Ligand-exchange mechanism, inner-sphere

Strongly binding anions, such as molybdate, sorb on oxides by a ligand-exchange mechanism that involves the exchange of an oxide-surface hydroxyl group (SOH) for an aqueous anion (A ) (Stumm, Kummert, and Sigg, 1980 Sposito, 1984 Stumm, 1992). This reaction results in the formation of an inner sphere complex and is illustrated as follows ... 

The presence of an inner sphere water molecule (or hydroxide ion) at the fourth ligand site for catalytically essential zinc ion introduces questions which are central to an understanding of the role(s) played by zinc ion in the catalytic mechanism. Does zinc ion activate the water molecule for direct participation in the chemical transformation — i.e., as an acid-base catalyst, or as a nucleophile or, does zinc ion function as a Lewis acid in the activation of substrate chemical bonds In the latter case, inner sphere coordination of substrate most probably would be preceded by displacement of the inner sphere water molecule, although ligand exchange mechanisms involving a penta-coordinate transition-state and/or penta-coordinate intermediate (s) can not be excluded a priori as possibilities. 

ZsR — Zp.) Equation 5.25b describes the inner-sphere complexation of an anionic species through the ligand exchange mechanism. In this reaction, actually a special case of Eq. 5.25a, SR comprises a permanent structural moiety S and an exchangeable ligand R the entity R in the general surface complex SR C does not exist, and so the symbol for the complex is shortened to SC, where C More generally, one could... 

Where solvent exchange controls the formation kinetics, substitution of a ligand for a solvent molecule in a solvated metal ion has commonly been considered to reflect the two-step process illustrated by [7.1]. A mechanism of this type has been termed a dissociative interchange or 7d process. Initially, complexation involves rapid formation of an outer-sphere complex (of ion-ion or ion-dipole nature) which is characterized by the equilibrium constant Kos. In some cases, the value of Kos may be determined experimentally alternatively, it may be estimated from first principles (Margerum, Cayley, Weatherburn Pagenkopf, 1978). The second step is then the conversion of the outer-sphere complex to an inner-sphere one, the formation of which is controlled by the natural rate of solvent exchange on the metal. Solvent exchange may be defined in terms of its characteristic first-order rate constant, kex, whose value varies widely from one metal to the next. 

The first two pathways (a) and (b) show, respectively, the influence of H+ and of surface complex forming ligands on the non-reductive dissolution. These pathways were discussed in Chapter 5. Reductive dissolution mechanisms are illustrated in pathways (c) - (e) (Fig. 9.3). Reductants adsorbed to the hydrous oxide surface can readily exchange electrons with an Fe(III) surface center. Those reductants, such as ascorbate, that form inner-sphere surface complexes are especially efficient. The electron transfer leads to an oxidized reactant (often a radical) and a surface Fe(II) atom. The Fe(II)-0 bond in the surface of the crystalline lattice is more labile than the Fe(III)-0 bond and thus, the reduced metal center is more easily detached from the surface than the original oxidized metal center (see Eqs. 9.4a - 9.4c). 

Rates of ligand exchange depend quite strongly on the coordina-tive environment of the metal center. The water exchange rate of Fe(H2O)5(OH)is almost three orders of magnitude higher than that of Fe(H20)g+, and follows a dissociative, rather than an associative exchange mechanism (20). Fe(1120)5(OH)has also been shown to form inner-sphere complexes with phenols (27), catechols (28), and a-hydroxycarboxylic acids (29) much more quickly than Fe(H20) +. The mechanism for complex formation with phenolate anion (A-) is shown below (27) ... 

If a magnesium cation is incorporated into the active site of a protein-based enzyme, it usually binds via an inner-sphere mechanism in which one or more water ligands in [Mg(OH2)e] are exchanged for organic ligands originating from side chains. The... 

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