Water Exchange with metal ions

Rates of water exchange at metal ions vary tremendously with the nature of the cation. The range is from almost 10 s, for Cu or Cr down to less than 10 s for Rh. The values for many metal aquocations are shown in Fig. 

Data for water exchange with Fe(H20)j+ are shown in Table 4.1. " The value for AV indicates an interchange dissociative mechanism, which is also reflected in data for the reaction of Fe + with tpy AV = -1-3.5 cm mol , ) and other ligands. " One of the earliest studies of substitution in a labile metal ion was of the reaetion of Fe + with bpy and phen in acid solution (Sec. 2.1.4). 

For a given metal ion the rates and the activation parameters for complex formation are similar to those for water exchange, with the complex formation rate constants usually about a factor of 10 lower than those for water exchange. 

Figure 4.14 — (A) Flow injection system for the preconcentration and determination of copper P peristaltic pumps A 0.5 M HNOj B sample q = 2.5 mL/min) C water (jq = 0.5 mL/min) E 1 M NaNOj/O.l M NaAcO, pH 5.4 q = 0.5 mL/min F 1 M NaAcO/2 x 10 M Cu pH 5.0 (9 = 1.0 mL/min) 3-5 valves ISE copper ion-selective electrode W waste I and II 2 and 3 mL of chelating ion exchanger for purification III 100 fil of chelating ion exchanger for metal ion preconcentration. (B) Scheme of the flow system for the determination of halides A 4 M HAcO/1 M NaCl/0.57 ppm F B 1 M NaOH/0.5 M NaCl C, mixing coil (1 m x 0.5 mm ID PTFE tube) Cj stainless-steel tube (5 cm x 0.5 mm ID) ISE ion-selective electrode R recorder. (Reproduced from [128] and [129] with permission of Elsevier Science Publishers and the Royal Society of Chemistry, respectively).

The interlayer separation directly depends on the number of water molecules present between the layers. In the anhydrous phases ZrM(P04)2 (M = Mg, Ca, Sr, or Ba) and the half exchanged phases ZrHM(P04)2 (M = Li, Na, K, Rb, or Cs) the interlayer separation is independent of cation size, indicating that the interlayer cavity, which contains one water molecule in the unexchanged compound, is sufficiently large to accommodate a single cation. In contrast, in the fully exchanged alkali metal ion compositions, the interlayer separation increases with increasing cation radius. 

The radical OH reacts with many inorganic anions at near-diffusion-controlled rates, but with metal ions there seems to be an upper limit of about 3 x 10 dm mol s , regardless of the reduction potential of M " ". There is no correlation between the measured values of 3.13 and the rates of exchange of water molecules coordinated to M" , which rules out ligand substitution as a general mechanism other possibilities are abstraction of H from a coordinated water molecule and OH entering the coordination shell to increase the coordination number by one. 

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