The synthesis and reactivity of geometrically constrained tricoordinate phosphorus (sigma(3)-P) compounds supported by tridentate triamide chelates (N[o-NR-C6H4](2)(3-); R = Me or iPr) are reported. Studies indicate that 2 (P{N[o-NMe-C6H4](2)}) adopts a C-s-symmetric structure in the solid state. Variable-temperature NMR studies demonstrate a low-energy inversion at phosphorus in solution (Delta G(exptl)(double dagger 298) = 10.7(5) kcal/mol), for which DFT calculations implicate an edge-inversion mechanism via a metastable C-2-symmetric intermediate. In terms of reactivity, compound 2 exhibits poor nucleophilicity, but undergoes oxidative addition at ambient temperature of diverse O-H- and N-H-containing compounds (including alcohols, phenols, carboxylic acids, amines, and anilines). The resulting pentacoordinate adducts 2 center dot[H][OR] and 2[H][NHR] are characterized by multinuclear NMR spectroscopy and X-ray crystallography, and their structures (which span the pseudorotation coordinate between trigonal bipyramidal and square planar) are evaluated in terms of negative hyperconjugation. At elevated temperatures, the oxidative addition is shown to be reversible for volatile alcohols and amines.