Inner-sphere character

The quantitative effects of steric encumbrance on the electron-transfer kinetics reinforce the notion that the inner-sphere character of the contact ion pair D+, A- is critical to the electron-transfer paradigm in Scheme 1. Charge-transfer bonding as established in the encounter complex (see above) is doubtless an important consideration in the quantitative treatment of the energetics. None the less, the successful application of the electron-transfer paradigm to the... 

N. Sutin, Brookhaven National Laboratory Strictly speaking, the outer-sphere and inner-sphere designations refer to limiting cases. In practice, reactions can have intermediate outer-sphere or inner-sphere character this occurs, for example, when there is extensive interpenetration of the inner-coordination shells of the two reactants. Treating this intermediate situation requires modification of the usual expressions for outer-sphere reactions — particularly those expressions that are based upon a hard-sphere model for the reactants. 

These considerations lead, for example, to the assignment of a predominantly outer sphere character to Cl, Br, F, CIO3, NO3, sulfonate, and trichloro-acetate complexes and an inner sphere character to F", IO3, SO, and acetate complexes of trivalent actinides and lanthanides. The variation in AH° and AS° of complexation of related ligands indicates that those whose pA), values are <2 form predominantly outer sphere complexes, while those for whom > 2 form predominantly inner sphere complexes with the trivalent lanthanides and actinides. As the pK increases above 2, increasing predominance of inner sphere complexation is expected for these metals. 

The changes in the potential profile of the interfacial region because specific adsorption do indeed affect the electrode kinetics of charge transfer processes, particularly when these have an inner sphere character [13, 26] (see Fig. 1.12). When this influence leads to an improvement of the current response of these processes, the global effect is called electrocatalysis. ... 

The first step is diffusion controlled while the second represents the formation of an outer sphere complex in which the metal ion and the ligand are separated by at least one molecule to water. In the final step, this outer sphere complex ejects the water and forms an inner sphere complex in which the metal and ligand are directly associated. Some ligands cannot displace the water and complexation apparently terminates with the formation of the outer sphere complex. Plutonium cations form both inner and outer sphere complexes, depending on the ligand pK. For trivalent plutonium, we can assign a predominant outer sphere character to the halide, nitrate, sulfonate and trichloroacetate complexes and an inner sphere character to the fluoride, iodate, sulfate and acetate complexes (23). A study of Am , Th and complexation... 

HP04 , and 804 should decrease from near 4 to about 3 in the order 804 > HP04 > 003 . Thus, covalency of bonding should be least for the sulfate complexes and greatest for the carbonate complexes, which are apparently for the most part of inner sphere character for cation EN values above 1.5. 

In contrast, similar plots of AG against AGet for the oxidants TCNE and hexa-chloroiridate(IV) deviate substantially from the simulated curve for outer-sphere electron transfer (see Figures 19B, C). Moreover, the most pronounced deviations are observed with the least hindered tetraalkyltin donors. The fact that steric effects are only observed in the latter cases, but not with the FeL3 acceptor, leads to the conclusion that the inner coordination spheres of the tetraalkyltin donors are perturbed by TCNE and hexachloroiridate in the ET transition state. In other words, the electron transfers to TCNE and iridate(IV) exhibit strong inner-sphere character and thus occur from wai i-five-coordinate precursor complexes reminiscent of a variety of trigonal-bipyramidal structures known for tin(IV) derivatives, i.e. 

The calculation results are shown in table II. We also report formation constants determined from spectrophotometry for Ho and Nd, and by solvent extraction for Er. Without going into detail for these two methods, it may be noted that the results show fair agreement. That fact points out the inner sphere character of orthophenanthrolinium lanthanous complexes. The main absorption band of Am (III] is modified by the presence of orthophenanthroline. We used these spectral variations to calculate the formation constants of Am (III], as described in the previous paragraph. As for azide complexes, we observed that Am monoorthophe-nanthroline is more stable than the equivalent lanthanide complex, and for the bis orthophenanthroline species, the difference... 

