Rapid water exchange

Enormous acceleration of substitution at ruthenium(II) can be obtained by appropriate choice of ligands. Following the demonstration of remarkably rapid water exchange and complex formation (7d or D in mechanism) at the organometallic aqua-ion [Ru(ri5-C5Me5)(H20)3]2+ (150), comparably rapid substitution has been demonstrated at... 

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. 

All of this describes just the thermodynamically favored directions of water transfer, for ideal crystalline solids. Many salt hydrate pairs seem to behave approximately ideally. However, if water activity is to be controlled close to the transition value, the rates of water release and uptake must be sufficient. Different salt pairs have very different rates of water exchange. It is difficult to give quantitative values, because the rates will depend on the size and shape of the crystals in each of the salt hydrate forms. This will depend on how they have been crystallized and handled subsequently. For example, cycling between hydrate forms, with gain and loss of water, will usually lead to a reduction in crystal size, and hence more rapid water exchange in future cycles. 

Complex reducing agents are V(ll), Cr(Il) and Fe(ll). The aqua ions of iron(II) and chromium(Il) with and t2g Cg configurations respectively (high spin d and high spin d ) are kinetically labile undergoing rapid water exchange... 

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