Outer-sphere effect

Ten years ago Rorabacher (13) observed the substitution rate constants for aquonickel(II) ion with different amines (Table II). There is a decrease in the rate constants by a factor of 14 in going from ammonia to dimethylamine. If nickel-(II) substitution reactions are dissociative, then why is the effect this large Is this a steric effect with some associative contribution or is it an outer-sphere effect There has been surprisingly little investigation of the nature of the entering ligand so far as its bulk or its nucleophilicity is concerned even for what have been generally considered as simple substitution reactions. 

If one goes to still lower temperatures, one usually sees evidence for outer-sphere effects, which is what you are referring to. In the cases which I have discussed here, either one does not see any evidence for outer-sphere effects or else one can choose to operate in a region where the outer-sphere effects are known to be unimportant. 

We have investigated the ferrocene/ferrocenium ion exchange to determine the effects of different solvents on electron-transfer rates. There is probably only a very small work term and very little internal rearrangement in this system. Thus the rates should reflect mostly the solvent reorganization about the reactants, the outer-sphere effect. We measured the exchange rates in a number of different solvents and did not find the dependence on the macroscopic dielectric constants predicted by the simple model [Yang, E. S. Chan, M.-S. Wahl, A. C. J. Phys. Chem. 1980, 84, 3094]. Very little difference was found for different solvents, indicating either that the formalism is incorrect or that the microscopic values of the dielectric constants are not the same as the macroscopic ones. 

Attempts were made to investigate the relaxivity enhancement produced by an MFI-type zeolite with framework-incorporated gadolinium (84). A relaxivity less than 3 s mM was found at room temperature and at 40 MHz. This indicates that the accessibility of gadolinium to water plays a very important role, and that the outer sphere effect might be very small. 

The strong hydration of halide ions in water is partly due to outer-sphere effects. Oxo anions in aqueous solution may complex with outer-sphere water molecules as in the hydrated chromate and permanganate ions ... 

Other factors, for example the charge on the complex or outer sphere effects such as ion pairing, will also have an effect on E° but it is not possible to predict the exact relationship of E° to these factors. Nevertheless, they should always be borne in mind. 

Outer-sphere effects occur also when an anion is acting as the... 

A common feature of self-ionizing solvents are the amphoteric properties of the solvent molecules, since they may act both as EPD and as EPA (27). It appears therefore that outer-sphere effects, similar to those existing between polarized solvent molecules of solvated ions, occur in the pure liquids resulting in mutual polarization of solvent molecules within associated units. From the functional point of view this is described in the following way ... 

The hydration of U(iv) in acid solutions has been followed by relaxation time (Tj and T2) measurements. (173) The relaxation rates observed in perchloric acid solutions at high temperature are governed by the exchange process of water molecules between the inner coordination sphere of U(iv) and the bulk water. In contrast, at low temperatures the rates are dominated by an outer sphere effect. NMR... 

Bass JD, Solovyov A, Pascal AJ, Katz A (2006) Acid-base bifunctional and dielectric outer-sphere effects in heterogeneous catalysis A comparative investigation of model primary amine catalysts. J Am Chem Soc 128 3737... 

Lancaster KM, Sproules S, Palmer JH, Richards JH, Gray HB (2010) Outer-sphere effects on reduction potentials of copper sites in proteins the curious case of high potential type 2 C112D/M121E Pseudomonas aeruginosa azurin. J Am Chem Soc 132 14590-14595... 

In a subsequent paper [100], the dual emission displayed by these Ir(III) complexes has been examined in greater detail. The conclusion was reached that the dual emission is related to the presence of Franck-Condon barriers originating from a combination of inner-sphere and outer-sphere effects analogous to those encountered in electron transfer processes. 

A subsequent study by Shu and Rorabacher [122] has been directed at outer-sphere effects in methanol—water mixtures. To reduce inner-sphere effects to a minimum, a quinquedentate ligand (N-hydroxyethylethylene-diamine-N,N, N -triacetate, HEEDTA) was complexed to Ni(II) leaving a single coordination site available for ammonia—solvent exchange... 

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