Series of ab initio modern valence bond calculations, based on spin-coupled (SC) theory, along the MP2(fc)/6-31G(d,p) minimum-energy paths, are used to examine the electronic rearrangements that take place during the gas-phase S(N)2 identity reactions of Cl- with RCl, where R is methyl, ethyl, or tert-butyl. The corresponding reaction of F- with CH3F is also considered. The SC descriptions of the two CH3X + X- reactions (X = F or Cl) are found to be qualitatively similar, but there is a significantly larger extent of bond formation at the transition state for the fluorine case, and the electronic rearrangements also start much sooner. Comparing CH3Cl + Cl- and CH3CH2Cl + Cl-, the SC calculations suggest that the electronic structure reorganization is largely unaffected by the presence of the additional methyl group. The description of the transition state for the corresponding gas-phase S(N)2 identity reaction of (CH3)(3)CCl is found to be radically different: it is held together by predominantly ionic interactions and most closely resembles a carbocation clamped" between two chloride ions.