkyl halide reductions has been the degree of bonding of the metal to the halide at the transition state, that is, whether the reaction is inner-sphere in the Taube sense (see Scheme 1). The Ms symbol refers to solid metal, and less is known about reactions that occur at solid surfaces than those in solution because of problems with studying kinetics (unless the surface is an electrode). Rieke and coworkers emphasize quantities that can be measured, the ln(A Rx/A -Rx) for secondary (j-RX) and tertiary (/-RX) halides relative to primary ones (n-RX), which they summarize for 14 reactions [31]. The Co /RI, Cr /RBr, and Cr 7 Cl cases are completely established as Taube inner-sphere reactions [10]. They show the largest selectivity, ln(A ,-Rx/A -Rx) and ln(A 5-Rx)/A n-Rx) values of over three and one, respectively. At the other extreme, reaction of RBr with magnesium is extremely unselective [41], and is now usually accepted to be outer-sphere. 

In principle, any of the steps in Scheme 2 can be rate limiting from the diffusion controlled formation of the association complex to the substitutional breakup of the successor complex. Because of the multiplicity of steps, a detailed interpretation of an experimentally observed rate constant can be extremely difficult. However, in some cases it has been possible to obtain direct or indirect information about the electron transfer step in an inner-sphere reaction using chemically prepared, ligand-bridged dimeric complexes. For example, reduction of the Co -Ru precursor to the product shown in equation (7) by Ru(NH3)6 + or Eu + occurs selectively at the Ru " site. The initial reduction to give Ru is followed by intramolecular electron transfer from Ru to Co which is irreversible since the Co site is rapidly lost by aquation. ... 

Mechanisms for the electrochemical processes at mercury electrodes in solutions of [Ni(cyclam)] + and CO2 have been proposed (see Scheme 5.1 ). Scheme 5.1 shows the formation of a carbon-bonded Ni(II) complex by reaction of CO2 with Ni(cyclam)+. The formation of such a complex is considered to be a fundamental step in the mechanism of the [Ni(cyclam)] +-catalyzed electrochemical reaction. The overall process for the transformation of CO2 into CO also involves inner-sphere reorganization. Scheme 5.1 includes the formation of sparingly soluble complex containing Ni(0), cyclam and CO which is a product of the reduction of [Ni(cyclam)] + under CO. Depositation of a precipitate of the Ni(0) complex on the mercury electrodes inhibits catalysis and removes the catalyst from the cycle. The potential at which the [Ni L-C02H] + intermediate (see lower left hand of Scheme 5.1) accepts electrons from the electrode. This potential is not affected by substitution on the cyclam ring, as shown by comparison of [Ni(cyclam)] + and [Ni(TMC)] " (TMC = tefra-iV-methylcyclam)... 

The reaction between [U02] + and Cr + has been monitored using flow techniques and results in the formation of a dinuclear [U Cr i] complex. The rate law is first order with respect to both and Cr with no hydrogen-ion dependence in the concentration range 0,3—2.0M. A possible mechanism postulated involves the rate-controlling transfer of an electron after the rapid formation of an inner-sphere complex (Scheme 5). 

Insertion of the multiple bonds usually is considered as an inner-sphere process that takes place after binding of the Het group and after coordination of the rmsaturated organic molecule. Depending on the nature of metal complex and on the type of reacting molecules, inner-sphere reaction can be depicted either as four-centered cycloaddition-like process (A, Scheme 3) or as an attack of the Het group on the coordinated multiple bond (B, Scheme 3). Preliminary coordination of the multiple bond in the form of tt-complex is usually required to carry out irmer-sphere reaction, and such coordination increases the reactivity of the unsaturated molecule (i.e., activation of C=C and C=C bonds). In both cases the same syn-addition product is expected and the reaction involving alkynes leads to vinyl derivatives (C, Scheme 3). 

The question of much interest is the difference between the transient structures A and B in the inner-sphere reactions of alkynes and alkenes (Schemes 3 and 4). On the moment it is unclear whether it is the same process just depicted in different ways or this really corresponds to a change in the mechanism depending on the nature of the reacting system. Future studies on the subject are anticipated to shed some light on this fascinating problem. 

Scheme 3 Inner-sphere reaction of alkynes (Het-heteroatom)...

The scheme in Fig. 9-5 above illustrates the case in which the bridging ligand, X, is transferred from metal center Mi to M2 in the course of the reaction. Although this is not a necessary consequence of an inner-sphere pathway, it is often observed, and provides one method for establishing the mechanism. 

A more interesting situation is found when the homogeneous redox reaction is combined with a chemical reaction between the electrocatalyst and the substrate. In this case, the catalytic process is called chemical catalysis. 3 This mechanism is depicted in Scheme 2 for reduction. The coupling of the electron transfer and the chemical reaction takes place via an inner-sphere mechanism and involves the formation of a catalyst-substrate [MC-S] complex. Here the selectivity of the mechanism is determined by the chemical step. Metal complexes are ideal candidates... 

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