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.

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Properties

The structure of molecular sodium phosphide is similar to that of molecular lithium phosphide, Li₃P, for both are trigonal pyramidal, with a lone pair of electrons on the central phosphorus atom. The ionic properties of Na₃P are also similar to Li₃P, which is known to be an ionic superconductor. The lowest energy conformation is the same for both molecules, non-planar C_3v symmetry. Due to its unique structure, Na₃P has the ability to invert itself. The inversion barrier energies for Na₃P and Li₃P are calculated to be 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.[2] Therefore, no chemical company manufactures Na₃P for distribution, and why processes requiring Na₃P as reactant must obtain it in situ.

References

1. Yunle, G.; Fan, G.; Yiate, Q.; Huagui, Z.; Ziping, Y. Mater. Res. Bull. 2002, 37, 1101-1106.

2. Francisco, J.S.; Khitrov, G. Chem. Phys. 1993, 171, 153-157.

3. Baudrimont. Ann. Chim. Phys. 1864, 2, 13.

4. Xie, Y.; Su, H.; Li, B.; Qian, Y. Mater. Res. Bull. 2000, 35, 675-680.

5. Jarvis, R.F.; Jacubinas, R.M.; Kaner, R.B. Inorg. Chem. 2000, 39, 3243-3246.

6. Peterson, D.J. 1967. Patent No. 3,397,039.

7. Khanna, P.K.; Eum, M.-S.; Jun, K.-W.; Baeg,J.-O.; Seok, S. I. Mater. Lett. 2003, 57, 4617-4621.

8. Atarashi, Y.; Fukumoto, O. Japanese Patent No. JP 42,006,269.

9. Morozova, M.P.; Bol'shakova, G.A.; Lukinykh, N.L. Zh. Obsch. Khim. 1959, 29, 3144-3145.

10. Dong, Y.; DiSalvo, F.J. Acta. Crystallogr. Sect E: Struct. Rep. Online. 2005, E61, i223-i224.