This dissertation describes the synthesis of several transition-metal polyphosphides including orthorhombic FeP 2 , cubic CoP 3 , cubic NiP 2 , monoclinic PdP 2 and monoclinic CuP 2. The investigation of these materials was initiated by the discovery of MP x formation upon reacting a metal halide with molecular yellow P 4 in superheated toluene. When these MP x products were annealed at moderate temperatures (350-500 °C), crystalline phosphorus-rich phases were produced. We found that these phases have been previously synthesized from high-temperature elemental reactions and that low temperature routes to these phase-pure polyphosphides using at least one non-elemental source were not found in the literature. The absence of low-temperature, "bottom up" routes to these materials encouraged us to investigate our initial findings further, as such methods provide unique chemical and structural flexibility in materials synthesis. Metal-rich counterparts to the aforementioned polyphosphides have been successfully produced via molecular solvothermal reactions. These reports consistently used an excess of phosphorus in their reactions but still afforded metal-rich products, and often produced materials with a combination of metal-rich and phosphorus-rich phases. These routes show the inability to dial in the phosphorus content of the produced MP x phases and so they were unable to use balanced stoichiometry to rationalize the chemistry, and rarely attempted to identify reaction byproducts. This prompted us to design a synthesis in which discrete amounts of a phosphorus reagent could be used to target specific compositions and phases and allow for byproduct identification. In order to determine reaction byproducts, a metal halide and yellow P 4 were reacted in together without solvent in sealed ampoules. The clear liquid byproduct was identified as PCl 3 or PBr 3 , indicating that stoichiometric and balanced reactions were possible. In these reactions, metal halides and phosphorus (red or yellow) were balanced such that the chloride was ideally removed has PCl 3 and any remaining phosphorus was used to form targeted, phase-pure MP x phases. Using this stoichiometry in solid-state reactions, all of the aforementioned polyphosphide phases were synthesized as phase pure products at moderate temperatures (500-700 °C). By pelletizing the metal halide reagents in reactions with yellow P 4 or by co-pelletizing the metal halide with red phosphorus, porous pellet products reminiscent of the reagent pellet could be afforded. The reaction stoichiometries used in solid-state reactions were adapted to solvothermal reactions. In these reactions, amorphous MP x products were synthesized from metal halides and yellow P 4 in various solvents (superheated toluene, 1-octadecene and hexadecane), and upon annealing (350-500 °C), targeted phase-pure CoP 3 , NiP 2 and CuP 2 were produced. Reactions substituting red phosphorus for yellow P 4 in hexadecane reactions also yielded the same crystalline phases. Solvothermal reactions were modified by...