Electron-Transfer Activation in Electrophilic Mechanisms. Cleavage of Alkylmetals by Mercury(II) Complexes

The disappearance of the transient charge-transfer (CT) absorption bands coincides with the electrophilic (SE2) cleavage of a homologous series of alkyltin compounds by various mercury(II) halides, cyanide, and carboxylates. The second-order kinetics for HgCl2 cleavages afford rate constants which vary in a rather unaccountable way with the structure of the alkyltin compound and with the polarity of the solvent. Furthermore, the relative reactivities of these alkyltin compounds in the analogous electrophilic cleavage by I2 or Br2 show poor correlations with HgCl2 cleavages, in different solvents. However, the description of the activation process as an electron transfer in the precursor complex, e.g, [R4Sn HgCl2] → [R4Sn+ HgCl2], stems from the CT transition energy and leads to a linear free energy relationship in which the activation free energy is equal to the driving force for the formation of the ion pair. The latter is readily dissected by eq 18 into separate changes in electronic, steric and solvation energies. With this mechanistic formulation, the reactivities of various alkyltin compounds follow a remarkably simple linear correlation with the ionization potentials and the solvent effects, in the comparison with I2 and Br2 cleavages. Moreover, the reactivities of the various mercury(II) derivatives relate directly to differences in their electron affinities.