Electron donor-acceptor complexes. 2. Quantitative evaluation of solvent effects on the reactivity of alkylmetals

The kinetics of the halogenolysis of various tetraalkyltin compounds are examined in nonaqueous media with solvents of differing polarity, spanning the range from hexane, carbon tetrachloride, methylene chloride, chlorobenzene, acetone, acetonitrile, to ethyl and methyl alcohol. The rates show widely divergent behavior with solvent changes, and there is no apparent correlation with the activation free energies. However, the inclusion of charge-transfer transition energies leads to a simple relationship (eq 4) with the solvation energies of the tetraalkyltin cations, which are determined independently from the gas-phase ionization potentials and the oxidations in solution of the tetraalkyltin compounds. The solvation energies evaluated in this manner correspond well with those calculated from the modified Born equation. They allow the activation process for the halogenolysis of tetraalkyltin compounds to be dissected separately into electronic, steric, and solvation effects. The relationship of this mechanistic formulation to the Marcus theory of inner-sphere electron transfer is established, and the applicability of the charge-transfer concept generally to the mechanism of electrophilic reactions is stressed.