Solvents donicities

The EPA properties of the metal ion decrease with increasing donicity of the solvent so that the stabilizing effect of the radical anion is decreased the redox potential is shifted to more negative values by increasing solvent donicity (Fig. 2). 

The changes in tin coordination are associated with an upheld shift in the tin resonance of the order of 200 ppm and an increase in the Sn—C one-bond coupling of ca 160-200 Hz. Bipyramidal forms are favored if the solvent donicity is increased, including structures in which the solvent molecules occupy the two axial positions of the five-coordinate TBP. 

The basic idea in defining the donicity is based on the work of Lindqvist (29), who obtained from calorimetric measurements an order of relative, solvent, EPD strengths toward SbClg. It was then supposed that the relative EPD properties toward different EPA units (neutral and cationic) could be predicted (at least qualitatively) from the trends observed toward SbClg. This is, indeed, true for a large number of both neutral and ionic substrates (25-28). Figure 1 shows the relationship between F chemical shifts of CF3I dissolved in various EPD solvents as a function of solvent donicity DN (30). Nucleophilic attack of an EPD at the iodine atom causes an electron shift from iodine to the... 

Figure 1 shows the linear correlation between F chemical shift and solvent donicity. No relationship exists between chemical shifts and dipole moments or polarizabilities of the solvent molecules. This result is particularly significant in that it shows that the functional approach can be successfully applied even to weak chemical interactions, for example, to interactions that are usually considered as being due to Van der Waals forces or intermolecular forces in the sense of Mulliken s theory. 

Use of donicity values as a measure of cation solvation and cation stabilization has been demonstrated by polarographic measurements on alkali and alkaline earth metal ions, and various transition and rare earth metal ions (17,16,34). This is illustrated in Fig. 2 which shows the variation of half-wave potentials for the reduction of Tl -vTF, Zn " " -> Zn°, and Eu -> Eu " as a function of solvent donicities. 

Sn(CH3)3l dissolved in nitrobenzene as a function of concentration of various EPD solvents added (35). In noncoordinating or weakly coordinating solvents, such as hexane, earbon tetrachloride, 1,2-dichloroethane, nitrobenzene, or nitromethane, Sn(CH3)3l is present in an unionized state (tetrahedral molecules). Addition of a stronger EPD solvent to this solution provokes ionization, presumably with formation of trigonal bipju amidal cations [Sn(CH3)3 (EPD)2J. Table II reveals that the molar conductivities at a given mole ratio EPD Sn(CH3)3l are (with the exception of pyridine) in accordance with the relative solvent donicities. No relationship appears to exist between conductivities and the dipole moments or the dielectric constants of the solvents. 

The situation becomes more complicated if a given ionization reaction is studied in solvents that differ both in their EPD and EPA properties. This may be illustrated for the complex formation between Co and Cl ions. Qualitatively, stabilities of cobalt-chloro complexes usually decrease with increasing EPD strength of the solvent (25, 26). Quantitative measurements reveal, however, a number of irregularities which cannot be understood by considering the differences in solvent donicities. Accurate thermodynamic data have recently been determined for the reaction... 

Dielectric continuum theory obviously cannot separate ls from lv for bis(dini-trogen) radical cations, because plots of fmax ( = /.) versus y for them are far from being linear. There is an obvious effect that correlates with solvent donicity, because good donor solvents like DMF and DMSO have larger fmax values than acetonitrile, which has a larger y value. We have found that for the rather limited set of solvents that we have employed, fmax can be converted to a linear relationship with y using the following relationship ... 

The rms fit values of Table 7 are small enough compared to our estimated +100 cm 1 accuracy of measuring tJmax to call fmr linear with y. Although we do not believe it can be stated that solvent donicity causes the changes in vmaK that are observed, use of the three-parameter Equation (17) produces A values that we argue are close to 7.v because of their dependence upon structure of the IV compound. As shown in Scheme 7, the z.v values obtained in Table 7 for similar structural units are... 

Covalent contributions should be considerably higher for solvation of soft univalent cations such as T1+ or Ag+. The same applies to solvation of transition metal ions, where even stronger covalent bonds are formed. Hence, one would expect there to be a relationship between solvent donicities and solvation enthalpies of a given metal ion. 

DN = 38.8) is a stronger EPD than DMSO (DN=29.8) but its ability to solvate anions is much smaller 43) compared to other aprotic solvents, and coordination of HMPA to Co2+ (as wdl as to several other metal cations) is definitely sterically hindered (see preceding section). The role of solvent donicity in complex formation is supported by polarographic studies of the reduction of Eu + to Eu2+ in different solvents using supporting electrolytes with anions of different EPD properties (Fig. 8)... 

