Donor properties, solvents

The basic premise of Kamlet and Taft is that attractive solute—solvent interactions can be represented as a linear combination of a nonspecific dipolarity/polarizability effect and a specific H-bond formation effect, this latter being divisible into solute H-bond donor (HBD)-solvent H-bond acceptor (HB A) interactions and the converse possibility. To establish the dipolarity/polarizability scale, a solvent set was chosen with neither HBD nor HBA properties, and the spectral shifts of numerous solvatochromic dyes in these solvents were measured. These shifts, Av, were related to a dipolarity/polarizability parameter ir by Av = stt. The quantity ir was... 

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. 

Thioethers lack the capacity to neutralize positive charge and display weak donor properties. Consequently, they do not readily displace strong donor solvents (water) or strongly bonding anions (such as halides) from the coordination sphere. As a consequence, many thioether complex syntheses employ aprotic or alcoholic solvents and precursor complexes with weakly bound solvents (such as DMSO or acetone) or anions (such as C+3S03 ). Despite the synthetic challenges, a wide range of complexes has been reported, particularly with the cyclic poly-thioethers, where the macrocyclic effect overcomes many of the above difficulties. 

Figure 7 shows the positions of FL absorption and fluorescence spectra (in centimeter-1) as a function of %. As it is seen, the graphs for different neutral solvents show linear dependence, whereas for protic solvents (alcohols), a different line can be drawn. Such graphs can be practically used for the determination of n in different samples and solvents. If the measured value will be placed on the line for alcohols, this means that the site of our probe incorporation possesses proton donor property. 

In solvents that have donor properties, solubility leads to complex formation to give species such as S A1C13 (where S is a solvent molecule). Beryllium chloride is soluble in solvents such as alcohols, ether, and pyridine, but slightly soluble in benzene. 

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. 

The major activity in gas-phase studies now depends on the use of modem techniques such as ion cyclotron resonance (ICR). Thus, as already mentioned (Section ELD). Fujio, Mclver and Taft131 measured the gas-phase acidities, relative to phenol, of 38 meta- or para-substituted phenols by the ICR equilibrium constant method, and their results for +R substituents led them to suggest that such substituents in aqueous solution exerted solvation-assisted resonance effects. It was later163 shown by comparison of gas-phase acidities of phenols with acidities of phenols in solution in DMSO that solvation-assisted resonance effects could also occur even when the solvent did not have hydrogen-bond donor properties. Indeed for p-NC>2 and certain other substituents these effects appeared to be larger than in aqueous solution. 

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. 

The redox equilibria can be considerably shifted by the presence of additional donor units. Thus the redox potential in a donor solvent will be influenced by the presence of anions and it may be different for a metal chloride and a metal iodide. The effect becomes more pronounced if the supporting electrolyte contains anions which have donor properties. Such donor anions will compete with solvent molecules for coordination. 

The precipitation of a coordination compound from solution depends on the characteristics of the solvent such as the dielectric constant and the donor property. Since water has a high dielectric constant and appreciable donor character toward the lan-... 

This diversity in solvent properties results in large differences in the distribution ratios of extracted solutes. Some solvents, particularly those of class 3, readily react directly (due to their strong donor properties) with inorganic compounds and extract them without need for any additional extractant, while others (classes 4 and 5) do not dissolve salts without the aid of other extractants. These last are generally used as diluents for extractants, required for improving then-physical properties, such as density, viscosity, etc., or to bring solid extractants into solution in a liquid phase. The class 1 type of solvents are very soluble in water and are useless for extraction of metal species, although they may find use in separations in biochemical systems (see Chapter 9). 

These chemical electron-transfer reactions are in contrast with the electrode reactions. For instance, stilbene gives a mixtnre of 1,2-diphenylethylene with 1,2,3,4-tetraphenylbutane on electrolysis (Hg) in DMF at potentials about that of the first one-electron wave. This solvent has faint proton-donor properties. The stilbene anion-radical is stable under these conditions it has enough time to diffuse from the electrode into the solvent and dimerize therein (Wawzonek et al. 1965). 

Nelsen et al. (2007) have revealed one more aspect of solvent control over charge localization. Solvents with marked electron-donor properties contribute to charge localization in cation-radicals, whereas anion-radicals experience the same changes in better electron-accepting solvents. Thus, naked (non-ion-paired) anion-radicals of 4,4 -dinitrostilbene and 4,4 -dinitrotolane show the spectra of delocalized species in HMPA and THF, but essentially spectra of localized species in DMF, DMSO, and MeCN. 

The solvatochromism of [Fe(phen)2(CN)2] and [Fe(phen)2(CN)2] has been discussed, with particular reference to solvent hydrogen-bond donor properties. Various solvation models have been applied to solvatochromism of [Fe(diimine)2(CN)2], and cormections between solvatochromism, electronic absorption spectra, and color perception parameters discussed in relation to [Fe(phen)2(CN)2]. Solvatochromic properties have been documented for [Fe(bipy)2(CN)2]>... 

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. 

Organic solvents can also be classified according to their ability to accept or transfer protons (i.e., their acid-base behavior) [20,21]. Amphiprotic solvents possess donor as well as acceptor capabilities and can undergo autoprotolysis. They can be subdivided into neutral solvents that possess approximately equal donor and acceptor capabilities (water and alcohols), acidic solvents with predominantly proton donor properties (acetic acid, formic acid), and basic solvents with primarily proton acceptor characteristics (formamide, N-methylformamide, and N,N-dimethylformamide). Aprotic solvents are not capable of autoprotolysis but may be able to accept protons (ACN, DMSO, propylene carbonate). Inert solvents (hexane) neither accept nor donate protons nor are they capable of autoprotolysis. 

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). 

It seemed likely that mechanism B would produce a symmetrical trigonal bipyramid and thus lead to racemization if an optically active substrate was used in the reaction. Mechanism A would certainly lead to total racemization, but mechanism C would not cause loss of optical activity. On the other hand it is possible to draw an asymmetric trigonal pyramid or an asymmetric tetragonal pyramid as a five-coordinate intermediate in mechanism B. The latter seem unlikely in organic solvents with weak donor properties. Furthermore, recent evidence suggests a symmetric trigonal pyramid as an intermediate in the racemization of trisacetylacetonate (50). 

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