Sodium Phosphide

Sodium phosphide is the inorganic compound with the formula Na₃P. It is a black solid. It is often described as Na⁺ salt of the P³⁻ anion. Na₃P is a source of the highly reactive phosphide anion. It should not be confused with sodium phosphate, Na₃PO₄.

In addition to Na₃P, five other binary compositions of sodium and phosphorus are known: NaP, Na₃P₇, Na₃P₁₁, NaP₇, and NaP₁₅.

Structure and Properties

The compound crystallizes in a hexagonal motif, often called the sodium arsenide structure. Like K₃P, solid Na₃P features pentacoordinate P centers.

Preparation

The first preparation of Na₃P was first reported in the mid-19th century. French researcher, Alexandre Baudrimont prepared sodium phosphide by treating molten sodium with phosphorus pentachloride.
: 8 Na_(l) + PCl₅ → 5 NaCl + Na₃P

Many different routes to Na₃P have been described. Due to its flammability and toxicity, Na₃P (and related salts) is often prepared and used in situ. White phosphorus is reduced by sodium-potassium alloy:
: P₄ + 12 Na → 4 Na₃P

Phosphorus reacts with sodium in an autoclave at 150 °C for 5 hours to produce Na₃P.

Alternatively the reaction can be conducted at normal pressures but using a temperatures gradient to generate nonvolatile Na_xP phases (x < 3) that then react further with sodium. In some cases, an electron-transfer agent, such as naphthalene, is used. In such applications, the naphthalene forms the soluble sodium naphthalenide, which reduces the phosphorus.

Uses

Sodium phosphide is a source of the highly reactive phosphide anion. The material is insoluble in all solvents but reacts as a slurry with acids and related electrophiles to give derivatives of the type PM₃:
: Na₃P + 3 E⁺ → E₃P (E = H, Me₃Si)
The trimethylsilyl derivative is volatile (b.p. 30-35 C @ 0.001 mm Hg) and soluble. It serves as a soluble equivalent to "P³⁻".

Indium phosphide, a semiconductor arises by treating in-situ generated "sodium phosphide" with indium(III) chloride in hot ''N,N’''-dimethylformamide as solvent. In this process, the phosphide reagent is generated from sodium metal and white phosphorus, whereupon it immediately reacts with the indium salt:

: Na₃P + InCl₃ → InP + 3NaCl

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

Precautions

Sodium phosphide is highly dangerous releasing toxic phosphine upon hydrolysis, a process that is so exothermic that fires result. The USDOT has forbidden the transportation of Na₃P on aircraft and trains due to the potential fire and toxic hazards.

References

{{Phosphides

Phosphides
Sodium compounds