Nano-corrals for capturing surface electrons are of interest in molecular electronics. Here we show that haloalkane molecules, e.g., 1-chlorododecane, physisorbed on Si(111)-(7 · 7) self-assemble to form dimers stable to 100 C which corral silicon adatoms. Corral size is shown to be governed by the haloalkane chain-length. Spectroscopic and theoretical evidence shows that the haloalkane dimer induces electron transfer to the corralled adatom, shifting its energy levels by 1 eV. Isolation of a labile pre-cursor points to a model for corral formation which combines mobility with immobility; monomers diffusing in a mobile vertical state meet and convert to the immobile horizontal dimers constituting the corrals.