Water exchange aqua ions table

The nine-coordinate Gd(III) poly(amino carboxylates) all have positive activation volumes as indicative of dissociatively activated water exchange, and much lower exchange rates in comparison to the Gd(III) aqua ion (Table 1). Several factors should be considered to rationalize the decreased water exchange rates. For many of these complexes only the coordination number of nine exists... 

Table 1.9 Rate Constants for Water Exchange in Metal Aqua Ions...

Fig. 4. Interpretation of volumes of activation (in cm3 mol-1) for water exchange on aqueous M(H20)g+ in terms of contributions from bond-making and -breaking (a) summary of volumes of activation for metal aqua ions (b) calculated curves for Al(III), Ga(III), and In(III) with use of the Connolly volumes in Table IV.

EXAFS study on Eu2+ and Sr2+ in both solid state and aqueous solution gave coordination numbers of 8.0 for strontium(II) and 7.2 for europium(II) (228). The water exchange rate measured on the divalent europium aqua ion is the fastest ever measured by 170 NMR (Table XVI) (2). The activation volume is much more negative (—11.7 cm3 mol-1) than those determined on trivalent lanthanide aqua ions clearly indicating an a-activation mechanism which is most probably a limiting... 

In the Re(V) and W(IV) aqua oxo complexes, comparison of both the complex formation of the [MO(OH2)(CN)4], by NCS ions and the water exchange (k iq) shows a relative increase in reactivity of approximately 3 orders of magnitude (Table II), which is in direct agreement with the previously (1, 2, 50) concluded dissociative mechanism. The increase in Lewis acidity of the Re(V) center compared to that of W(IV) is expected to result in a much less reactive system in a dissociative activated mode. 

Table V summarizes rate constants, and activation volumes for water exchange on [M(0H)(H20)5] together with those of [M(H20)6] . Accuracy of exchange rate constants /jqh and its activation parameters AH and AS relies on the knowledge of AHj , and ASa° (39,52). The hydrolysis of aqueous M " " ions is complicated by oligomerization and ranges of hydrolysis constants have been reported for example for Al (91,92) and Ga (93-95). As a general trend a strong increase in the lability of the coordinated water molecules is observed for the hydrolyzed species when compared to the hexa-aqua ions. Even at very low pH, where the mole fraction of the hydrolyzed species is extremely small, the water exchange observed by NMR on the bulk water can be dominated by the fast exchange on [M(0H)(H20)5] and not by the much slower exchange on...

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. 

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. 

Table 1 Rate constants and activation parameters for water exchange on organometallic aqua-ions and related hexaaqua-ions...

Ligand substitution kinetics on water-soluble porphyrins have been measured.18,105 106 The Co111 ion of the porphyrin exchanges axial ligands several orders of magnitude more readily than do simple amine or aqua complexes. Relatively low ligand discrimination ratios and positive activation entropies support a dissociative mechanism (equation 18, Table 7). 

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