Complex outer-sphere complexes

These three types of surface species—inner-sphere complex, outer-sphere complex, and diffuse-layer—represent three modes of adsorption of small aqueous ions that contribute to the formation of the electrochemical double layer on clay mineral surfaces. No inference of special planes containing adsorbed ions is required by these surface chemical speciation concepts, nor is detailed molecular structure implied, other than the general notions of surface complexes and vicinal dissociated ions. It is sometimes convenient, although not necessary, to group the two types of surface complex into a Stern layer to distinguish them from diffuse-layer ions [18]. This geometric partitioning of surface species, however, should not be taken to mean that diffuse-layer ions necessarily approach a particle surface less closely than do Stern-layer ions. 

The data in Figure 12.24 show the potentiometric titrations of a metal-exchange synthetic resin (Dowex 50W-X8) known to produce relatively weak metal-surface complexes (outer-sphere complexes). These data show that when the resin was saturated with Ca2+, no titration plateau (region of Na+, NH4 or NH3 adsorption) was exhibited. When the resin was saturated with Cu2+, up to two titration plateaus were exhibited, depending on the type of titrant used. When NaOH was the titrant, one apparent titration plateau was exhibited, whereas when NH4OH was the titrant, two... 

Outer and Inner Sphere Complexes. Outer sphere complexation involves interactions between metal ions and other solute species in which the co-ordinated water of the metal ion and/or the other solute species are retained. For example, the initial step in the formation of ion pairs, where ions of opposite charge approach within a critical distance and are then held together by coulombic attractive forces, is described as outer sphere complex formation. 

Kr, Xe, etc). Metal ions (Cu, Ag) form hydroxy-complexes. Outer-sphere complexes of anions can be detected in aqueous solutions. Inorganic salts- KCl, Fe- salts, etc. can not be complexed. 

Fig. 5-3. Simulation cells used in the molecular d mamics analysis of the sorption of Ba to various goethite surfaces. Configurations represent Ba sorbed on the (110) goethite surface as an inner sphere complex, outer. sphere complex, and fully. solvated. solution species. Morlilicd from Liang and C ygan (2(K)2).

For the silica gel (Figure 3A), the solution was removed slightly less effectively, and more Cs was left (ca. 0.0020 atoms/A2). The spectral behavior is quite similar to that of boehmite, except that there is a peak due to surface Cs coordinated by only water molecules and not in contact with the surface oxygens (so-called outer sphere complexes)at 30% RH. Complete dynamical averaging among sites at frequencies greater than ca. 10 kHz occurs at 70% RH and greater. 

For kaolinite the sample permeability was very low and the solution was poorly removed. The spectra (Figure 3C) are consequently complex, containing peaks for inner and outer sphere complexes, CsCl precipitate from resMual solution (near 200 ppm) and a complex spinning sideband pattern. Spectral resolution is poorer, but at 70% RH for instance, inner sphere complexes resonate near 16 ppm and outer sphere complexes near 31 ppm. Dynamical averaging of the inner and outer sphere complexes occurs at 70% RH, and at 100% RH even the CsCl precipitate is dissolved in the water film and averaged. 

Differentiation between inner- and outer-sphere complexes may be possible on the basis of determination of activation volumes of dediazoniations catalyzed by various metal complexes, similar to the differentiation between heterolytic and homolytic dediazoniations in DMSO made by Kuokkanen, 1989 (see Sec. 8.7). If outer-sphere complexes are involved in a dediazoniation, larger (positive) volumes of activation are expected than those for the comparable reactions with inner-sphere complexes. Such investigations have not been made, however, so far as we are aware. 

Utilization of the Pfeiffer effect and outer-sphere complexation for the prediction of absolute configurations of optically active metal complexes. S. Kirschner and I. Bakkar, Coord. Chem. Rev., 1982,43, 325-335 (27). 

Outer sphere complexes of tervalent cobalt. V. E. Mironov, Russ. Chem. Rev. (Engl. Transl), 1970,39, 319-333 (232). 

Complexation of Pu is discussed in terms of the relative stabilities of different oxidation states and the "effective" ionic charge of Pu0 and Pu02+2. An equation is proposed for calculating stability constants of Pu complexes and its correlation with experimental values demonstrated. The competition between inner v outer sphere complexation as affected by the oxidation state of Pu and the pKa of the ligand is reviewed. Two examples of uses of specific complexing agents for Pu indicate a useful direction for future studies. 

Two common limiting forms of the rate law for mechanism (1) are encountered experimentally. In the event that the equilibrium constant, K, for outer sphere complexation is small in relation to the concentration of MX and Y, the rate law... 

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... 

When the entering ligand, L, is uncharged, the stability of the outer-sphere complex M OH2 L2+ may be so low that its concentration does not differ significantly from that arising from diffusive collisions between M OH2m+ and L. Under these conditions, entry of L into the... 

Thus, although the rate of substitution should be very dependent on the nature of Lx, distinguishing the operation of an A mechanism according to Eq. (8) from ligand substitution proceeding through an outer-sphere complex on the basis of rate laws is usually not feasible. This does not, however, preclude the operation of an A mechanism within an outer-sphere complex. 

Fig. 5. A model for S042- substitution on [Be(H20)4]2+ proceeding from the outer-sphere complex on the left through the transition state at center to the inner-sphere complex at the right of the figure (16, 66).

However, there are a number of other miscellaneous biological roles played by this complex. The [Co(NH3)6]3+ ion has been shown to inhibit the hammerhead ribozyme by displacing a Mn2+ ion from the active site.576 However, [Co(NH3)6]3+ does not inhibit ribonuclease H (RNase),577 topoisomerase I,578 or hairpin ribozyme,579 which require activation by Mg2+ ions. The conclusions from these studies were that an outer sphere complex formation between the enzyme and Mgaq2+ is occuring rather than specific coordination of the divalent ion to the protein. These results are in contrast to DNase I inhibition by the same hexaammine complex. Inhibition of glucose-induced insulin secretion from pancreatic cells by [Co(NH3)6]3+ has been found.580 Intracellular injection of [Co(NH3)6]3+ into a neurone has been found to cause characteristic changes to the structure of its mitochondria, and this offers a simple technique to label neuronal profiles for examination of their ultrastructures.581... 

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. 

Detailed kinetic data are rare for natural aquatic ligands. For simple, not strongly binding ligands, it has been shown [164,165,171] that the dehydration of M(H20)q+ subsequent to the formation of an outer sphere complex or ion pair (Eigen-Wilkens mechanism, equation (27)) is often the rate-limiting step in the formation of the metal complex, ML. This mechanism has often been applied to natural ligands [5,167,171] without further confirmation of its validity. 

As Fig. 2.4 illustrates, a cation can associate with a surface as an inner sphere, or outer-sphere complex depending on whether a chemical, i.e., a largely covalent bond, between the metal and the electron donating oxygen ions, is formed (as in an inner-sphere type solute complex) or if a cation of opposite charge approaches the surface groups within a critical distance as with solute ion pairs the cation and the base are separated by one (or more) water molecules. Furthermore, ions may be in the diffuse swarm of the double layer. 

The anions Cl, NO3, CIO, for some oxides also SO " and SeO are considered to adsorb mainly in outer-sphere complexes and as diffuse ion swarm. 

The Homogeneous Case. Margerum (1978) and Hering and Morel (1990) have elaborated on mechanisms and rates of metal complexation reactions in solution. In the Eigen mechanism, formation of an outer-sphere complex between a metal and a ligand is followed by a rate limiting loss of water from the inner coordination sphere of the metal, Thus, for a bivalent hexaaqua metal ion... 

The stability constant for the outer sphere complex depends on the charge of the reacting species and the ionic strength of the medium, and can be readily calculated from electrostatic considerations. 

We can simplify by considering that k.- k.w, and by setting k k-i = Kos. Kos is the equilibrium constant of the outer sphere complex. For the rate of the formation of MeLJ2 n)+ inner-sphere complex (now written without water), we have... 

We can now calculate the outer-sphere complex formation constant according to Eq. (4.24) ... 

In a more restrictive sense, the term "ion exchange" is used to characterize the replacement of one adsorbed, readily exchangeable ion by another. This circumscription, used in soil science (Sposito, 1989), implies a surface phenomenon involving charged species in outer-sphere complexes or in the diffuse ion swarm. It is not possible to adhere rigorously to this conceptualization because the distinction between inner-sphere and outer-sphere complexation is characterized by a continuous transition, (e.g., H+ binding to humus). 

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