Donor number general

Gutmann proposed that acid-base characteristics could be represented by a donor number, generally designated as DN (a single symbol, not a product) and an acceptor number AN. DN represents the base properties of the molecule and is essentially a molar heat of reaction with a reference acid, under standardized conditions. AN represents the acid properties of the molecule. The asterisk in AN indicates that the original Gutmann values have been corrected for dispersion force interactions and are expressed as molar enthalpies. Jensen has discussed limitations of the Gutmann DN/AN theory. ... 

Generally the magnitudes of solvent exchange rate constants increase in the sequence NH3 > H20 > DMF > MeCN > MeOH, which is largely independent of the nature of the metal ion (108,109). This sequence cannot be readily identified with specific characteristics such as dielectric constant, donor number, electric dipole moment, or stereochemistry, and it appears to reflect the overall solvent characteristic. There may be a correlation between the AH for solvent exchange on [M(solvent)6]2+ and the heat of dissociation of solvent from this species (110). 

Since monocyclopentadienyl rare-earth complexes require four to six a-donor ligands to reach the stable coordination numbers 7 to 9, monomeric complexes must bind several neutral donor molecules. Generally, some of these donor molecules are easily lost, and dinuclear and polynuclear complexes are formed. This accounts for the fact that monocyclopentadienyl rare-earth complexes exhibit rich structural complexity. Figure 18.4.4 shows the structure of CpErCl2(THF)3. 

Solvents can be classified as EPD or EPA according to their chemical constitution and reaction partners [65]. However, not all solvents come under this classification since e.g. aliphatic hydrocarbons possess neither EPD nor EPA properties. An EPD solvent preferably solvates electron-pair acceptor molecules or ions. The reverse is true for EPA solvents. In this respect, most solute/solvent interactions can be classified as generalized Lewis acid/base reactions. A dipolar solvent molecule will always have an electron-rich or basic site, and an electron-poor or acidic site. Gutmann introduced so-called donor numbers, DN, and acceptor numbers, AN, as quantitative measures of the donor and acceptor strengths [65] cf. Section 2.2.6 and Tables 2-3 and 2-4. Due to their coordinating ability, electron-pair donor and acceptor solvents are, in general, good ionizers cf. Section 2.6. 

Equation (67), which describes the change in the experimental rate constant k [228] in this model, is formally very similar to the equation which was used earlier to describe the inhibition [294, 295] of electrode reactions. Such an equation was used hy Kisova [296]. Later it was found that Eq. (68) better describes the rate constant-solvent composition dependence. However, it fails to describe this dependence at high concentrations of an organic solvent which has a donor number lower than that of water. In general, it fails when the rate constant-solvent composition dependence exhibits a minimum. 

The enthalpy term in this equation can be replaced by the free energy if the entropy term in the Gibbs-Helmholtz equation is assumed to be negligible. In the experience of the authors this is generally a reasonable assumption. If the acceptor and donor number for the probe molecules are known then Ka and Kb, the acid and base number of the surface can be computed. 

Various attempts have been made to classify solvents, e.g. according to bulk and molecular properties empirical solvent parameter scales hydrogen-bonding ability and miscibility >. In table I solvents are divided into classes on the basis of their acid-base properties which can be used as a general chemical measure of their ability to interact with other species. Detailed information on these and other solvents, their symbols, fusion and boiling pointe and Gg), bulk properties (6,Ti, q), and currently-used correlation parameters DN (donor number), Ej-value, and AN (acceptor number) is given in Appendix A-1. 

Solubilities of transition metal halides are frequently low The solubilities of nickel chloride and cobalt chloride are NiCl2 < C0CI2 and are known to increase in general with increasing donor number of the solvent ... 

With dihalides of cobalt(II) and nickel(II), such as the chlorides, bromides and iodides, as well as with the thiocyanates, tetrahedral complexes with general formula MeXa (HMPA>2 are formedAs may be expected these compounds are non-conductors in a solvent of very low donor number, such as nitromethane. Likewise the disolvates of certain metal nitrates, such as cobalt(II), nickel(II) or copper(II) appear to have the nitrate groups located within the coordination spheres. The compound Co(N03)2 (HMPA)2 contains Co(II) in pseudotetra-hedral C2V environment. Cu(N03)2(HMPA)2 is thought to be a planar complex, while in Ni(N03)2(HMPA)2 a distorted tetrahedral structure appears to be present... 

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