Homolytic substitution (SH2) of tetramethyltin by an iodine atom is shown to be the rate-limiting step in the photochemically initiated radical chain process for iodinolysis in carbon tetrachloride solutions. The reactivities of various tetralkyltin compounds with different electronic and steric properties are measured from the kinetics of the iodinolysis. The activation energy, △G‡, for the (SH2) process is found to be quantitatively related to the driving force for ion-pair formation, i.e., R4Sn + I. →[R4Sn+.I-], by evaluating (1) the free-energy change, △G°, for the oxidation of the tetraalkyltin compound and (2) the interaction energy, △E, of the ion pair. The latter is obtained independently from the charge-transfer interaction of the alkyltin compound with iodine. The formation of the ion pair as a reactive intermediate in (SH2) reactions is supported by selectivity studies, in which intramolecular competition i n the homolytic displacement of alkyl ligands from unsymmetrical tetraalkyltin compounds is found to be the same as that observed in the fragmentation of the cation radical R4Sn+â. generated independently by electron transfer to hexachloroiridate(IV). Solvent studies support a rather polar transition state for the (SH2) iodinolysis of alkylmetals, in accord with the charge-transfer mechanism in Scheme II. The direct comparison of the activation energies for the cleavages of alkylmetals by an iodine atom and molecular iodine in the same solvent allows parallels to be drawn for homolytic (SH2) and electrophilic (SE2) mechanisms.