Fig. 9. Relationship between solvent donicity DN and chemical shift of the Na nucleus in solutions of NaC104 or Na[BPh4] in various EPD solvents reference saturated aqueous NaCl solution...

The applicability of donicities to cation-solvent interactions is most convincingly demonstrated by the polarographic reduction of various metal ions in solvents of different donicity. The observed variation of half-wave potentials with solvent donicity can be explained neither in terms of the Born equation nor by simple microscopic electrostatic models in view of the magnitude of the dipole moments of solvent molecules. The concept also provides the basis for an interpretation of complex formation reactions and the behaviour of electrolytes (ion pair equilibria) in a large number of EPD solvents. 

An increase in solvent polarity (as manifested in the dielectric constant, e) for solvents with roughly similar solvent donicity (from the donor number D ), that is, comparing ethyl acetate to acetone and toluene to dichloromethane, results in an increase in both /cq and Kq for both 1 and 2. However, /c g for 1 shows very little solvent dependence (a decrease of only ca. 1.2-fold is observed from ethyl acetate to acetone, and 1.5-fold from toluene to dichloromethane) whereas for /cre for 2 a one order-of-magnitude increase is observed. We conclude that the reductive elimination is inhibited in solvents of increased polarity, whereas oxidative addition is favoured. As a result for 2 spans a larger range of values compared to 1. 

Figure 27 Gutmann s solvent donicity number, DN, plotted against for corresponding solvents.

Correlation of with Gutmann s DN. The relationship between the scale and Gutmann s Solvent Donicity (DN) numbers provides a context for several additional interesting correlations. Gutmann has defined donicity as the negative A -value in kcal mole for the interaction of the nucleophilic solvent (Lewis base) with SbCIs in a highly diluted solution in dichloroethane. DN values have been reported for 17 HBA solvents whose /5 values are known (3,7,9,11,13,14,18,23,24,25,26,28,29,37,50,52,61) (175). 

However, donor-acceptor interactions are affected not only by the Lewis acid and base strengths, but also by other, steric and electron structural, factors. Thus, even in systems where either solely the donor or the acceptor property of the solvent is manifested, solvents with different space requirements may interact to different extents because of the steric properties of the reference solute and a reference acceptor with a tendency for dative 7c-bonding (back-coordination) will interact more strongly with jr-acceptor solvent molecules (e.g., acetonitrile) than would be expected from their basicity. The solvent donicity investigations by Burger et al [Bu 71, 74] with transition metal complex reference acceptor model systems have clearly shown the great extent to which such secondary effects may distort the solvent scale. 

The Taft method was also used to examine relationships between Gutmann s solvent donicity (DN) and acceptor number (AN) and the tt, a, and jS [Ta 81], It is shown that the AN for nonprotonic solvents correlates well with n and for protonic solvents with a linear combination of tc and a. It was therefore concluded that AN, represented as a measure of the solvent s ability to serve as an electron-pair acceptor, is, in fact, a combined measure of solvent polarity/polarizability and... 

Electron Pair Donicity and AbiUty to Accept a Hydrogen Bond The ability of solvent molecules to donate a free electron pair from their donor atoms (O, N, or S) to coordinate with acceptor atoms of solutes is a measure of the solvent donicity. It can also be construed as its basicity in the Lewis and the Brbnsted senses, because it also describes the ability of the solvent molecules to accept a proton from a Bronsted acid to be protonated or to form a hydrogen bond. 

The kinetics of formation and/or dissociation of the mono-complex of Ni with isoquinoline have been measured in a variety of solvents (Table 1). Neither the dissociation rate constant nor the overall equilibrium constant for complex formation correlates with Gutmann s donicity scale for solvents, but the activation enthalpy for dissociation does show a linear relationship with solvent donicity. This, conclude the authors, can be rationalized from a mechanistic point of view by assuming that the solvent molecules co-ordinated to the metal ion help to stabilize the transition state. A similar correlation between A/f J donicity (Table 1) is found ... 

Taft, R.W., Pienta, N.J., Kamlet, M.J. and Arnett, E.M. (1981) Linear solvation energy relationships. 7. Correlations between the solvent-donicity and acceptor-number scales and the solvatochromic parameters n, a, and / . J. Org. Chem., 46, 661-667. 

Ej.(30) becoming a mixed function of n and a. Later, a number of twin probes were tried, perhaps the most important being those from which the DN ( solvent donicity ) and AN ( acceptor number ) scales of Gutmann [40] were generated. These involve electron donation by solvent to probe, for example, to SbClj, and electron donation by probe, for example, EtjPO to the protonic solvent, respectively. Quite good correlations exist between and DN [5], and between a and AN [1],... 

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