Donor properties of solvents

In relation to the above, the objective of the work was to investigate correlation between electron-donor properties of solvents and C6o solubility. 

The donor number (DN), introduced by Gutmann and Mayer [15-19], represents a measure of the donor properties of solvents and is defined as the numerical value of the heat of adduct formation between the donor molecule and the reference acceptor SbCl5 in dilute 1,2-dichloroethane solution ... 

The data collected by Gritzner [79] and his analysis led him to the conclusion that different interactions of hard and soft cations with donors cannot be accounted for by using only one parameter. Gritzner [79] also tried other correlations. No correlation was found with the acceptor number and the other Lewis acidity parameter, x, introduced by Dimroth and Reichardt [19, 83]. Only those parameters which represent donor properties of solvents are correlated with the change of E /2 potentials for the electroreduction of cations. 

Many other properties of solutes and solutions are affeeted by solvents. Here, we will discuss material stability and stabilization, some aspeets of reactivity (more information on this subject appears in Chapter 13), physical properties, some aspeets of eleetrieal and electrochemical properties (more information on this subject appears in Chapter 11), surface properties, and polarity and donor properties of solvents. 

The majority of useful solvents have donor properties, and the ligands to be coordinated to an acceptor molecule or to an acceptor ion will have to compete for coordination with solvent molecules. Such reactions in solution may be represented as replacement reactions of solvent molecules coordinated to ions or molecules by competitive ligands L or X which may be neutral or charged. The occurrance of replacement or ligand exchange reactions will depend on the relative donor properties of solvent molecules D and competitive ligands L or X towards the ion or molecule under consideration ... 

While the formation of an anionic complex is supported by a low donor number of the solvent and by high donor properties of X, the formation of solvated cations (ionization) is favoured by a high donor number of the solvent. Autocomplex formation will be expected to occur, when the donor properties of solvent and anionic ligand are similar. 

Table 3. Dielectric, Electron Acceptor, and Donor Properties of Solvents Used for Carbocationic Polymerization...

Solvents such as organic liquids can act as stabilizers [204] for metal colloids, and in case of gold it was even reported that the donor properties of the medium determine the sign and the strength of the induced charge [205]. Also, in case of colloidal metal suspensions even in less polar solvents electrostatic stabilization effects have been assumed to arise from the donor properties of the respective liquid. Most common solvent stabilizations have been achieved with THF or propylenecarbonate. For example, smallsized clusters of zerovalent early transition metals Ti, Zr, V, Nb, and Mn have been stabilized by THF after [BEt3H ] reduction of the pre-formed THF adducts (Equation (6)) [54,55,59,206]. Table 1 summarizes the results. 

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. 

Hydrogen bond donor solvents are simply those containing a hydrogen atom bound to an electronegative atom. These are often referred to as protic solvents, and the class includes water, carboxylic acids, alcohols and amines. For chemical reactions that involve the use of easily hydrolysed or solvolysed compounds, such as AICI3, it is important to avoid protic solvents. Hydrogen bond acceptors are solvents that have a lone pair available for donation, and include acetonitrile, pyridine and acetone. Kamlet-Taft a and ft parameters are solvatochromic measurements of the HBD and HBA properties of solvents, i.e. acidity and basicity, respectively [24], These measurements use the solvatochromic probe molecules V, V-die lliy I -4-n i in tan iline, which acts as a HBA, and 4-nitroaniline, which is a HBA and a HBD (Figure 1.17). 

Gutmann introduced3 the concepts of donor number (donicity) and acceptor number (acceptivity), as dimensionless numbers, for the characterization of donor properties of bases independently of the solvent. 

It is apparent that for complex formation in solution the donor properties of the solvent should be as low as possible. Thus a good ionizing solvent will be a poor medium for complex formation since its molecules will compete for coordination with the ligands added to the solution. 

The donor properties of the ligandes I, Br-, Cl", NCS" and N3 are much hi ier than that of nitromethane and the conversion into [C0X4 j2" is complete at the stoichiometric amounts of X" in this solvent. The stabilities of [C0X4 ]2"... 

In a donor solvent the iodide ions is much more strongly solvated than the neutral donor and hence the donor properties of the iodide ion are lowered in solution. This event has been described as the thermodynamic solvatation effect. It becomes increasingly important with an increase of the ratio of the free enthalpy of solvation to the free enthalpy of the ligand exchange reaction. 

In this plot a characteristic curve is found for each ion. The half-wave potentials of Na+, K+, Rb+ and Cs+ (Fig. 22) are similar in each of the solvents. The curve in the j2-DNdiagram reveals that in strong donor solvents E, j2 remains nearly constant at increasing donicity 120 121F This observation suggests that these ions cannot utilize the strong donor properties of such solvents and that solvation is mainly due to electrostatic forces between ion and solvent dipoles. 

Parameters of the Kamlet-Taft solvatochromic relationship. These parameters measure the contributions to overall solvent polarity of the hydrogen bond donor, the hydrogen bond acceptor, and the dipolarity/polarizability properties of solvents. 

The reaction involves the transfer of an electron from the alkali metal to naphthalene. The radical nature of the anion-radical has been established from electron spin resonance spectroscopy and the carbanion nature by their reaction with carbon dioxide to form the carboxylic acid derivative. The equilibrium in Eq. 5-65 depends on the electron affinity of the hydrocarbon and the donor properties of the solvent. Biphenyl is less useful than naphthalene since its equilibrium is far less toward the anion-radical than for naphthalene. Anthracene is also less useful even though it easily forms the anion-radical. The anthracene anion-radical is too stable to initiate polymerization. Polar solvents are needed to stabilize the anion-radical, primarily via solvation of the cation. Sodium naphthalene is formed quantitatively in tetrahy-drofuran (THF), but dilution with hydrocarbons results in precipitation of sodium and regeneration of naphthalene. For the less electropositive alkaline-earth metals, an even more polar solent than THF [e.g., hexamethylphosphoramide (HMPA)] is needed. 

Overall, the review deals mainly with the chemistry in aqueous media, with occasional mention to work in organic solvents. Cyanometallate complexes are known to display profound changes in their electronic structure and reactivity when dissolved in solvents with different acceptor capability, associated with the donor properties of the exposed electron pairs at the cyano ligands (15). These specific interactions are also related to the role of cationic association in the thermodynamics and kinetics of the reactions involving cyano complexes (16). 

The redox potentials of many metalloporphyrin complexes are sensitive to the nature of the electrolyte in which they are measured. This is because one or both axial positions are coordinated by either solvent or anionic ligands. Such dependencies of E° values have been extensively studied by Kadish and coworkers 29 notably good correlation between E° data and Donor/Acceptor Number properties of solvents have been observed. 

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