Metal-solvent interactions, donicity

The metal-solvent interaction is expected to depend on the donicity of the solvent the higher the donor number of the solvent the stronger the solvent-metal interaction should be. Hence, a correlation between the contact potential difference A>// (a = 0) and the donor number of the solvent should be observed. However, this correlation for the Hg electrode is rather poor, with the most deviant point having been found for water, that is, for the case of the strongest dipole-dipole interaction in the bulk. The correlation is better when acceptor numbers of solvents are taken into account. ... 

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. 

Furthermore, the applicability of the donicity rule may be unexpected for the solvation of alkali metal ions, where a complete explanation of the observations may be provided by considering electrostatic interactions between ion and dipolar solvent molecules. 

Interactions between alkali metal ions and solvent molecules are considered to be essentially electrostatic in nature. Although covalent contributions to the solvate bonds cannot be completely neglected (see Sect. 5d and 6), one woidd not expect the donicities to be generally applicable to the solvation of alkali metal ions. This problem will be discussed in detail in the following section. 

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. 

An interesting observation in this process concerns the influence of the solvent. We used benzonitrile as the solvent, whereas Cacchi s method uses acetonitrile. With benzonitrile electronic factors rule the selectivity, while with acetonitrile the steric aspects seem to predominate. A plausible explanation is based on the difference of polarity between the two solvents as well as their donicity number (a tendency of the solvent to interact with a Lewis acid). As acetonitrile is more polar than the benzonitrile, transition states without the interaction between the palladium and the carbonyl such as 59 and 60 are favored. The opposite phenomenon is expected for the benzonitrile, with its lower donicity. The cationic palladium intermediate is internally solvated by the carbonyl oxygen, thus favoring the cyclic intermediate. With decreased solvation of the cationic palladium, a closer contact of the carbOTiyl moiety to the metal is facilitated (Scheme 17). These data suggest that these parameters explain the key role of the benzonitrile in the Heck-Matsuda reaction. 

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