Water Exchange from the First Coordination Shell

Water Exchange from the First Coordination Shell 

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Water Exchange from the First Coordination Shell 141... 

Water Exchange from the First Coordination Shell 153 Tab. 4.6. Selected properties of reactants/products, transition states, and intermediates. 

The simplest reaction on a metal ion in aqueous solution is the exchange of a water molecule between the first and second coordination shells. This reaction is fundamental in understanding not only the reactivity of metal ions in chemical and hiological systems hut also the metal-water interaction. The replacement of a water molecule from the first coordination shell represents an important step in complex-formation reactions of metal cations and in many redox processes (1). 

Replacing one or more water molecules from the first coordination shell of a di- or a trivalent transition metal ion by a kinetically inert mono-or multi-dentate ligand can have a strong effect on the exchange rate constant of the remaining water molecules. In general the remaining water molecules become more labile (Tables VII and VIII) the acceleration can... 

Fig. 4.1J. Visualization of water exchange between the second coordination shell and bulk solvent on [CrlHaOje] obtained from MD simulation (1) selected first sphere water molecule (2), (3) second sphere water molecules before exchange (4) exchanging outer sphere water molecule.

In relation with the ongoing discussion, if Cu in aqueous solution has five or six water molecules in its first coordination shell (9,116,117), it is interesting to compare water exchange rates measured on five-coordinate copper complexes. Rates of water exchange on five-coordinate complexes of copper(II) are drastically reduced from the rate of exchange on aqua ions of copper(II) (Table VII) (113). The mechanism of water exchange is of associative character in all examples studied to date with the exception of [Cu(tpy)(H20)2]. For that complex the water exchange is very rapid compared to the other complexes and the mechanism is a Id. 

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) ... 

From molecular dynamics simulations, information on water exchange is available for the three regions of coordination. The simulation qualitatively confirmed the lability maximum of the first hydration shell found in the middle of the series [70]. From the trajectories obtained from the simulations, all transitions of water molecules between the first hydration shell and the bulk were identified and grouped into pairs of water molecules that together form a coupled exchange [70]. 

More sophisticated quantum mechanical models began to be applied in the 1980s. Rode et al. calculated the hydration energies of these metal ions by including effects from two hydration shells beyond the first coordination sphere. The stabilization per water molecule in the first coordination sphere, AE(l), and in the second coordination sphere, A (II), were taken into account. With some rather arbitrary adjustments, these quantities were found to correlate reasonably well with the AG (2S°C) for water exchange. 

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