The proposed SN2 reactions of a hindered organophosphorus reactant with aliphatic and aromatic nucleophiles [Ye et al., Org. Lett. 19, 5384–5387 (2017)] were studied theoretically in order to explain the observed stereochemistry of the products. Our computations indicate that the reaction with the aliphatic nucleophile occurs through a backside SN[email protected] pathway while the reaction with the aromatic nucleophile proceeds through a novel SN[email protected] mechanism, followed by a frontside SN[email protected] mechanism. To the best of our knowledge, this is the first time that a SN[email protected] mechanism is reported. We also found that on reducing the bulkiness of substituents around the phosphorus atom, the backside SN[email protected] mechanism is preferred. The conclusions made from investigating the steric effect should help experimentalists to decide for the organophosphorus reactant to achieve the products of desired stereochemistry.