Sodium phosphide

Sodium phosphide, Na3P, is a black, ionic salt containing the alkali metal sodium and the phosphide anion.1 Na3P is used in many chemical reactions requiring a highly reactive phosphide anion. It should not be confused with sodium phosphate, Na3PO4.

Properties
The structure of sodium phosphide is similar to lithium phosphide, Li3P, for both are trigonal pyramid (chemistry), with a lone pair of electrons on the central phosphorus atom occupying a molecular orbital. The ionic properties of Na3P are also similar to Li3P, which is known to be a superionic conductor. The lowest energy conformation is the same for both molecules, non-planar C3v symmetry. Due to its unique structure, Na3P has the ability to invert itself. The inversion barrier energies for Na3P and Li3P are 7.5 kcal/mol and 4.9 kcal/mol respectively.²

History
Preparation of sodium phosphide originally began in the mid-19th century. French researcher, Alexandre Baudrimont prepared sodium phosphide by reacting molten sodium with phosphorus pentachloride.³

8Na(l) +  PCl5  →  5NaCl + Na3P

Preparation
Since the original preparation was performed there have been many different ways of preparing sodium phosphide. Yellow phosphorus reacts with sodium in an autoclave at 150 °C for 5 hours to produce Na3P.4

P4 + 12Na → 4Na3P

If the autoclave technique cannot be utilized the reactants may be placed under vacuum and heated from 30 minutes at 170 °C, then 30 minutes at 350 °C, and finally for 5 hours at 480 °C.5

The previous two preparations require high temperatures, which are not always desirable. For this reaction to proceed at ambient conditions a catalyst is used. The catalyst used in one method was naphthalene due to its ability to form radical (chemistry) anions and dianions and act as an electron bridge for the binding of sodium to phosphorus.6

Uses
Sodium phosphide has a variety of uses because it contains the highly reactive phosphide anion. Due to its flammability and toxicity, Na3P is prepared in situ. One such application is in the preparation of indium phosphide, a nanoparticle superconductor. Na3P was prepared by reacting sodium metal and white phosphorus in N,N’-dimethylformamide, DMF. Indium(III) chloride was then added to the reaction.7



Sodium phosphide is also employed commercially as a catalyst in conjunction with zinc phosphide and aluminum phosphide for polymer production. When Na3P is removed from the ternary catalyst polymerization of propylene and 4-methyl-1-pentene is not effective.8



Precautions
There are inherent hazards when reacting sodium and any allotrope of phosphorus to produce sodium phosphide. The USDOT has forbidden the transportation of Na3P on passenger aircraft, cargo only aircraft, and trains due to the potential fire and toxic hazards. Therefore, no chemical company manufactures Na3P for distribution, and why processes requiring Na3P as reactant must obtain it in situ.