Ammonia, alkyl amines, and aryl amines are found to undergo rapid intermolecular N-H oxidative addition to a planar mononuclear sigma(3)-phosphorus compound (1). The pentacoordinate phosphorane products (1.[H][NHR]) are structurally robust, permitting full characterization by multi. nuclear NMR spectroscopy and single-crystal X-ray diffraction. Isothermal titration calorimetry was employed to quantify the enthalpy of the N-H oxidative addition of n-propylamine to 1 ((PrNH2)-Pr-n + 1 -> 1.[H][(NHPr)-Pr-n], Delta H-alpha n(298) = -10.6 kcal/mol). The kinetics of n-propylamine N-H oxidative addition were monitored by in situ UV absorption spectroscopy and determination of the rate law showed an unusually large molecularity (nu = k[1][(PrNH2)-Pr-n](3)). Kinetic experiments conducted over the temperature range of 10-70 degrees C revealed that the reaction rate decreased with increasing temperature. Activation parameters extracted from an Eyring analysis (Delta H double dagger = 0.8 +/- 0.4 kcal/mol, Delta S double dagger = 72 +/- 2 cal/(mol.K)) indicate that the cleavage of strong N-H bonds by 1 is entropy controlled due to a highly ordered, high molecularity transition state. Density functional calculations indicate that a concerted oxidative addition via a classical three-center transition structure is energetically inaccessible. Rather, a stepwise heterolytic pathway is preferred, proceeding by initial amine-assisted N-H heterolysis upon complexation to the electrophilic phosphorus center followed by rate-controlling N -> P proton transfer.