Negative ions formed in the reaction A** + BC (--->) A('+) + BC('-) where A** is a highly-excited Rydberg atom and BC is an electron-attaching molecule exhibit very long lifetimes against autodetachment. This has led to speculation that these negative ions may be stabilized by a post-attachment interaction with the Rydberg atom ion core. In such a stabilization process the Rydberg ion core would carry away some of the vibrational energy of the negative ion. To investigate this possibility the kinematics of Xe(nf) - SF(,6) collisions have been studied and the data compared to the results of a Monte Carlo calculation. These comparisons indicate that few electron transfer reactions lead to significant energy transfer to the Rydberg ion core, implying that the observed long lifetime of the SF(,6)('-) ion is a consequence of the very low initial translational energy of the Rydberg electron, rather than a post-attachment interaction.