Gas phase identity ion pair S(N)2 reactions at nitrogen LiX + NH2X --> XNH2 + LiX (X = F, Cl, Br and I) are investigated at the G2M(+) level. Two reaction mechanisms, inversion and retention, are proposed. Results indicate the inversion mechanism is favorable for all halogens. Including Li in the anionic S(N)2 model will raise the overall barriers for the LiX + NH2X (X = F, Cl and Br) reactions and lower the overall barrier for the LiI + NH2I reaction. The stabilization energies for complexes (XLiNH2X)-N-... increase in the order: F < Cl < Br < I and show a good correlation with halogen electronegativities. (C) 2003 Elsevier Science B.V. All rights reserved.