Outer distinction from inner sphere

Traditionally, electron transfer processes in solution and at surfaces have been classified into outer-sphere and inner-sphere mechanisms (1). However, the experimental basis for the quantitative distinction between these mechanisms is not completely clear, especially when electron transfer is not accompanied by either atom or ligand transfer (i.e., the bridged activated complex). We wish to describe how the advantage of using organometals and alkyl radicals as electron donors accrues from the wide structural variations in their donor abilities and steric properties which can be achieved as a result of branching the alkyl moiety at either the a- or g-carbon centers. 

Section 3, are commonly classified as inner-sphere (contact) ion pairs (Kochi, 1988). Accordingly, in organic and organometallic processes a strong distinction must be made in their behaviour from that of other less common outer-sphere ion pairs that are pertinent to the Marcus treatment of electron-transfer dynamics (Eberson, 1987 Lee et al., 1991). 

A simpler situation from the point of view of a theoretical treatment -although more difficult to study experimentally - is electron exchange between ions which constitute two halves of the same redox couple, e.g. MnO /MnO -, Co(NH3) +/Co(NH3)6+ etc. Two distinct types of mechanism have been postulated. In the outer-sphere mechanism, the coordination spheres of both oxidant and reductant remain intact as electrons are transferred, and the oxidation numbers of the central atoms change. The inner-sphere mechanism describes a situation where a bridged binuclear complex is formed as an intermediate, and the bridging ligand - which may be Cl-, OH etc. or an ambidentate ligand like NCS" - provides a pathway for electron transfer. 

Examination of the structures of Ln(III) hydrates in crystals and our knowledge of Ln(III) complexes in solution now throws up a problem which the above equations do not readily meet. There is no certain distinction between inner and outer sphere for ions such as Ln(III). Firstly the inner sphere is constantly switching between 8- and 9-coordination but 9-coordination is not far from 6-innermost water molecules which can distort to an octahedron and 3-outermost water molecules. The steps of kinetics can involve multiple re-arrangements of the cation hydration shell which is itself variable in the series of Ln(III). The model equations above are only guides to thinking. 

The temperature-dependent Raman spectra are depicted in Fig. 4-27a, b. Figure 4-27a shows the spectra of H2O-I (the water molecules in the inner coordination sphere) from 133-223 K. Figure 4-27b shows the spectra of H2O-II (the water molecules in the outer sphere). The spectra above 223 K are not shown because of the overlap with fluorescence that is observed with the 514.5 nm excitation. Plots of the variations of band frequency with temperature are illustrated in Fig. 4-28a, b for H2O-I and H2O-II. Two discontinuities are observed at 195 5K and 140 5K, indicative of three distinct phases occurring in the temperature range studied, as indicated in Fig. 4-28a. The higher-frequency OH stretch region, as shown in Fig. 4-28b does not show any discontinuities for H2O-I. A plot of full width at half maximum intensity (FWHM) vs. T for H2O-I shows a discontinuity at 140 K (Fig. 4-28c, d). Additional support for these phase transitions was found from the temperature dependences of the UO vibrational mode, lattice vibrations and the NO3 ion vibrations (translations and rotations). 

Most importantly, the organometallic donor-acceptor complexes and their electron-transfer activated reactions discussed in this review are ideal subjects to link together two independent theoretical approaches, viz. the charge-transfer concept derived from Mulliken theory [14-16] and the free-energy correlation of electron-transfer rates based on Marcus theory [7-9]. A unifying point of view of the inner-sphere-outer-sphere distinction applies to charge-transfer complexes as well as electron-transfer processes in organometallic chemistry. 

Redox processes between metal complexes are divided into outer-sphere processes and inner-sphere processes that involve a ligand common to both coordination spheres. The distinction is fundamentally between reactions in which electron transfer takes place from one primary bond system to another (outer-sphere mechanism) and those in which electron transfer takes place within a primary bond system (inner-sphere mechanism) (Taube, 1970). 

The importance of Marcus theoretical work on electron transfer reactions was recognized with a Nobel Prize in Chemistry in 1992, and its historical development is outlined in his Nobel Lecture.3 The aspects of his theoretical work most widely used by experimentalists concern outer-sphere electron transfer reactions. These are characterized by weak electronic interactions between electron donors and acceptors along the reaction coordinate and are distinct from inner-sphere electron transfer processes that proceed through the formation of chemical bonds between reacting species. Marcus theoretical work includes intermolecular (often bimolecular) reactions, intramolecular electron transfer, and heterogeneous (electrode) reactions. The background and models presented here are intended to serve as an introduction to bimolecular processes. 

The inner-sphere X can be calculated from the measured force constants of the redox molecule in each oxidation state. It is possible to have distinctly different Aig values for each oxidation state. The outer sphere X for heterogeneous electron transfer can be estimated from the dielectric continuum theory (Eq. 20) [238] ... 

It should be pointed out that the distinction between inner- and outer-sphere complexes is not the same as that of nonspecific versus specific sorption. In an outer-sphere complex, the adsorbate is separated from the surface for (at least) one layer of water molecules, preventing any type of bonding, thus this is always nonspecific adsorption. On the other hand, an adsorbate in an inner-sphere complex may form some sort of chemical bond, but not necessarily for example, in micas K+ ions are often found as inner-sphere complexes, but the interaction is mainly electrostatic no chemical bond is formed. Thus, specific adsorption implies inner-sphere complex, but the opposite is not true. 

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