Solvent Exchange at the Metal Ion

In this and the next three sections, we shall only consider work involving the simple, uncomplexed metal ion the effect of ligands already bound to the metal will be discussed separately in Section 6. 

It will be apparent that any attempt to elucidate the mechanism of substitution at labile metal ions must involve itself in a consideration of the kinetics of the solvent exchange process. Since the pioneering work of Swift and Connick, the n.m.r. line-broadening and pulse techniques have been used more and more frequently for this purpose. 

Reuben and Fiat have studied the concentration and temperature dependence of the transverse relaxation time in aqueous solutions of the perchlorates of Tb +, Dy +, Ho +, Er +, and Tm +. Lower estimates for the rate constants of water exchange were found to be in the range 0-3—2 6 X 10 s S and an upper limit of 5 kcalmoL was estimated for the activation enthalpy of this process. These results are of especial interest in the light of the variation in overall complex formation rate kf found with the members of the lanthanide series, discussed in Section 5. 

The comparatively large water-exchange rates for these trivalent ions have been attributed to their having co-ordination numbers greater than six. 

An interesting practical point is also made in Reuben and Fiat s paper. They suggest that spherically-shaped samples are preferable for studies involving line-widths in paramagnetic solutions in order to minimize inhomogeneous broadening. They point out that this artefact may be a cause of some of the inconsistencies found in the literature. 

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Bain-Ackerman and Lavallee " have measured the rate parameters for the entry of five bivalent metal ions into A-methyltetraphenylporphyrin in dmf solution. All reactions follow a second-order rate law and with the exception of Mn(II), the order of the porphyrin metalation rate constants (Cu(II) > Zn(II) > Co(II) > Ni(II)) coincides with that of solvent exchange at the metal ions. The values of k for this deformed porphyrin (Table 6.4) are all larger than for the corresponding metal ion reacting with planar porphyrins. The authors favor a mechanism which involves solvent dissociation from the metal ion as an important rate-determining factor but they also point out that porphyrin deformation... 

Two preliminary accounts have appeared which report new approaches to the problem of the mechanism of complex formation in water. The reaction between Nd " and S04 was investigated by ultrasonics in H2O and D2O and it was concluded from the fact that the reaction was 2.3 times slower in D2O than in HgO (whereas the dissociation step was accelerated) that solvent exchange at the metal ion cannot be the controlling step in lanthanoid complexation reactions. A high-pressure cell with spectrophotometric detection was used in conjimction with a laser temperature-jump apparatus to measure the volumes of activation of the reaction of Co + and Ni + with the bidentate ligand pyridine-2-azo-p-dimethylaniline (1), and of Ni + with NH3. In all cases the value of AV was ca. 8 cm mol, a value which corresponds to a considerable fraction of the molar volume of water ... 

Ultrasonic studies have been reported on solutions of copper(ii) nitrate and perchlorate in ethylene glycol. In each case a single, concentration-independent relaxation effect is observed which is probably due to cation desolvation coupled with the diffusive approach of the two solvated ions. Tanaka has proposed a modification to the Bennetto-Caldin scheme for solvent exchange at bivalent metal ions. 

An idealized temperature dependence of the relaxation rates is shown in Figure 10.4. The parameter plotted, is the difference between the relaxation rate in the presence of the exchanging species and the rate for the pure solvent, divided by the metal ion concentration. In the high-temperature limit at the left of Figure 10.4, exchange is fast and relaxation is controlled by the nuclear relaxation rate in the inner coordination sphere of the metal ion, T2m - temperature is lowered, exchange becomes... 

The n.m.r. technique has been used to measure the kinetics of solvent exchange at the beryllium and magnesium ions in trimethyl phosphate (tmp). In both ca, a temperature range could be found over which the H signal of tmp in the first coordination sphere of the metal appears as a doublet downfield from the doublet signal of tmp in the bulk solvent. With Be +, the co-ordination number was found to be four and the rate law... 

Solvent Structure. There has been some discussion of the importance of solvent structure in kinetics, for example in connection with aquation of cobalt(m) complexes in binary aqueous mixtures. There are difficulties in squaring the kinetic parameters for dimethylformamide and for dimethyl sulphoxide exchange at iron(u) with Bennetto and Caldin s model of solvent structural effects, but this model proved useful in discussion of the aquation of [Co(NH3)s(DMSO)] + in binary aqueous mixtures. Bulky hydrophobic groups in solvent molecules have an effect on solvent structure which is reflected in the kinetics of complex formation. - For dissociative solvent exchange at some M + ions, activation enthalpies appear to be determined by the solvation enthalpy of the metal ion and the solvent structure as manifested in its enthalpy of vaporization. In the reaction of Ni + with malonate, the range of solvent variation of activation parameters is comparable with their likely errors, preventing the authors from discussing their results in terms of Bennetto and Caldin s theories. ... 

The stereochemistry of hydrogen-deuterium exchange at the chiral carbon in 2-phenylbutane shows a similar trend. When potassium t-butoxide is used as the base, the exchange occurs with retention of configuration in r-butanol, but racemization occurs in DMSO. The retention of configuration is visualized as occurring through an ion pair in which a solvent molecule coordinated to the metal ion acts as the proton donor... 

The smaller contribution to solvent proton relaxation due to the slow exchanging regime also allows detection of second and outer sphere contributions (62). In fact outer-sphere and/or second sphere protons contribute less than 5% of proton relaxivity for the highest temperature profile, and to about 30% for the lowest temperature profile. The fact that they affect differently the profiles acquired at different temperature influences the best-fit values of all parameters with respect to the values obtained without including outer and second sphere contributions, and not only the value of the first sphere proton-metal ion distance (as it usually happens for the other metal aqua ions). A simultaneous fit of longitudinal and transverse relaxation rates provides the values of the distance of the 12 water protons from the metal ion (2.71 A), of the transient ZFS (0.11 cm ), of the correlation time for electron relaxation (about 2 x 10 s at room temperature), of the reorienta-tional time (about 70 x 10 s at room temperature), of the lifetime (about 7 x 10 s at room temperature), of the constant of contact interaction (2.1 MHz). A second coordination sphere was considered with 26 fast exchanging water protons at 4.5 A from the metal ion (99), and the distance of closest approach was fixed in the range between 5.5 and 6.5 A. 

Examinations of possible correlations between the volume of activation and the entropy of activation for series of similar reactions have been reported for reactions of transition metal coordination compounds, such as solvent exchange, ligand substitution, or isomerization.163 167 A limiting factor in a potential correlation may be the lack of precision that often attends experimental determination of the entropy of activation. Attention has been drawn specifically to the qualitative nature of the correlations between the two parameters for solvent exchange at some 3 + cations, and at square planar Pd2+ and Pt2+ ions.168... 

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