A series of structurally constrained phosphenium ions based on pyridinylmethylamidophenolate scaffolds are shown to undergo P(III)/P(V) oxidative addition with C-H bonds of alkynes, alkenes, and arenes. Nonactivated substrates such as benzene, toluene, and deactivated chlorobenzene are phosphorylated in quantitative yields. Computational and spectroscopic studies suggest a low-barrier isomerization from a bent to a T-shaped isomer that initiates a phosphorus-ligand-cooperative pathway …