Transition state structures have been found in ab initio calculations of identity reactions of lithium fluoride and chloride and of sodium chloride with methyl fluoride and chloride. The retention transition state can be considered as an S(N)i type in which a methyl cation interacts with a metal halide triple ion, MX(2)(-). The inversion transition states are highly bent from the linear X...C...X bond angles found for the ionic analogs. The inversion TS can also be considered as an ionic assembly. These bond angles become more linear and the activation energies are reduced for reactions with ion pair aggregates; such transition states can be considered as a (XCH(3)X)(-) triple ion interacting with a (MXM)(+) triple ion.