The present work is concerned with odd electron reactivity and uses ab initio computations and VB mixing modeling to develop effective means for conceptualizing and predicting reactivity patterns in the nucleophilic substitution reactions on sigma-cation radicals. All critical species along the reaction profile were characterized for both the backside and frontside substitution pathways. The backside and frontside transition states (TSs) in the case of H2S + C2H6.+ were located also in two solvents. The computational data were then processed by a VB analysis which models the state of the reaction complex along the entire reaction profile in terms of the contributing VB configurations. Subsequently, the VB information was converted into VB mixing diagrams and reactivity paradigms were formulated. The resulting analysis provides insight into the electronic structure and bonding in the TS as well as into the origins of the barrier and stereospecificity in nucleophilic substitutions on cation radicals. Mechanistic predictions are made and isotope effect probes developed for the stereochemistry and the electronic structure of the TSs.