The reactions of O,O-diphenyl Z-S-phenyl phosphorothiolates with X-pyridines have been studied kinetically in acetonitrile at $35.0{^{circ}C}$. The Hammett plots for substituent (Z) variations in the leaving group (log $k_2$ vs. $sigma$ Z) are biphasic concave downwards with breaks at Z = H. The large magnitudes of ${ ho}X( ho_{nuc})$, ${eta}X( ho_{nuc})$, and the cross-interaction constant, $ ho$XZ, suggest frontside nucleophilic attack toward the leaving group. The sign reversal of $ ho$Z from positive in $sigma$ Z $leq$ 0 to negative in $sigma$ Z $geq$ 0 is interpreted as the change in mechanism from concerted to stepwise with rate-limiting expulsion of the leaving group. The anomalous negative sign of $ ho$ Z for leaving groups with electron-withdrawing substituents is interpreted as the intramolecular ligand exchange process of the leaving group from the equatorial position in the intermediate to the apical position in the TS.