Inner-sphere species

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. 

Illustrate solution, cation exchange, and outer- and inner-sphere species around a soil particle. 

Maass and Eigen have shown for the zinc acetate system fos " 32 — 3.2 X 107 sec."1 The rate of loss of water from zinc equals the rate of loss of a carboxylate group from zinc. This means that the ion pair and inner sphere species of zinc acetate are present in solution in equal amounts. If we now examine the zinc oxalate system we see that we have similar rate steps. 

The effect of pressure on a number of inner-sphere electron transfer reactions has also been investigated. By way of example, the reaction of Co(NH3)5X2-" with Fe(H20)6 -" exhibits Avalues of +10.7 (X = F), +8.7 (X = Cl), +6.4 (X = Br), and +13.0 (X = N3 ), which are mainly ascribed to the release of a solvent molecule during the formation of the bridged inner-sphere species, [(NH3)5Co-X-Fe(H20)5] (24). Other examples of pressure effects on inner-sphere electron transfer reactions, also including some intramolecular reactions induced by pulse radiolysis, have been reported in the htera-ture (i, 25, 26). 

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... 

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. 

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... 

It is thought that exchange can occur through the species Fe(DMSO) and Fe(DMSO) possibly via an inner sphere mechanism the exchange occurs in the absence of water. 

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). ... 

The species (MS03)" represents an inner-sphere complex between sulphite and the oxidant formed by ligand-displacement. (MS03)" is formed by abstracting an electron from... 

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. 

Iron(III)-catalyzed autoxidation of ascorbic acid has received considerably less attention than the comparable reactions with copper species. Anaerobic studies confirmed that Fe(III) can easily oxidize ascorbic acid to dehydroascorbic acid. Xu and Jordan reported two-stage kinetics for this system in the presence of an excess of the metal ion, and suggested the fast formation of iron(III) ascorbate complexes which undergo reversible electron transfer steps (21). However, Bansch and coworkers did not find spectral evidence for the formation of ascorbate complexes in excess ascorbic acid (22). On the basis of a combined pH, temperature and pressure dependence study these authors confirmed that the oxidation by Fe(H20)g+ proceeds via an outer-sphere mechanism, while the reaction with Fe(H20)50H2+ is substitution-controlled and follows an inner-sphere electron transfer path. To some extent, these results may contradict with the model proposed by Taqui Khan and Martell (6), because the oxidation by the metal ion may take place before the ternary oxygen complex is actually formed in Eq. (17). 

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