The gas-phase ion pair S(N)2 reactions at nitrogen, LiY + NH2X --> NH2Y + LiX (Y, X = F, Cl, Br, or I), with inversion mechanisms are investigated at the level of modification of G2 theory, and the results are compared with two related reactions (Y- + NH2X and LiY + CH3X)-Calculated results show that the LiY + NH2X reactions are exothermic only when the nucleophile is a heavier lithium halide. The complexation enthalpies are found to depend primarily on the identity of nucleophile LiY and only to a small extent on the identity of NH2X, decreasing in the order LiI > LiBr > LiCl > LiF. Including a Li cation in the anionic S(N)2 model will raise the overall barriers for the LiY + NH2X (Y, X = F-Br) reactions and lower the overall barriers for the LiI + NH2X (X = F-1) reactions. Another interesting feature of the ion pair reactions at nitrogen is the good correlation between the reaction barriers with the geometrical looseness of Li-Y and N-X bonds in the transition-state structures. The data for the reaction barriers show good agreement with the prediction of the Marcus equation and its modification. Kinetic, thermodynamic investigations and NBO analyses predict that the nucleophilicities of LiY in the gas-phase ion pair S(N)2 reactions at nitrogen decrease in the order LiI > LiBr > LiCl > LiF.