Phosphinylidenes are a family of organophosphorus compounds that are of biological and pharmaceutical importance. They are also most commonly employed as ancillary ligands in homogenous catalysis. These include phosphinates, H-phosphonates, Hphosphinates and secondary phosphine oxides. Such compounds can exhibit a prototropic (i.e., proton migration from one atom to another) tautomerism process involving structures with a tetracoordinate pentavalent phosphorus P(V) and a trivalent phosphorus P(III) atom (see Scheme 1A). Due to the availability of a free lone pair of electrons, the P(III) hydroxide form is known to coordinate with the metal in transition metal complexes. However, the P(V) oxide form is known to be the most stable species and the tautomeric equilibrium displayed in Scheme 1A is strongly shifted toward P(V) if the R were electron-donating groups. On the contrary, with electron-withdrawing fluororganyl groups, only the P(III) tautomer was isolated and detected spectroscopically, and in some cases, a solvent-dependent equilibrium was observed between the tautomeric forms. Although the reactivity of phosphinylidenes suggests the presence of both oxide and acid tautomers, the direct intramolecular proton transfer (PT) for their interconversion is unfavorable at room temperature due to its high activation barrier of ~ 60 kcal mol -1 . Alternatively, an intermolecular PT mechanism between two tautomers of the same species, a base-or water-catalyzed PT has been proposed. Recently, a detailed computational study on the tautomerization of simple secondary phosphine oxides predicted that the two-water bridged PT provides the most energetically favorable path in the gas phase, as compared to intra-and intermolecular PT involving one, two, or three tautomers. Scheme 1. Tautomerization reaction in phosphinylidene compounds showing A).