Inner sphere reactions

If this complex now collapses, it will be the labile Co-Cl bond which is broken, as opposed to the inert Cr-Cl bond. The labile cobalt(ii) complex reacts further with bulk water to generate [Co(H20)6] (Eq. 9.37). The key feature is that a necessary consequence of this inner-sphere reaction is the transfer of the bridging ligand from one center to the other. This is not a necessary consequence of all such reactions, but is a result of our choosing a pair of reactants which each change between inert and labile configurations. In the reaction described above, the chloride... 

For the Co(III) complex Co(NH3)jN02 , Halpern and Nakamura have obtained spectrophotometric evidence for the inner-sphere reaction occurring via Co(CN)sONO which isomerises to give the product Co(CN)sN02 . The species Co(NH3)5CN also reacts in this manner to give Co(CN)sNC " and finally Co(CN). ... 

Candlin and Halpern comment that the sequence of rapid rates observed for Cr " " as a reductant i.e. Co(NH3)5p > Co(NH3)sBr > Co(NH3)5CI > Co(NH3)5F ) is contrary to that found for the slow reactions of Fe (ref. 126) and Eu (ref. 113). All three reductants would appear to favour inner-sphere mechanisms, but in the case of Fe and Eu the order of reactivity seems to be connected with the stability of the product halide complex (FeX or EuX ) which increases in the order X = 1 to X = F . Or in other words, as pointed out by Halpern and Rabani, in the generalised inner-sphere reaction... 

Electrode reactions are inner-sphere reactions because they involve adsorption on electrode surfaces. The electrode can act as an electron source (cathode) or an electron sink (anode). A complete electrochemical cell consists of two electrode reactions. Reactants are oxidized at the anode and reduced at the cathode. Each individual reaction is called a half cell reaction. The driving force for electron transfer across an electrochemical cell is the Gibbs free energy difference between the two half cell reactions. The Gibbs free energy difference is defined below in terms of electrode potential,... 

The Electron Transfer Step. Inner-sphere and outer-sphere mechanisms of reductive dissolution are, in practice, difficult to distinguish. Rates of ligand substitution at tervalent and tetravalent metal oxide surface sites, which could be used to estimate upward limits on rates of inner-sphere reaction, are not known to any level of certainty. 

In the inner-sphere reactions, the process involves a transition state in which a mutual strong penetration of the coordination spheres of the reagents occurs (and, therefore, strong interaction between reagents), whereas in the outer-sphere reactions there is no overlap of the coordination spheres of the reagents (and, therefore, there is weak interaction between reagents). 

It will be very difficult to detect Cu(II)X and Eu(III)X as intermediates because of their marked lability, and therefore hard to characterize Cu+ and Eu + as inner-sphere reductants by product identification. It is easier to detect Fe(III) and V(III) species, by flow methods, and a number of reactions of Fe + with Co(III) complexes and with V(IV), Co(III), and Cr(III) complexes. Table 5.2, have been shown to progress via the intermediate required of an inner-sphere reaction. 

Examination of the data for (5.49) and (5.50) in Tables 5.7 and 5.8 shows that there is some general order of reactivity for the various ligands L. Containing an unshared electron pair after coordination appears a minimum requirement for a ligand to be potential bridging group, for it has to function as a Lewis base towards two metal cations. Thus CofNHj) and Co(NH3)jpy + oxidize Cr by an outer-sphere mechanism, giving Cr " as the product, at a much slower rate than for the inner-sphere reactions. 

As might be foreseen, there are a (limited) number of systems where the energetics of the outer- and inner-sphere reactions are comparable and where therefore both are paths for the reaction. An interesting example of this behavior is the reaction of Cr(H20) with IrClg which has been studied by a number of groups and is now well understood. At 0°C, most of the reaction proceeds via an outer-sphere mechanism. The residual inner-sphere process utilizes a binuclear complex, which can undergo both Cr —Cl and Ir —Cl cleavage ... 

For most of the other reactions the mechanism is uncertain. Among these, the reduction of CofNHs X by Fe+2 and Eu+2 exhibit the reverse trend (—i.e., Br < Cl < F). We think, but are not certain, that these are inner sphere reactions. 

I would appreciate Dr. Halpern s comments on whether he thinks this provided a valid criterion for inner sphere reactions in those cases. 

The concept of electrocatalysis and its relation to chemical surface bonding of reactive intermediates is closely related to that of heterogeneous catalysis. Following the previous section, simple Gibbs energy curves can illustrate the essential ideas of how adsorption of intermediates and their associated Gibbs energy affect the rate of an inner-sphere reaction. 

Inner Sphere Inner sphere reactions are more complicated than outer sphere reactions because, in... 

The rate-determining step in most inner sphere reactions is ihe electron transfer step, noi the formation of the bridged complex. If dissociation of a reBcianl complex were rale determining, firsi-order kinelics would be expected 54 Haim. A. Prog. Inorg. Ckem. 19 3,30, 273-357. 

It is important to notice that the rate of a given outer sphere electrode redox reaction should be independent of the nature of the metal electrode if allowance is made for electrostatic work terms or double layer effects which will, of course, be dependent on the nature of the electrode material. Inner sphere reactions, on the other hand, are expected to be catalytic with kinetics strongly dependent on the electrode surface due to specific adsorption interactions. 

A related example of inner-sphere reaction is shown in reaction (5), where an additional mechanistic subtlety appears.10 As shown by the products, inner-sphere electron transfer also occurs but now by remote attack in which the sites of bridging ligand binding to the reductant and oxidant are at different atoms on the bridging ligand. 

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. 

---