Water exchange kinetic parameters

Solvent Exchange.—Kinetic parameters for water exchange at some d and d complexes of formula [ML5(OH2)] + are given in Table 21. A comparison of... 

Direct kinetic studies of water exchange on [Pd(H20)4]2+ and [Pt(H20)4]2+, the parameters for which appear in Table XIV (267-274), were first reported well after the chemistry outlined in the preceding paragraph had become established. Solvent and ligand exchange on... 

The first experimental information on the kinetic parameters for water exchange on a tetravalent metal ion was published in 2000 for U4+ and Th4+ (265,268,271). The coordination numbers for these two complexes were determined by EXAFS to be 10 1. Based on the high coordination number (there are no complexes known with unidentate ligands and coordination numbers larger than 10) a limiting associative mechanism (A) is unlikely and a d-activated mechanism is probable. Surprisingly,... 

Kinetic Parameters for Fe /Fe Self-Exchange in Water at 298K. 

This parameter helps distinguishing the relative importance of interfacial kinetics and bulk transport. For LpEM < Tpem water transport through the PEM is dominated by interfacial water exchange, whereas for LpEM > bulk permeation of water prevails. The data obtained in Monroe et al. yield Lpem -100-300 im. This indicates that the interfacial vaporization resistance exceeds the resistance due to bulk transport in the membrane when the membrane thickness is LpEM < 100 im. 

Kinetic parameters are shown in Table 4.1 for the exchange of the first-row (and one second-row ) divalent transition metal ions in water. Since AV for a D mechanism is K,... 

Table 4.1 Kinetic Parameters for Water Exchange of Divalent Transition Metal Ions, M(H20) + at 25 °C Refs. 23-25...

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). 

In another investigation,425 the exchange between [Ce(edta)aq] and hydrated Pb2+, Ni2+ or Co2+ ions again show reaction by dissociation of protonated [Ce(Hedta)aq] as well as by the direct attack of metal ions on [Ce(edta)aq] or [Ce(Hedta)aq]. The kinetic parameters for the Ni2+ or Co2+ ions could be related to the relatively slow (k - 2.6 x 106s 1 for Co2+ and 3.4 x 104 s-1 for Ni2+) water exchange reactions of these ions. The direct attack was interpreted in terms of an intermediate in which one of the carboxylate groups was coordinated to the incoming ion rather than to Ce3+. These reactions were followed by spectrophotometry at 280 nm, where the absorbance of Ce3+aq is much lower than the edta complex. 

Kinetic Parameters for Bridge-Formation and Water-Exchange Reactions of Ammine Complexes at 25°C in 1.0 A/(Na,H)CI04 ... 

In the last few years several macromolecular systems have been studied by variable temperature and pressure 170 NMR in order to determine the rate and mechanism of water exchange. The kinetic parameters characterizing the water exchange on some macromolecular Gd(III) complexes are presented in Table 2. 

The kinetic effect on (1/72—1/7)) is proportional to (Amagnetic field as shown in Fig. 7.23. From the temperature and pressure dependence, the kinetic parameters presented in Table 7.13 were obtained. The two activation parameters, namely the entropy and the volumes of activation, are negative and also are of the same magnitude for all the lanthanide ions. These activation parameters imply a common water exchange mechanism for all the lanthanides studied and possibly an associative activation path of exchange. The activation volume, AV of —6.0 cm3 mol-1 probably reflects the difference between a large negative contribution due to the transfer of a water molecule electrostricted in the second coordination sphere to the first coordination sphere and a positive contribution due to the difference in partial molar volumes of N + 1 coordinated transition state and N coordinated aquo lanthanide ion. It should be noted that the latter difference (in partial molar volumes of Fn(H20)w+i and Fn(H20)jv is due to the increase in Fn-O bond distance (Fig. 7.16). 

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