Water exchange reactions

In the IPCM calculations, the molecule is contained inside a cavity within the polarizable continuum, the size of which is determined by a suitable computed isodensity surface. The size of this cavity corresponds to the molecular volume allowing a simple, yet effective evaluation of the molecular activation volume, which is not based on semi-empirical models, but also does not allow a direct comparison with experimental data as the second solvation sphere is almost completely absent. The volume difference between the precursor complex Be(H20)4(H20)]2+ and the transition structure [Be(H20)5]2+, viz., —4.5A3, represents the activation volume of the reaction. This value can be compared with the value of —6.1 A3 calculated for the corresponding water exchange reaction around Li+, for which we concluded the operation of a limiting associative mechanism. In the present case, both the nature of [Be(H20)5]2+ and the activation volume clearly indicate the operation of an associative interchange mechanism (156). 

The simplest reactions to study, those of coordination complexes with solvent, are used to classify metal ions as labile or inert. Factors affecting metal ion lability include size, charge, electron configuration, and coordination number. Solvents can by classified as to their size, polarity, and the nature of the donor atom. Using the water exchange reaction for the aqua ion [M(H20) ]m+, metal ions are divided by Cotton, Wilkinson, and Gaus7 into four classes ... 

In the simplest case, it can be assumed that the water exchange reaction with a complexing site of a heterogeneous ligand will be governed by the Eigen-Wilkens mechanism as for complexes with simple ligands. In this case, equations (26) and (29) can be combined to yield ... 

NiO O) (H). Recently Merbach and coworkers (12) have cited evidence for a gradual changeover of mechanism from 1 to 1 for the water exchange reactions of divalent metal ions in going... 

To test this, we list in Table I all those water-exchange reactions of metal ions for which a positive AV has been found. 

Pressure-decelerated water exchange reactions and evidence for dissociative interchange in corresponding net substitution reactions... 

In the literature there are only few studies on the water-exchange processes of the manganese(II) species in general (33,38- 1), and the only seven-coordinate Mn(II) complexes studied are [Mn(EDTA) (H20)] and its derivatives (38,39,42,43). Such studies are essential for understanding the mechanism of the manganese-containing SOD mimetics. The volume of activation for the water-exchange reaction... 

Activation Parameters and Rate Constants for The Water-Exchange Reaction... 

Activation volumes for water exchange on [M(0H)(H20)5] (Table V) are all more positive than those measured on the corresponding hexa-aqua ions indicating a more dissociative character for the water-exchange reaction. The decrease in the positive charge at the metal center loosens the metal-water bonds and facilitates rupture of the M-0 bond. 

Water exchange reaction mechanism 332 Water NMRD in diamagnetic systems 33-9 Water protein relaxation rate 149 Wigner rotation matrices 65, 67 Wild type azurin 122... 

A number of important structural aspects of zinc complexes as found in enzymes are introduced in this section to serve as background information for the subsequent sections. Aquated Zn(II) ions exist as octahedral [Zn(H20)6] + complexes in aqueous solution. The coordinated water molecules are loosely bound to the Zn + metal center and exchange rapidly with water molecules in the second coordination sphere (see Figure 1) with a rate constant of ca 10 s at 25 °C extrapolated from complex-formation rate constants of Zn + ions with a series of nucleophiles. The mechanism of the water exchange reaction on Zn(II) was studied theoretically, from which it was concluded that the reaction follows a dissociative mechanism as outlined in Figure 2. ... 

The above mechanistic interpretation is in contrast with the one appearing in the coordination chemistry of NO on the very labile Fe(III) porphyrins and hemoproteins, which show water substitution-controlled kinetics at the iron(III) center (22,25). The latter Fe(III) moieties are, however, high-spin systems, whilst the cyano-complexes are low-spin. There is strong experimental evidence to support the dissociative mechanism with the Fe(III)-porphyrins, because the rates are of the same order as the water-exchange reactions measured in these systems (22d). Besides, the Fe(III) centers are less oxidizing than [Fein(CN)5H20]2- (21,25). 

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 case of Gd3+, there is a rapid water exchange with respect to the relaxation 7im [28]. For water exchange reaction of Gd3+ aquo ion the water tumbling time is 7 x 10 11 s. When Gd3+ is bound to a macromolecule, part of the hydration sphere is substituted by a protein molecule, the effective correlation changes (i.e.) effectively it becomes the electron relaxation time [29] which is about 10-9 s. By virtue of binding to a macromolecule, a net enhancement in proton relaxation rate, Eq is observed which is characteristic of the Gd3+ complex and depends on the resonance frequency and temperature. Some data on the enhancements obtained for Gd3+ protein complexes are given in Table 11.5. 

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