Solvation of cation

The layer of solvent molecules not directly adjacent to the metal is the closest distance of approach of solvated cations. Since the enthalpy of solvation of cations is nomially substantially larger than that of anions, it is nomially expected that tiiere will be insufBcient energy to strip the cations of their iimer solvation sheaths, and a second imaginary plane can be drawn tlirough the centres of the solvated cations. This second plane is temied the outer Helmholtz plane (OHP). 

In mixed solvent systems the difference in the solvating abilities of solvent molecules S and S2 causes a selective solvation of cations and anions [119,120],... 

The best-known solvent parameters are the donor number [21] and acceptor number [22] proposed by Gutmann and coworkers. The donor number (DN) for a donor solvent D is defined as the positive value of the enthalpy difference AH (kcalmol ) for the reaction of D with an acceptor-halide SbCls (D + SbCls D SbCls) in an inert medium such as 1,2-dichloroethane. DN is a fair measure for the donor properties of a solvent. The correlations of DN with the solvation energies are known to be good particularly for solvation of cations. A typical example [19] is shown in Fig. 3. 

The acidity dependence of the activation enthalpies and entropies, All and AS. of the hydration of 1,3-cyclohexa- and 1,3-cyclooctadienes was ascribed30 to a dielectric solvation effect in dilute acids, which is overcome by increasing solvent structure as the availability of water decreased in concentrated acids. This suggestion was one of the early premises of a more recent interpretation31 of acidity effects in terms of water activity and solvation of cationic species. 

Both steric and electronic effects of substituents upon experimental pXa-Vcdues provide information about protonation sites. In view of the small steric requirement of the proton, steric effects in protonation arise most often from steric inhibition of solvation of cations, which results in a reduction of their stability (pA a smaller). 

B) The AG -values derived from formation constants of complexes in methanol (cf. Table 2) are not identical with the free energies of transfer AG, as defined by Eq. (30), because the cations are already solvated by the nonaqueous solvent prior to the complexation. These values are representative, none the less, since the solvation of cations in water and methanol is very similar (95). 

Crown ethers are large-ring cyclic ethers with several O atoms. A typical example is 18-crown-6 ther, Fig. 14-l(a). The first number in the name is the total number of atoms in the ring, the second number is the number of O atoms. Crown ethers are excellent solvaters of cations of salts through formation of ion-dipole bonds. 18-Crown-6 ether strongly complexes and traps K, [from, e.g., KF, as shown in Fig. 14-1(6)]. 

Solvation of cation and ligand. The solvation free energy increases in the order K+ < Na+ < Ca2+, hence less energy is required to (partially) desolvate K+ in order to bind it. 

Although solvents may be classified as donor solvents (Lewis bases) and acceptor solvents (Lewis acids), most of the more widely used nonaqueous solvents are donor solvents. Some acceptor solvents, such as S02, BrF3, AsC13, or the liquid hydrogen halides, have proved to be useful in coordination chemistry.13"16 Ionization is promoted in a donor solvent by solvation of cations and in an acceptor solvent by solvation of anions. For example, arsenic(m) iodide is ionized in a donor solvent D according to the reaction... 

The solvation of cations and anions by water molecules is possible due to the dipolar nature of water in the sense that oxygen and hydrogen have partial negative and positive charges respectively. 

While there are many indications that oxygen coordinated solvation of cations in NMA is predominant, there have been suggestions that nitrogen coordination solvation of cations also may occur216,217). Much of the evidence for this is based on the interpretation of vibrational spectral data for solid solvates and these data have shown a remarkable flexibility toward interpretation217 218). It is quite possible that the type of coordination may depend strongly on the particular ion involved. 

The dissociation constants of trityl and benzhydryl salts are KD 10 4 mol/L in CH2C12 at 20° C, which corresponds to 50% dissociation at 2-10-4 mol/L total concentration of carbocationic species (cf. Table 7) [34]. The dissociation constants are several orders of magnitude higher than those in analogous anionic systems, which are typically KD 10-7 mol/L [12]. As discussed in Section IV.C.l, this may be ascribed to the large size of counterions in cationic systems (e.g., ionic radius of SbCL- = 3.0 A) compared with those in anionic systems (e.g., ionic radius of Li+ 0.68 A), and to the stronger solvation of cations versus anions. However, the dissociation constants estimated by the common ion effect in cationic polymerizations of styrene with perchlorate and triflate anions are similar to those in anionic systems (Kd 10-7 mol/L) [16,17]. This may be because styryl cations are secondary rather than tertiary ions. For example, the dissociation constants of secondary ammonium ions are 100 times smaller than those of quaternary ammonium ions [39]. 

Several aspects of solvation phenomena will be considered solvation of cations, interaction of cations with other groups, and phenomena of electrolytic dissociation. The essential general features will be covered if we consider on the one hand a solvent of high dielectric constant, such as water, and on the other, remark on the differences in the state of an elec-trol3d produced by dissolving it in solvents of low dielectric constant. Special emphasis will be given to the subject of hydration of ions, because most of the work on redox reactions has been done with water as solvent. 

There seems little doubt that in radiation induced polymerizations the reactive entity is a free cation (vinyl ethers are not susceptible to free radical or anionic polymerization). The dielectric constant of bulk isobutyl vinyl ether is low (<4) and very little solvation of cations is likely. Under these circumstances, therefore, the charge density of the active centre is likely to be a maximum and hence, also, the bimolecular rate coefficient for reaction with monomer. These data can, therefore, be regarded as a measure of the reactivity of a non-solvated or naked free ion and bear out the high reactivity predicted some years ago [110, 111]. The experimental results from initiation by stable carbonium ion salts are approximately one order of magnitude lower than those from 7-ray studies, but nevertheless still represent extremely high reactivity. In the latter work the dielectric constant of the solvent is much higher (CHjClj, e 10, 0°C) and considerable solvation of the active centre must be anticipated. As a result the charge density of the free cation will be reduced, and hence the lower value of fep represents the reactivity of a solvated free ion rather than a naked one. Confirmation of the apparent free ion nature of these polymerizations is afforded by the data on the ion pair dissociation constant,, of the salts used for initiation, and, more importantly, the invariance, within experimental error, of ftp with the counter-ion used (SbCl or BF4). Overall effects of solvent polarity will be considered shortly in more detail. 

Another problem with the MSA is that it does not distinguish between the solvation of cations and anions of the same size. Thus, although the K and F ions have approximately the same radius, the F anion is more strongly solvated than the K cation (table 3.4). This can be understood in terms of the effect that each ion has on local water structure. The K ion disrupts this structure more so that the stabilizing effect of the local ion-dipole interactions is offset by the work done to break up the water structure, that is, to disrupt attractive dipole-dipole interactions and hydrogen bonding between local water molecules. This means that the parameter 8s should be different for cations and anions in the same solvent (table 3.5). 

The value of X for a typical polar solvent is approximately two. This equation was introduced earlier in the development of the MSA for ion-solvent interactions (section 3.5). It was seen that the MSA gives an improved description of ion solvation parameters with respect to the Born model. However, it fails to distinguish between the solvation of cations and anions of the same size. In other words, it fails to distinguish between the short-range chemical interactions which stabilize ions of differing charge. 

The superiority of liquid ammonia and hexamethylphosphotriamide over all other solvents in dissolving alkali metals and serving as a medium for the generation of solvated electrons is linked to their high donor activity and strong solvation of cations as well as to their sufficiently high dielectric constants. 

Solvation of cations by poly(ethylene oxide) chain is highly cooperative, showing the phenomenon "nothing or everything", i.e. the cation is either not solvated or fully solvated using its complete coordination ability ... 

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