Outer-sphere species

Particular use was made of conductivity measurements of cobalt(iii) and platinum(ii) complexes which allowed a facile determination of the number and type of ions present in solution. For example, the compounds Co(NH3) Cl3 would give a monocation and an monoanion (n=4), a dication and two monoanions (n = 5) and a trication and three monoanions (n=6) respectively. In some cases, it was also possible to distinguish chemically between inner and outer sphere chloride by precipitation of the outer sphere species as AgCl. 

Cations attracted to colloid surfaces through their waters of hydration are said to be outer-sphere species, whereas those that interact directly with the oxygen atoms present on the surface are called inner-sphere species. Of the two, the latter species will be more strongly bonded and harder to extract than will be the outer-sphere species. 

Several different types of species are illustrated in Figure 6.1. The potassium cation (K+) at the top of the figure is separated from the soil surface by water molecules and would thus be considered an outer-sphere species. The potassium cation near the bottom of the figure is directly connected to the soil particle by an ionic charge and is therefore an inner-sphere species. Above this is an inner-sphere phosphate directly bonded to a soil surface aluminum. Also shown are potassium cations attached (inner sphere) to colloidal clay (CC) and colloidal soil organic matter (COM). Each of these is a different species. 

Complexation reactions are assumed to proceed by a mechanism that involves initial formation of a species in which the cation and the ligand (anion) are separated by one or more intervening molecules of water. The expulsion of this water leads to the formation of the inner sphere complex, in which the anion and cation are in direct contact. Some ligands cannot displace the water and complexation terminates with the formation of the outer sphere species, in which the cation and anion are separated by a molecule of water. Metal cations have been found to form stable inner and outer sphere complexes and for some ligands both forms of complexes may be present simultaneously. 

The first step is diffusion controlled while the second represents the fast formation of the outer sphere complex. The final step involves the conversion of the outer to the inner sphere complex. This is the rate determining step and is dependent on the equilibrium concentration of the outer sphere complex. Consequently, calculations of rate constants by the Eigen model involves estimation of the formation constant of the outer sphere species. 

The formation of a blocking film on an electrode surface will decrease the capacitance compared to that of the bare electrode, since the distance of closest approach of the counter ions, d, will be increased by the thickness of the layer [see (13.3.2) and Figure 14.5.5]. The extent of blocking by the monolayer and the presence of pinholes can be assessed in a number of ways (88). To obtain the aggregate area of the pinholes one can, for example, compare the sizes of voltammetric peaks for the bare and filmed electrode (such as those for the formation and reduction of an oxide layer on Au). To obtain the spatial distribution, one can deposit a metal like Cu, then strip the film and perform microscopy on the resulting surface. A frequently used procedure is to observe the chronoamperomet-ric or cyclic voltammetric behavior of an outer sphere species like Ru(NH3) in solution... 

Complex formation is proposed in the mechanism of oxidation of hydrazinium ion by hexachloroiridate(rv). The observed rate law, -d[Iriv]/(j/ rjfj-jriv]. [N2H5+]/([H+] + T[N2H5+]), is consistent with a mechanism involving the rapid formation of the outer-sphere species [N2H4lrCl ] with a slow subsequent redox step ... 

The inertness of the surface raises interesting questions. The aqueous solvent window is pushed out as a result of water electrolysis being an inner-sphere mechanism. As a result, it is often stated in the literature that BDD can detect species which other electrodes cannot due to the extended solvent window. This is certainly true of outer-sphere species, but care must be taken when considering inner-sphere species. Heterogeneous ET will be retarded for many of these species on BDD, as there are no favorable adsorption sites, pushing out their electrochemical detection potential. Therefore, each species should be considered on a case-by-case basis, in combination with the effect of surface termination. For example, both oxidation [89] and reduction, in... 

Another aspect which is dealt with in this review refers to the complexation reactions between the metal ion, R, and different neutral or charged ligands, L" . In general, two types of complexes may be formed in solution outer-sphere species in... 

A powerful application of outer-sphere electron transfer theory relates the ET rate between D and A to the rates of self exchange for the individual species. Self-exchange rates correspond to electron transfer in D/D (/cjj) and A/A (/c22)- These rates are related through the cross-relation to the D/A electron transfer reaction by the expression... 

