During the course of optimization, we also identified hypophosphite (H2PO2 -), a species containing phosphorus in the +1 oxidation state, as a product of ball milling. In addition, a phosphorus recovery rate of only 81% suggested formation of insoluble or gaseous side products (presumably elemental phosphorus and phosphine gas, respectively). These findings pointed toward a non-negligible portion of Na3P3O9 undergoing overreduction to yield species in the lower oxidation states. Overreduction consumes hydride equivalents, thereby reducing the yield of phosphite, and also poses a danger to the environment as phosphine gas is known to be highly toxic. In pursuit of better yield and selectivity of reduction to phosphite, we explored the scope of phosphate sources (Tables and). We first tried linear phosphates (pyrophosphate and triphosphate) that are industrially accessible by dehydration of hydrogen or dihydrogen phosphate at 300-900 • C. Unlike cyclic phosphates, each polyphosphate molecule PnO3n+1 (n+2)-contains only (n-1) P-units that are susceptible to hydride attack, namely, in theory, a polyphosphate with n P-units can only produce (n-1) equivalents of phosphite with one equivalent of orthophosphate as byproduct: