Anion water exchange

In the pyroaurite structure the brucite layers are cationic. However, on oxidation the resultant brucite layers in y - NiOOH are anionic. To preserve electroneutrality, cations and anions are exchanged in the intercalated layer during the oxidation-reduction process. This is illustrated in Fig. 4. In the case of Mn-substituted materials, some Mn can be reduced to Mn(II). This neutralizes the charge in the brucite layer this part of the structure reverts to the P - Ni(OH)2 structure and the intercalated water and anions are expelled from the lattice. With this there is a concomitant irreversible contraction of the interlayer spacing from 7.80 to 4.65A [72]. 

Cation resins exchange positive ions (e.g., calcium, magnesium or iron) from water, while anion resins exchange negative ions (e.g.,... 

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... 

HzPO . The values are 0.24, 0.21, 0.16 and 0.13, respectively. The values span a range of a factor of two which must be admitted to be a little larger than the experimental uncertainty and also easily within the differences among the anions in their probability of occupancy of the crucial outer sphere site adjacent to the leaving water molecule. All are nearly a factor of five below the water exchange rate. These results conform neatly to the predictions. 

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) ... 

Water exchange on cationic lanthanide chelates can also be influenced by the nature of the counter-anions (170,171). Anions like halides, sulfate, nitrate, acetate, and fluoroacetate impose different order on the second coordination shell around the chelate by influencing the hydrogen bond network. Anions with a high charge density like CU and S04 can break up the hydrogen bond network between water molecules around the metal center and by that, slow down the water exchange rate of the inner shell water molecule (171). 

A review of iron(III) in aqueous solution covers hydrolysis and polymerization, the formation and dissociation of binuclear species, and kinetics and mechanisms of water exchange and complex formation. " Kinetic and equilibrium data for hydrolytic reactions of iron(III) have been conveniently assembled. Reviews of hydrolysis of Fe aq, and subsequent precipitation of hydrated oxide-hydroxide species, cover a very wide range of media, from geochemistry to biology to human metabolism. Added anions or pH variation can affect which form... 

A mass-transfer model was proposed to account for the function of the [BMIM]PF6/water biphasic system in the reaction (Scheme 19). According to this model, a small quantity of [BMIM]PF6 dissolved in water exchanged its anion with H2O2 to form Q OOH , which was transferred into the [BMIM]PF6 phase to initiate the epoxidation reaction. In this reaction system, the ring-opening... 

Although OH reacts at near-diffusion-controlled rates with inorganic anions [59], there seems to bean upper limit of ca. 3 x 10 dm mol sec in the case of simple hydrated metal ions, irrespective of the reduction potential of M"". Also, there is no correlation between the measured values of 43 and the rates of exchange of water molecules in the first hydration shell of, which rules out direct substitution of OH for H2O as a general mechanism. Other mechanisms that have been proposed are (i) abstraction of H from a coordinated H2O [75,76], and (ii) OH entering the first hydration shell to increase the coordination number by one, followed by inner-sphere electron transfer [77,78]. Data reported [78] for M" = Cr, for which the half-life for water exchange is of the order of days, are consistent with mechanism (ii) ... 

In the case of sulfate and dihydrogen phosphate where the ion pair association constants have been clearly identified, the anation rates are known for the 1 1 outer sphere complex. These rate values vary somewhat, and this perhaps indicates participation of the entering group. But there may be another way to interpret what is going on. These two rates of anion entry are smaller than the rate of water exchange of the aquopentammine complex. In fact, the univalent anion enters the complex from the outer sphere at approximately one-eighth of the rate of water exchange, and the divalent anion enters the complex from the outer sphere about twice as fast. 

We can now make sensible guesses as to the order of rate constant for water replacement from coordination complexes of the metals tabulated. (With the formation of fused rings these relationships may no longer apply. Consider, for example, the slow reactions of metal ions with porphyrine derivatives (20) or with tetrasulfonated phthalocyanine, where the rate determining step in the incorporation of metal ion is the dissociation of the pyrrole N-H bond (164).) The reason for many earlier (mostly qualitative) observations on the behavior of complex ions can now be understood. The relative reaction rates of cations with the anion of thenoyltrifluoroacetone (113) and metal-aqua water exchange data from NMR studies (69) are much as expected. The rapid exchange of CN " with Hg(CN)4 2 or Zn(CN)4-2 or the very slow Hg(CN)+, Hg+2 isotopic exchange can be understood, when the dissociative rate constants are estimated. Reactions of the type M+a + L b = ML+(a "b) can be justifiably assumed rapid in the proposed mechanisms for the redox reactions of iron(III) with iodide (47) or thiosulfate (93) ions or when copper(II) reacts with cyanide ions (9). Finally relations between kinetic and thermodynamic parameters are shown by a variety of complex ions since the dissociation rate constant dominates the thermodynamic stability constant of the complex (127). A recently observed linear relation between the rate constant for dissociation of nickel complexes with a variety of pyridine bases and the acidity constant of the base arises from the constancy of the formation rate constant for these complexes (87). 

Removal of citrate ion by an anionic ion-exchange process can be accomplished by exchange with hydroxyl ion and the subsequent formation of water, which is a component of juice and which can be removed by evaporation hence, it should be preferable to a method that relies on the addition of a neutralizing substance to a citrus juice. The ion-exchange process is illustrated in the following equation ... 

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