Photo-excitation of chromophoric metal complexes electrostatically adsorbed to tin-oxide semiconductor nanoparticles is often accompanied by injection of electrons from the complexes into the semiconductor conduction band. The mechanism of back electron transfer (semiconductor particle to adsorbed molecule) for a family of tris-bipyridyl ruthenium and osmium complexes has been examined by evaluating the kinetics of transfer to derivatives featuring alkyl substituents of varying length, methyl to pentyl. The substituents serve to change the electron transfer (ET) distance under conditions of weak chemical interaction with the semiconductor surface. Accompanying increases in alkyl substituent length, and therefore transfer distance, are systematic decreases in back ET rate. The decreases are indicative of nonadiabatic ET, i.e. electronic rather than nuclear control of the reaction dynamics. Further analysis points to trap-mediated transfer, rather than direct transfer from the conduction band, as the most probable back-reaction pathway.