Oxidation—Reduction. Redox or oxidation—reduction reactions are often governed by the hard—soft base rule. For example, a metal in a low oxidation state (relatively soft) can be oxidized more easily if surrounded by hard ligands or a hard solvent. Metals tend toward hard-acid behavior on oxidation. Redox rates are often limited by substitution rates of the reactant so that direct electron transfer can occur (16). If substitution is very slow, an outer sphere or tunneling reaction may occur. One-electron transfers are normally favored over multielectron processes, especially when three or more species must aggregate prior to reaction. However, oxidative addition... 

Outer-sphere. Here, electron transfer from one reactant to the other is effected without changing the coordination sphere of either. This is likely to be the ea.se if both reactants are coordinatively. saturated and can safely be assumed to be so if the rate of the redox process is faster than the rates observed for substitution (ligand tran.sfer) reactions of the species in question. A good example is the reaction. 

The superb elegance of this demonstration lies in the choice of reactants which permits no alternative mechani.sm. Cr" (d ) and Co" (d ) species are known to be substitutionally labile whereas Cr" (d ) and Co " (low-spin d ) are substitutionally inert, Only if electron transfer is preceded by the formation of a bridged internrediate can the inert cobalt reactant be persuaded to release a Cl ligand and so allow the quantitative formation of the (then inert) chromium product. Corroboration that electron transfer does not occur by an outer-sphere mechanism followed by los.s of CP from the chromium is provided by the fact that, if Cl is added to the solution, none of it finds its way into the chromium product. 

Now we can proceed to assemble the positive evidence for the path (I II -> IV, Fig. 7). Once the outer sphere complex, (II), is formed, all replacements of water should occur at the same rate, k - lO- If the ion pairing constant Ka is known, or a limiting rate of anion entry corresponding to saturation of the association is observable, the rates of conversion of (II) into (IV) may be compared for various X. All should be equal to / -h20 if the activation mode is d, but they will not equal the rate of water exchange which was identified with on the D path. The reason is that species (II) has a number of solvent molecules in its... 

During this study, an intermediate absorbing at 425 m/i was detected and shown in a further study to be a dimer (VOV " ), with nearly two-thirds of the V(IV)-V(II) reaction proceeding via this species in an inner-sphere step, the remainder reacting via an outer-sphere pathway. The mechanism proposed for the reaction was... 

Comparison of equations (2.11) and (2.15) reveals q and r to be kikilk i and A 2//r i, respectively. This enables k to be calculated from qjr. In its simplest forms the structure of the reactive intermediate can be viewed as V(OH)Cr " (when n is 1) or as VOCr (when n is 2). Similar species which have been characterized or implied kinetically are CrOCr (ref. 33), Np02Cr (ref. 37), U02Cr (ref. 31), VOV " (ref. 34), U0Pu02 + (ref. 41), Pu02pe + (ref. 42) and FeOFe + (ref. 38). Predictions on the rate of the V(III)- -Cr(lI) system, based upon Marcus theory", have been made by Dulz and Sutin on the assumption that an outer-sphere process applies. The value arrived at by these authors is 60 times lower than the experimental value. 

The ML species may interact with a species in its second coordination sphere. Therefore one distinguishes inner-sphere charge-transfer and outer-sphere charge-transfer states. 

Here we mention as an example that in the coordination-chemistry field optical MMCT transitions between weakly coupled species are usually evaluated using the Hush theory [10,11]. The energy of the MMCT transition is given by = AE + x- Here AE is the difference between the potentials of both redox couples involved in the CT process. The reorganizational energy x is the sum of inner-sphere and outer-sphere contributions. The former depends on structural changes after the MMCT excitation transition, the latter depends on solvent polarity and the distance between the redox centres. However, similar approaches are also known in the solid state field since long [12]. 

After an extensive review of MMCT transitions involving ions in solids, it seems wise to start this paragraph with some molecular species, because many of these have been investigated in much more detail than their counterparts in non-molecular solids. It is suitable to make a distinction between outer-sphere charge-transfer (OSCT) and inner-sphere charge-transfer (ISCT) transitions [1], In the former the metal ions do not have ligands in common, in the latter they are connected by a common ligand. Studies are usually performed on metal-ion pairs in solution. 

Two types of electron transfer mechanisms are defined for transition metal species. Outer-sphere electron transfer occurs when the outer, or solvent, coordination spheres of the metal centers is involved in transferring electrons. No reorganization of the inner coordination sphere of either reactant takes place during electron transfer. A reaction example is depicted in equation 1.27 ... 

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