Formation constants of charge-transfer complexes. Steric effects in alkylmetal donors

The formation constants of charge-transfer (CT) complexes of iodine with various alkylmetals, including a series of homologous tetraalkyltin compounds, are measured in methylene chloride solutions. The Benesi-Hildebrand method could not be used because of complications from a competing cleavage of alkylmetals by iodine (iodinolysis), but a technique involving the kinetic analysis of the iodine absorbance is described for the determination of the formation constants K. From the values of K and the measurements of the heats of formation, the alkylmetal-iodine system can be considered as forming weak CT complexes. The formation constants (log K) for a series of dialkylmercury compounds are linearly correlated with their vertical ionization potentials, but the tetraalkyltin compounds show considerable scatter in the same plot of the data. The importance of steric effects on the formation constants of CT complexes is analyzed with the aid of Mulliken CT theory, in which K is expressed in terms of the resonance energy β by the application of second-order perturbation. The resonance energies, after correction for steric effects, are shown to be a linear function of the ionization potentials for all alkylmetals. Finally, the magnitude of the steric effect accords with that obtained directly from the experimental transition energies described in the foregoing study.