Mixed Outer- and 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  

The outer-sphere rate constant for the Cr(H20)5+/IrClg reaction can be estimated, using /larcus equation, as =10 M s . A value of this magnitude can obviously be competitive dtb that for the inner-snhere nath. which is more usual with the hiehlv labile CrtH,0)i  

A rather involved, but interesting, example of a reaction which proceeds by both inner-outer-sphere pathways is summarized in the scheme 

The outer-sphere pathway k ) produces the final products directly, as shown by a rapid increase in absorbance at 474 nm, which is a maximum for Ru(NH3)5pz. At the same time, a rapid inner-sphere (A ,) production of the binuclear complex takes place. A slower absorbance increase at 474 nm arises from the back electron transfer in the binuclear intermediate This produces the original reactants which then undergo outer-sphere reaction (Atj). 

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The role of the outer solvation shell in mixed solvents was also studied using the CoEn system as a model [302] (En = ethylenediamine). In this system the inner sphere of the substrate (CoEn ) and the product (CoEnl ) was not changed in the course of the one-electron electrode reaction. Therefore, the changes in the rate constant (determined by chronocoulometric method), observed when the composi-... 

Relaxation of complicated ligands may occur as a step in both pathways. Diebler and Eigen 461 indicated the ways in which such mechanisms could be analysed using fast reaction methods. Several studies of Ln(III) complex formation and of the formation of Ln(III) mixed complexes have been analysed. Generally the dissociative mechanism is considered to dominate and we are then concerned with the water exchange rate. Several studies have shown that the rate decreases from La(III) to Lu(III) but there seems to be a minimum rate around Tm(III). This is also seen in the rate of rotation of ligands on the surface of the ions, Fig. 7. There may be a small crystal field term, or another contribution to a tetrad -like effect from the 4f electron core. However in the hydrate the precise relationship between the inner and outer sphere water may also be important as we saw when we discussed the heat and entropy of complex ion formation. 

The EM and titration data are also essential to evaluate the possibility of mixed reaction mechanisms. For example, both FTIR and EXAFS can be used to determine the presence of specific bonds or complexes. Pure inner-and outer-sphere complexes may in fact be endmembers in some systems, while a mixture of hydrated and nonhydrated bonds may result in average hydration values intermediate between zero and one. Indifferent ions such as Cl, CIO/, NOj, Na and do not shift the PZC of oxides (6). In contrast, 804 has been found to adsorb specifically and produce a shift in the PZC of hematite... 

It was shown that, in addition to the trihalo complexes that bind the solvent in the inner coordination sphere, tetrahalo complexes with a regular tetrahedral inner coordination sphere are also formed, and that solvent molecules are bound in the outer coordination sphere of the latter complexes. This outer-sphere solvent coordination can also be followed by means of observing its effect on mixed complex formation reactions of the following type [Ye 74] ... 

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