Secondary solvent shell

In this simple model for ionic hydration, there is no reason to postulate any major disruption of the secondary solvent shell around ions. This is a popular concept, but it seems to me to be quite unnecessary, and certainly the spectroscopic evidence is against the concept. Such a disordered region would have to give rise to an increase in the number of broken bonds and hence in the concentrations of (LP)f and (OH)f j units. This is clearly not observed. 

Before we consider substitution processes in detail, the nature of the metal ion in solution will be briefly reviewed.A metal ion has a primary, highly structured, solvation sheath which comprises solvent molecules near to the metal ion. These have lost their translational degrees of freedom and move as one entity with the metal ion in solution. There is a secondary solvation shell around the metal ion, but the solvent molecules here have essentially bulk dielectric properties. The (primary) solvation number n in M(S)"+ of many of the labile and inert metal ions has been determined, directly by x-ray or neutron diffraction of concentrated solutions, from spectral and other considerations and by examining the exchange process... 

A Primary solvation shell B Secondary solvation shell C Disordered region D Bulk solvent... 

The equilibria considered up to now have all involved inner sphere complexes. There is the possibility that an inner sphere complex may react with free ligands in solution this includes the solvent itself, to give an outer sphere complex where the ligand enters the secondary solvation shell of the inner sphere complex. If the two species involved in this type of interaction are of opposite sign, which is the situation where this type of complex formation is expected to be most effective, the outer sphere complex is called an ion pair. Fuoss65 has derived an expression (equation 38) for the ion pair formation constant, XIP, from electrostatic arguments ... 

On treating ion solvation it is useful to differentiate between primary and secondary solvation shell or between chemical and physical solvation, respectively The electrostatic calculation of ion solvation is quite often less accurate because specific ion-solvent interactions have to be considered. In the primary solvation shell specific ion-solvent interactions are of much more importance than those with solvent molecules... 

First, immediately after ionization, contact ion pairs are formed, in which no solvent molecules intervene between the two ions that are in close contact. The contact ion pair constitutes an electric dipole having only one common primary solvation shell. The ion pair separated by the thickness of only one solvent molecule is called a solvent-shared ion pair In solvent-shared ion pairs, the two ions already have their own primary solvation shells. These, however, interpenetrate each other. Contact and solvent-shared ion pairs are separated by an energy barrier which corresponds to the necessity of creating a void between the ions that grows to molecular size before a solvent molecule can occupy it. Further dissociation leads to solvent-separated ion pairs Here, the primary solvation shells of the two ions are in contact, so that some overlap of secondary and further solvation shells takes place. Increase in ion-solvating power and relative permittivity of the solvent favours solvent-shared and solvent-separated ion pairs. However, a clear experimental distinction between solvent-shared and solvent-separated ion pairs is not easily obtainable. Therefore, the designations solvent-shared and solvent-separated ion pairs are sometimes interchangeable. Eventually, further dissociation of the two ions leads to free, i.e. unpaired solvated ions with independent primary and secondary solvation shells. The circumstances under which contact, solvent-shared, and solvent-separated ion pairs can exist as thermodynamically distinct species in solution have been reviewed by Swarcz [138] and by Marcus [241],... 

A somewhat different mechanism for the exchange of secondary alcohols and water was put forward by Grunwald et al. (1957) on the basis of results for the exchange and racemization of optically active 1-phenyl-ethanol (2). In OOlN perchloric acid they found exch/ rac = 0 82 004 at two different temperatures. The result is interpreted in terms of the ion pair hypothesis of Winstein and co-workers (1956). It is assumed that the protonated alcohol ionizes to give a planar carbonium ion, but the leaving water molecule does not equilibrate immediately with the solvent. Instead it is held for some finite time in the solvent shell of the carbonium ion, and therefore has a greater chance of being recaptured by the carbonium ion than the water molecules in the bulk of the solution. The chances of return, as compared to escape from the solvent shell, depend... 

Ion-solvent interaction causes orientation of the neighbouring inner solvent molecules and extends with greater or less attenuation into the bulk solution Primary, and in some cases also secondary, solvation shells are chosen as the basis of models. Solvent mixtures introduce the possibility of preferential ion solvation... 

It is only rarely possible to probe the composition or structure of secondary solvation shells. One situation where this is possible is [Cr(NCS)6] in solution in aqueous acetonitrile, described in Section 3 of Chapter 4. Control over solvent shell composition in mixed solvents is most readily achieved by the use of mixtures of co-ordinating and non-co-ordinating solvents. Examples of applications of this include the study of DMSO exchange at nickel(ii) in DMSO-nitromethane and DMSO-methylene chloride mixtures, and of aquo-ions in methylene chloride. ... 

The most common model of ion solvation is the concentric shell model, postulating the existence of several coordination shells around the solvated ion (Enderby and Neilson 1981). However, there remains controversy over the precise definition of primary and secondary solvation shells and numbers, especially for those ions where movement of solvent molecules between shells and bulk solvent is very fast. To avoid ambiguity, the term inner (or first) coordination sphere is used here to refer to both the solvent and the ligand molecules which are in direct contact with the central ion. 

Solvated ions have a complicated structure. The solvent molecules nearest to the ion form the primary, or nearest, solvation sheath (Fig. 7.2). Owing to the small distances, ion-dipole interaction in this sheath is strong and the sheath is stable. It is unaffected by thermal motion of the ion or solvent molecules, and when an ion moves it carries along its entire primary shell. In the secondary, or farther shells, interactions are weaker one notices an orientation of the solvent molecules under the effect of the ion. The disturbance among the solvent molecules caused by the ions becomes weaker with increasing distance and with increasing temperature. 

Since anions are much less solvated in dipolar aprotic solvents (23) than in water, the hydrogen ion will be more highly solvated in the mixed solvent because it is preferentially solvated by monoglyme in the monoglyme-water mixtures rather than in the pure aqueous medium. The selective solvation is an important factor in an understanding of solute-solvent interactions in mixed solvent systems. Unfortunately, the detailed compositions of the primary solvation shell and the secondary mode of solvation (ion-dipole interaction) in mixed solvents are not yet clearly understood. 

Uranyl nitrate has been extracted into solvents such as diethyl ether as the entity [U02(N03)2(H20)4] solvated by two to six molecules of the organic solvent.254-256 The solvation is usually considered to be of the secondary type, in which the solvent molecules are attached by hydrogen bonding to the water molecules in the primary hydration shell. However, IR data have been presented257 258 to support Muller s earlier hypothesis259 that only two of the water molecules are directly bound to the uranyl cation, the remaining coordination sites being occupied by solvent molecules. 

An alternative two-step mechanism involving a spin-paired diradical intermediate has also been considered for 1,3-cycloadditions.18,68,69 However, ab initio calculations70-72 on a wide variety of 1,3-dipoles and dipolarophiles are found to coincide essentially with a synchronous 1,3-cycloaddition mechanism.15,17 On the other hand, a two-step mechanism passing through two transition states separated by an intermediate has been derived using the MINDO/3 method, and found to be compatible with substituent and solvent effects as well as stereospecificity observed in 1,3-cycloadditions.73 However, several factors beyond FMO interactions, such as closed shell repulsions, geometrical distortions, polarization, and secondary orbital interactions, all influence mechanisms, rates, and regioselectivities in cycloaddition reactions.74... 

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