Inner-sphere hydration numbers

Kimura T, Kato Y (1998) Luminescence study on determination of the inner-sphere hydration number of Am(III) and Nd(ni). J Alloys Compd 271 867-871... 

Assuming that the interaction between the primary hydration sphere and the secondary water molecules of hydration are strongly electrostatic for both f-element series, Fourest et al. (1984) were able to estimate the inner-sphere hydration numbers of the trivalent actinide ions by interpolation using the values for the lanthanide elements (Habenschuss and Spedding 1979a, b, 1980). The interpolated values are listed in the last column of table 3 and shown in fig. 3. 

It will be seen from Table 5 that the variation of parameters with atomic number is by no means always monotonic. The explanation of this must again be connected with variation of coordination number along the series of lanthanides. If the strongly coordinating organic ligands are reasonably assumed to exert their full ligancy, then any variation must be in the extent of inner-sphere hydration of the various species, as it is rather unlikely that there is a... 

Rizkalla and Choppin (1991, 1994) have reviewed the hydration of lanthanide ions. They report that experimentally determined values (by electrophoresis and diffusion) for the hydrated radii of the lanthanides increase from La to Dy but i jparently level off for the heavier lanthanides (fig. 7). Replicate determinations by different authors place the uncertainty on these experimental values at 0.02-0.03 A. The apparent discontinuity near Tb is curious, but is paralleled by the heats and fiee energies of formation of the aquo cations. The similarity suggests that the observed trend represents a real chemical characteristic of the ions, perhaps related to the change in the inner-sphere coordination number or the balance of inner-sphere/second-sphere hydration. A simple analysis of the ions based on these hydrated radii indicates hydration numbers of 12-15 across the series (Lundqvist 1981). David and Fourest (1997) offer a more detailed interpretation that suggests a larger number of waters associated with the lanthanide cations. 

Proton relaxation NMR using Gd(IH) support these conclusions. Spin-lattice relaxation (Ti) values are inversely related to the average number of inner sphere hydration waters of the lanthanide cation (50-52). Chang reported hydration values of 3 and 1 for Gd complexes of EDTA and DTPA, respectively. Both NMR relaxation studies and luminescence lifetime data indicate a total coordination number between 8 and 10 for the lanthanides in aminopolycarboxylate complexes. 

Three papers have dealt with manganese(n) systems. Gale and co-workers proposed a simple method to estimate the inner-sphere hydration state of the Mn(ii) ion in coordination complexes and metal-loproteins. The method makes use of the 0 linewidth measurements for bulk water in the presence and in the absence of Mn(n), which allows the determination of transverse 0 relaxivity. Doing this as a function of temperature and finding the maximum yields a quantity which is directly proportional to the number of inner-sphere water ligands. Molnar et... 

It is clear from the above equations that numerous parameters (proton exchange rate, kcx = l/rm rotational correlation time, tr electronic relaxation times, 1 /rlj2e Gd proton distance, rGdH hydration number, q) all influence the inner-sphere proton relaxivity. Simulated proton relaxivity curves, like that in Figure 3, are often used to visualize better the effect of the... 

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) ... 

Excitation spectra have been of considerable use recently in studying both hydration numbers (by lifetime measurements) and inner-sphere complexation by anions (by observing appearance of the characteristic frequencies for e.g. the Eu3+ 5D0-+ 7F0 transition for the different possible species). Thus using a pulsed dye laser source, it was possible to demonstrate the occurrence of inner sphere complexes of Eu3+ with SCN, CI or NO3 in aqueous solution, the K values being 5.96 2, 0.13 0.01 and 1.41 0.2 respectively. The CIO4 ion did not coordinate. Excited state lifetimes suggest the nitrate species is [Eu(N03)(HzO)6,s o.4]2+ the technique here is to compare the lifetimes of the HzO and the corresponding D20 species, where the vibrational deactivation pathway is virtually inoperative.219 The reduction in lifetime is proportional to the number of water molecules complexed.217 218... 

All Gd(III) chelates approved for contrast agent application have one inner sphere water molecule. The inner sphere proton relaxivity is linearly proportional to the hydration number q (Eq. (5)), thus a higher q would result in increased relaxivities. However, ligands that leave space for more than one water molecule in the first coordination sphere form complexes of reduced stability,... 

X-ray diffraction studies on hydrated triorganotin compounds H2O — SnRsX have provided several examples of water engaged in inner-sphere coordination to tin in five-coordinate molecules. For inner-sphere coordination, the complexes tend to pack in arrangements in which the number of hydrogen bonding interactions involving the coordinated water molecule is maximized. For example, o-phenanthroline monohydrate, which chelates to a plethora of transition metals, forms with triphenyltin chloride an outer-sphere coordination complex in which the coordinated water molecule [O — Sn = 2.42 A]... 

The ESR spectra of monolayer hydrates of Cu-doped Ca-beidellite and Ca-montmorillonite, with the samples comprising layers oriented perpendicularly to the direction of the applied magnetic field, is essentially of the four-peak type, whereas that when the sample layers are oriented parallel to the magnetic field is of the one-peak type. These results indicate that the symmetry axis of the Cu2+ complex is perpendicular to the smectite siloxane surface [39,41]. The ESEM modulation pattern [41] shows further that the number of nearest-neighbor water molecules is 4, with a Cu2+-D+ distance of 0.29 nm, which is consistent with the 0.198-nm Cu—O distance observed for solvated Cu2+ in aqueous solution [20]. These results and the ESR data are consistent with the inner-sphere surface complex illustrated on the left in Fig. 8. The bivalent cation is coordinated to siloxane surface oxygen ions along the symmetry axis of the complex and to four... 

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