Exchange water

The exchange of coordinated H2O by isotopically labelled water has been investigated for a wide range of octahedral [M(H20)6] + species (Co + is not among these because it is unstable in aqueous solution, see Section 21.10). Reaction 

First order rate constants for the exchange of coordinated water show the following trends  

First order rate constants, k, for reaction 25.21 vary greatly among the hrst row J-block metals (all high-spin 

In Section 14.8, we described the complex [Fe(N0)(H20)5] + in association with the brown ring test for the nitrate ion. The binding of NO is reversible  

The kinetics of the reversible binding of NO to [Fe(H20)g] can be followed by using flash photolysis and monitoring changes in the absorption spectrum. Irradiation of [Fe(H20)5(N0)] at a wavelength of 532 nm results in rapid dissociation of NO and loss of the absorptions at 336, 451 and 585 nm, i.e. the equilibrium above moves to the left-hand side. Following the flash , the equilibrium reestablishes itself within 0.2 ms (at 298 K) and the rate at which [Fe(H20)5(N0)] reforms can be determined from the reappearance of three characteristic absorptions. The 

Suggest why the complex shown below undergoes water exchange at a rate 10 times faster than [Pt(OH2)4] .  

Most studies of the mechanism of substitution in octahedral metal complexes have been concerned with Wemer-type complexes. Organometallic complexes have entered the research field more recently. Among the former, the popular candidates for study have been Cr(lll) (d ) and low-spin Co(III) (d ) species. These complexes are kineti-cally inert and their rates of reaction are relatively slow and readily followed by conventional techniques. Both Rh(lll) and Ir(lII) (both low-spin d ) also undergo very slow substitution reactions. There is no universal mechanism by which octahedral complexes undergo substitution, and so care is needed when tackling the interpretation of kinetic data. 

As was pointed out in Section 26.2, for M and M ions of the if-block metals, data for reaction 26.24 indicate a correlation between rate constants and electrcMiic configuration. Table 26.4 lists activation volumes for reaction 26.24 with selected first row J-block metal ions. The change from negative to positive values of AV indicates a change 

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Fertilization of ponds to increase productivity is the next level of intensity with respect to fish culture, followed by provision of supplemental feeds. Supplemental feeds are those that provide some additional nutrition but caimot be depended upon to supply all the required nutrients. Provision of complete feeds, those that do provide all of the nutrients required by the fish, translates to another increase in intensity. Associated with one or more of the stages described might be the appHcation of techniques that lead to the maintenance of good water quaUty. Examples are continuous water exchange, mechanical aeration, and the use of various chemicals used to adjust such factors as pH, alkalinity, and hardness. 

Table 4. Water Exchange Rates and Activation Parameters of Hexaaqua Complexes at 25°C, ...

Important intermedia transfer mechanisms that must be considered where significant surface water contamination is expected include transfers to ground water where hydrogeology of the area indicates significant surface water-ground water exchange, transfers to biota where waters contaminated with lipophilic substances support edible biotic species, and transfer... 

The feed to a distillation tower is normally heated either by indirect heat exchange with hot products and/or in a furnace. The products must be condensed and cooled. This is accomplished in part by heat exchange with other petroleum streams and in part by cooling water exchange. The arrangement and relative... 

McFadden, E. R., Jr. (1983). Respiratory heat and water exchange physiological and clinical implications./. Appl. Vhysiol. 54, 331-336. 

Water exchange kinetics in labile aquo and substituted aquo transition metal ions by means of 170 n.m.r. studies. J. P. Hunt, Coord. Chem. Rev., 1971, 7,1-10 (29). 

P56 0.58 Computed from the P content of surface layer given in Table 14-3 and a water exchange rate between surface and deep ocean of 2 m/yr (Broecker, 1971)... 

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. 

Fig. 1. Mean lifetimes of a single water molecule in the first coordination sphere of a given metal ion, th2o> and the corresponding water exchange rate constants, h2o- The tall bars indicate directly determined values, and the short bars indicate values deduced from ligand substitution studies. References to the plotted values appear in the text.

Fig. 2. The effect of applied pressure, P, on the ratio of the rate constants for water exchange on [M(H20)6]2+ observed at applied and ambient pressures, kPlk0 (6).

The application of pressure to a d-activated exchange process produces a decrease in n kP/k0) because the approach to the transition state requires an increase in volume, as indicated qualitatively by the one or two descending superscript arrows on kP in Fig. 3. The opposite is the case for an a-activated exchange process where the approach to the transition state requires a decrease in volume, indicated qualitatively by the one or two ascending superscript arrows on kP in Fig. 3. On this basis, it is clear that when M = V and Mn, water exchange on [M(H20)6]2+ is a-activated, but when M = Fe, Co, and Ni, it is d-activated. The origins of these differences are considered in more detail in Section III,B. 

On the basis of the preceding discussion, the systematic trend in AV observed for water exchange on [M(H20)6]2+ may be rationalized through a More-O Ferral type of diagram (4, 7) as shown in Fig. 4. The bond-making and bond-breaking contributions to AV are plotted on the two axes, which are scaled to AVt for a D mechanism being... 

Fig. 4. Bond-making and bond-breaking contributions to the volumes of activation for water exchange on [M(H20)6]z+, AV-t- (7).

Fig. 7. The variation of AG-i- for water exchange on high-spin [M(H20)6]2+ and low-spin [M(H20)6]2+,3+ at 298.2 K with dn, where the closed squares represent directly determined values, and the open squares represent estimated values. The LFAE calculated for D and A mechanisms are indicated by open and closed circles, respectively. The AV are indicated by circles enclosing the rM of the metal ions in picometers.

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