The reactions of sodium trimetaphosphate (Na3P3O9) with KH displayed a higher degree of overreduction (with up to 7% yield of hypophosphite; Table , entries 1-6) compared to the reactions of pyrophosphate and triphosphate. When Na3P3O9 was treated with substoichiometric KH (H:P = 1:3), phosphorylated phosphites of various chain lengths were the major reduction products instead of free phosphite, pointing toward a mechanism of hydride phosphorylation through ring opening followed by stepwise cleavage of terminal phosphates (Figure ). The ring opening reactivity of trimetaphosphate was further demonstrated by the facile mechanochemical reaction between Na3P3O9 and Na2HPO3, affording phosphorylated terminal phosphites of various chain lengths (Figure (vii)), which were assigned based on a combination of 1D-and 2D-31 P NMR spectroscopic techniques (see SI). These results suggest that the ring-opening reactivity of trimetaphosphate provides additional pathways to form phosphorylated phosphites and thus significantly increases the degree of overreduction.