Activation volumes for water exchange

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. 

Tab. 4.2. Rate constants and activation volumes for water exchange on trivaient hexa-aqua transition metal ions and their conjugate base species. 

On the basis of a semi-empirical model. Swaddle and Mak estimated the limiting activation volumes for water-exchange reactions on di- and trivalent transition metal aqua complexes to be AV - -13cm mol for a D mechanism and AV 13cm mol for an A mechanism (20,21). Values of AV within these extreme values are usually interpreted in terms of Id, I, or la mechanisms, respectively (22). With these values as a guideline, assignments which type of mechanism holds can be made with significant confidence. 

The differences in solvent exchange in HjO and DMF are also reflected in the exchange mechanisms. The activation volumes for water exchange are negative and, within experimental error, essentially constant for the different lanthanides this suggests a concerted associative (I ) mechanism. By contrast, the data for DMF exchange are more consistent with a gradual transition from an to a D mechanism between Tb " and Yb ". ... 

Kinetics and mechanism of water exchange at iron(III), in the form of Fe aq and of FeOtf aq, have been much studied recently. It is now clear that water exchange at the former takes place by an associative mechanism, at the latter by a dissociative mechanism. Rate constants, " activation enthalpies and entropies, and activation volumes have been obtained from variable-temperature and variable-pressure 0 nmr experiments. The activation volumes for water exchange at Fe Caq) and at FeOH (aq) are -5.4 and 4-7.0 cm moL respectively the former is almost exactly the same as that reported for the isoelectronic Mn (aq) ion. If acidic (HCIO4) aqueous solutions containing iron(III) are heated for a time, then water exchange at the polynuclear product is over a... 

The determination of partial molar volumes for [M(NH3)5(OH2)], M = Co, Rh, Ir, and Cr, permits an initial-state/transition-state split of the effects of the nature of the metal ion on activation volumes for water exchange. The initial state dominates, which is surprising for a series in which the transition state is believed to range from associative to dissociative. In other words, the partial molar volume of the transition state appears... 

Confirmation of this classihcation comes from studies on the partial molar volumes of ions, and from the compressibility and viscosity of electrolyte solutions and from the activation parameters for water exchange found from NMR and ultrasonic methods. 

Table 2.6.1 provides data on activation volumes for the exchange of water coordinated to some transition metal ions. 

Fig. 4. Bond-making and bond-breaking contributions to the volumes of activation for water exchange on [M(H20)6]z+, AV-t- (7).

Activation Volumes, AV t, for Water Exchange and Ligand Substitution0... 

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.

An attempt is made to account quantitatively for the volumes of activation, AV, of ligand substitution processes. Causes of the pressure-dependence of AV include solvational change, for which a versatile analysis is developed. The pressure-independent AV values of solvent exchange reactions are good measures of the non-sol-vational components of AV for related net reactions. For water exchange, one can predict... 

The volume of activation is probably the easiest parameter to understand conceptually. Consider again the water exchange of Cr(lll), Sec. 2.3.3. The AF"" value of —10 cm mol for Cr(H20)g+ indicates that an associative process pertains (If) since CrflljO) " plus one H2O will occupy more volume than the activated complex which has seven waters associated with the Cr. The volume of coordinated water has been estimated as anywhere between 5 and 9 cm moC, so that AK < — 9 cm mol" for an associative mechanism. Conversely, the value of +2.9 cm moC for water exchange on Cr(H20)50H + suggests a dissociative activation mode for the exchange.More success in interpreting AK values is likely for reactions in which the is small, or at least a small component of the overall AFjj,. The... 

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