Outer- versus inner-sphere complexes

The concentration, counterion and pH dependences of 6( Sc) in aqueous solutions of scandium salts have revealed equilibria between outer-sphere versus inner-sphere complexes (contact ion pairs versus direct coordination of the anion), in addition to the equilibria between [Sc(H20)g], which is present at low pH, its deprotonation ([Sc(H20)5(0H)]2+) and oligomerization products ([Sc2(H20),(0H)2l N (Sc3(H20) (0H)4l +). present at pH values up to 4, and [Sc(OH)4]", the species that is formed at high pH. 

The type of surface complex, i.e., outer-sphere versus inner-sphere (Figure... 

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. 

Redox potential-structure relationships in metal complexes. Part 2. The influence of trans-substituents upon the redox properties of certain dinitrogen complexes of molybdenum and tungsten and some carbonyl analogues inner-sphere versus outer-sphere electron transfer in the alkylation of co-ordinated dinitrogen J.Chem.Soc. Dalton Trans./ (1980)/ 121-127... 

While lanthanide phosphate and carbonate stability constants increase substantially between La and Lu, the complexation behavior of lanthanides with sulfate changes very little across the lanthanide series. This difference in complexation constant trends is consistent with inner sphere (COj ) versus outer sphere (SO ) complexation behavior (Byrne and Li 1995). Stability constants for lanthanide sulfate complexes at 25 C and zero ionic strength could be well represented as logso4l8i(M) = 3.60d=0.08. The recommended stability constants (25 C, 0.7 mol kg ionic strength) shown in table 5 are based upon the works of Spedding and Jaffe (1954) and Powell (1974). Following the activity coefficient estimates of Millero and Schreiber (1982) and Cantrell and Byrne (1987a), lanthanide sulfate stability constants, expressed in terms of free-ion concentrations, were calculated as log 504 1 = log SO4/3 - 1.67. 

Choppin [24] examined some aspects of lanthanide-organic ligand interaction in aqueous solutions. An interpretation of thermodynamic parameters (AG, AH and AS) of complexation have been given in terms of hydration, inner versus outer sphere character, stability vs. chelate ring size and ligand charge polarization. 

Similar discrepancies as documented in the previous section for macroscopic data can be found for the spectroscopic approaches, which are now available for studying the structure of surface complexes in situ (i.e., wet samples). With respect to inverse modeling these studies would attempt to resolve the stmcture of the surface complexes in a certain system and to impose such structures in the surface complexation model. This would avoid extensive discussion about the mode of bonding (e.g., inner versus outer sphere monodentate versus polydentate). 

Surface species and surface complex stability constants These aspects are, to some extent, interrelated. The term surface species might comprise such aspects as the polydentate character of the surface complex, inner- versus outer-sphere siuface complexation, or charge distribution. Whether surface species are plausible can be checked either by surface spectroscopy or by appUcation of the bond-valence principle, which can be coupled with specfroscopic information [110]. The latter can also help to estimate reasonable values for the charge distribution factors in charge distribution models. All of this certainly allows a restriction in the many options which... 

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