The ∆G ‡ for the two-water bridged 12w pathway has the lowest barrier (26.0 kcal mol -1 ); albeit slightly high for a RT reaction in the absence of tunneling, we will see the latter's influence below. The ∆G ‡ lowering for 12w stems from the relaxed seven-membered ring TS, which makes it efficient for the PT instead of the sterically hindered TS adopted in the above cases. Similar mechanistic analysis on 1H,OH also revealed the same trend in the activation barriers and reaction energies, giving the lowest activation free barrier (22.6 kcal mol -1 ) with two-water assisted PT (see Table in the Supporting Information). The detailed analysis of these reaction mechanisms suggests that the two-water assisted PT is the most kinetically preferred path, consistent with previous study predicted for other phosphinylidenes. Therefore, calculations on all the tautomerization reactions considered in this study were carried out with the two-water bridge acting as a mediator for the PT. Notably, in the absence of water, the intramolecular tautomerization process in all these cases yields prohibitively high barriers of 52-64 kcal mol-1, rendering the reaction forbidden at RT (see Table in the Supporting Information). For the electron-donating substituted systems 1-4, the reaction is exothermic in the direction of the P(V)2w oxide form. In contrast, for the electron-withdrawing groups (entries 7 and 8), the acid P(III)2w