Primary solvation numbers

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... 

Table III. Primary Solvation Numbers. Comparison of Results from the Variation of B (vise.) with the Most Probable Values by Other Methods...

Because NH3(1) has a much lower dielectric constant than water, it is a better solvent for organic compounds but generally a poorer one for ionic inorganic compounds. Exceptions occur when complexing by NH3 is superior to that by water. Thus Agl is exceedingly insoluble in water but NH3(1) at 25°C dissolves 207 g/100 cm3. Primary solvation numbers of cations in NH3(1) appear similar to those in H20 (e.g., 5.0 0.2 and 6.0 0.5 for Mg2+ and Al3+, respectively), but there may be some exceptions. Thus Ag+ appears to be primarily linearly 2-coordinate in H20 but tetrahedrally coordinated as [Ag(NH3)4]+ in NH3(1). It has also been suggested that [Zn(NH3)4]2+ may be the principal species in NH3(1) as compared to [Zn(H20)6]2+ in H20. 

In 1938, Passynski made the following argument, which reiates the compressibii-ity of a solution to the sum of the primary solvation numbers of each ion of an electrolyte. Primary here means ions that are so compressed hy the ions field that they themselves have zero compressibility. 

Let the g-moles of salt be n, these are dissolved in n, g-moles of solvent. Then, there are cai /n2 g-moles of incompressible solvent per g-mole of solute. This was called by Passynski (not unreasonably) the primary solvation number of the salt, although it involves the assumption that water held so tightly as to be incompressible will qualify for primary status by traveling with the ion. 

To obtain individual ionic values, one has to make an assumption. One takes a large ion (e.g., larger than T) and assumes its primary solvation number to be zero," so that if the total solvation number for a series of salts involving this big anion is known, the individual hydration numbers of the cations can be obtained. Of course, once the hydration number for the various cations is determined by this artifice, each cation can be paired with an anion (this time including smaller anions, which may have significant hydration numbers). The total solvation numbers are determined and then, since the cation s solvation number is known, that for the anion can be obtained. 

It has been stressed that solvation is a far-reaching phenomenon, although only the coordination number and the primary solvation number can be determined. However, there are effects of ions on the properties of solutions that lie outside the radius of the primary hydration sheath. These effects must now be accounted for, insofar as they relate to the solubility of a nonelectrolyte. Let the problem be tackled as though no primary solvation had withdrawn water from the solution. One can write... 

Analyze all this and produce a clear and reasoned judgment, buttressed by reasonings in a multipage analysis. Include in your discussion a ranking of methods for primary solvation numbers. 

H2O molecules in the hydration spheres with the bnlk H2O solvent varies by many orders of magnitude depending on the metal ion. This, combined with the use of many different experimental techniqnes, has resulted in discrepancies in the literatnre, particnlarly for metal ions with a weakly bonnd, rapidly exchanging hydration sphere. The Na+ cation, for example, has had qnoted values for the primary solvation number ranging from 2 to 13, although a value close to six is generally accepted." ... 

In the field of structural determination NMR results agree well with those of other techniques, (although there have been conflicting results as for example the site of protonation of amides,which seem now to have been resolved ). This is not quite the case for determinations of primary solvation numbers of cations, where a wide range of techniques have given a wide range of values. However, values determined by different NMR techniques agree with one another, and with ultrasonic values, and are entirely reasonable. 

There is fairly extensive contradiction and uncertainty in the literature in connection with the solvation number [De 69]. The introduction of the concept of the primary solvation number is associated with Bockris [Bo 49]. According to this definition, the solvation number is the number of solvent molecules which are so strongly attached to the dissolved ion that they lose their degree of translational freedom and move together with the dissolved ion in the course of the Brownian movement. 

The primary solvation number may be determined by means of various mutually independent methods. However, it must be noted that the different methods do not yield identical values in every case. Padova [Pa 63b, Pa 64a] calculated the solvation numbers (n) of certain electrolytes from the molar volumes. He used the assumption that the solute ion gives rise to such a strong electrostatic field that the solvate sheath consisting of solvent molecules bound in the first coordination sphere becomes incompressible. Thus, the molar volume (cm /mole), of the solvated electrolyte can be described by the equation... 

So far as I know, this is one of the best methods for estimating primary solvation numbers experimentally. In principle, neutron diffraction should be superior, although it gives somewhat different information, but it has not yet been used very successfully for systems such as those under consideration here. Using these procedures, solvation numbers for a range of probe solutes have been obtained (Table 3.2). It is noteworthy that the solvation number for water... 

The hydrogen bond donating powers of water molecules and alcohols are comparable, both for the monomers in inert solvents and for the bulk solvents. Nevertheless, our results show that the primary solvation number for hydrogen bond acceptor solutes (such as Mc2CO, MeCONMc2 or anions) is usually at a maximum for aqueous solutions, but considerably less for methanolic solutions. 

Primary solvation number of magnesium (I I) in liquid ammonia 243... 

---