The interaction of trimethylamine and boron trifluoride on the Si(001) surface has been investigated using X-ray photoelectron (XP) spectroscopy. XP spectra show the TMA forms a dative bond with the clean Si(001) surface, while on the clean surface BF3 bonds dissociatively, yielding Si-BF2 and Si-F species. If the surface is first "saturated" with TMA. however, BF3 will continue to adsorb, giving rise to new, F(1s) XP peaks. Based on an analysis of the energies and thermal behavior of these peaks, we propose that the adsorption of TMA and BF3 on the Si(001) surface leads to the formation of a novel surface-mediated donor-acceptor complex of the structure TMA-Si-Si-BF3.