Spectroscopic, conductimeiric, and kinetic evidence have been used to establish the degree to which a complex is of outer- or inner-sphere character (cf. Nancollas 1966 Shaw et al. 1991). As noted above, the formation constants of many divalent metal-sulfate complexes are practically identical, consistent with a dominantly outer-sphere character. In fact, the sulfate pairs of divalent Mg, Zn, Ni, Co, Mn, and Be (log Ar ssocCBeSO ) = 1.95) have been shown to have about 10% inner-sphere character (Nancollas 1966). The greater stabilities of PbSO (log / assoc 2.69) and UO2SO4 (log /iTassoc =... 

Williams order, suggesting that they have significant inner-sphere character. 

ET.7-9,12,13 The inner-sphere character in organic ET reactions is established by their high sensitivity to steric effects.14... 

The thermodynamic data for complexation of trivalent lanthanide and actinide cations with halate and haloacetate anions are reported. These data are analyzed for estimates of the relative amounts of inner (contact) and outer (solvent separated) sphere complexation. The halate data reflected increasing inner sphere character as the halic acid pKa increased. Use of a Born-type equation with the haloacetic acid pKa values allowed estimation of the effective charge of the carboxylate group. These values were, in turn, used to calculate the inner sphere stability constants with the M(III) ions. This analysis indicates increasing the inner sphere complexation with increasing pKa but relatively constant outer sphere complexation. 

In summary, the halate data reaffirm the tendency of increased inner sphere character in Ln(III) complexes as the ligand pKa increases. 

Analysis of the entropy changes, indicates essentially 100% inner sphere formation for the Ac, 3 ClPr and ClAc complexes, 50% inner sphere for the CI2AC complexes. However, a study of 139La nmr shifts (18) was interpreted to show only 50% inner sphere character for LaClAc+2 and 20-25% for LaCl2Ac+. In light of this lack of agreement, we have analysed the complexation by another approach which would seem to be more justified than the entropy based estimations. 

The differences in behavior of the Eu ion in the ground and excited states may also account for the disagreement in the extent of outer-sphere complexation by chloroacetates as obtained from La-NMR data (Choppin 1980), solvent extraction (Choppin 1980) and luminescence measurements (Barthelemy and Choppin 1989a). Although NMR shift data and solvent extraction studies gave estimates of 60% and 25% inner-sphere character in the EulClAc) " and Eu(a2Ac) complexes, respectively, luminescence lifetimes data provided estimates of 100% and 50% inner-sphere character for the two complexes. 

The complexes of C1 CH3 C02 ligands were studied by La-NMR spectroscopy (Rinaldi et al. 1979). The shift data provided estimates for inner-sphere character of 100% (LaAc +), 50% (LaClAc +), 22% (LaCl2Ac +) and 0% (LaCl3Ac +). These fractions agreed well with those calculated from thermodynamic data for Eu(III) and Am(III) (Choppin 1980,1983b) using the extended Born equation (see section 3). 

Let us now examine the reaction of vic-dibromoalkanes with the low oxidation state iron and cobalt porphyrins. A first indicati< i of their inner-sphere character is the feet that they react... 

Several criteria help to distinguish between inner- and outer-sphere complexes, although not always unambiguously. For instance, Choppin (1971) and Choppin and Bertha (1973) have used the thermodynamic A// and A5 parameters. They have assigned, in aqueous solutions, a predominantly outer-sphere character to CL, Br, L, ClOj, NOj, sulfonate and trichloroacetate complexes and an inner-sphere character to F, lOj, SO and acetate complexes. Moreover, these authors have related this ordering to the pKa values of the ligands. On the other hand, a group of Russian authors have postulated that iimer- and outer- sphere complexes may be studied separately by spectrophotometric methods, but this is subject to some doubt (vide infra). 

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