ELECTRON-TRANSFER WITH ORGANOMETALS - STERIC EFFECTS AS PROBES FOR OUTER-SPHERE AND INNER-SPHERE OXIDATIONS OF HOMOLEPTIC ALKYLMETALS WITH IRON(III) AND IRIDATE(IV) COMPLEXES

An outer-sphere mechanism is shown for electron transfer to iron(III) complexes from a variety of substitution-stable alkylmetals, particularly tetraalkyltin compounds. In accord with Marcus theory, the second-order rate constant (log k) is linearly related to the reduction potentials E° of a series of substituted trisphenanthroline complexes of iron(III), with a theoretical slope of 8.5. For a given iron(III) oxidant, the ionization potentials IDof a wide variety of homoleptic alkylmetals of four-coordinate tin and lead as well as two-coordinate mercury are also linearly related to log k over more than 108. Electron transfer to iron(III) is not subject to steric effects, and even the highly hindered tetraneopentyltin is included in this correlation. On the other hand, the rates of electron transfer from the same alkylmetals to hexachloroiridate(lV) can be from 10 to 105 times faster than those expected on the basis of E°ircl6-alone. The contribution from an inner-sphere pathway is also indicated by the high susceptibility to steric effects. Thus, a plot of log k for hexachloroiridate(IV) oxidation shows strong, systematic deviations of those tetraalkyltin compounds with α-or β-branched alkyl groups. An inner-sphere mechanism proposed for electron transfer to hexachloroiridate(lV) involves a precursor or activated complex in which the configuration of tetraalkyltin is partially distorted to a quasi-five-coordinate structure. Outer-sphere and inner-sphere processes described in this manner probably represent the extremes of a continuum of mechanisms for electron transfer from alkylmetals. Finally, the products of oxidation are derived by a sequence of fast subsequent steps in which the unstable alkylmetal cation radical undergoes fragmentation, and the resultant alkyl radical is oxidized by a second equivalent of iron(l 11). Selectivity studies with methylethyltin compounds indicate that electron transfer with iron(111) and iridate(l V) produces structurally related cation radicals which are distinct from those generated in the gas phase by electron impact. © 1979, American Chemical Society. All rights reserved.