Phosphine: Preparation and Properties

Phosphine, $\text{PH}_3$, is the phosphorus analogue of ammonia and, like $\text{NH}_3$, is

manufactured and studied mainly as a laboratory reagent rather than a bulk industrial chemical.

It is prepared in the laboratory by boiling white phosphorus with concentrated sodium hydroxide

solution in an inert atmosphere (to exclude air, since the gas evolved is spontaneously flammable

when it contains traces of an impurity, diphosphine $\text{P}_2\text{H}_4$):

This is a disproportionation reaction: phosphorus (oxidation state $0$ in $\text{P}_4$) is

simultaneously reduced to $-3$ in $\text{PH}_3$ and oxidised to $+1$ in sodium hypophosphite,

$\text{NaH}_2\text{PO}_2$.

Structurally, phosphine is, like ammonia, a trigonal pyramidal molecule: phosphorus forms

three $\sigma$ bonds to the three hydrogen atoms and retains one lone pair. However, the

$\text{H-P-H}$ bond angle in $\text{PH}_3$ (about $93.6^\circ$) is markedly smaller than the

$\text{H-N-H}$ angle in $\text{NH}_3$ ($107.8^\circ$), and closer to a "pure" $90^\circ$ angle

between unhybridised p orbitals than to a tetrahedral angle. The usual explanation is that

phosphorus, being much less electronegative and considerably larger than nitrogen, holds its

bonding electron pairs less tightly and further from the nucleus, so the mutual repulsion between

the three P-H bonding pairs (and between them and the lone pair) is weaker than the corresponding

repulsion in the more compact, more electronegative $\text{NH}_3$; the bonding in $\text{PH}_3$

therefore uses hybrid orbitals with much greater p-character (closer to pure, un-hybridised p

orbitals, which are naturally at $90^\circ$ to one another) rather than the near-perfect $sp^3$

description that fits $\text{NH}_3$ well.

Phosphine is a colourless, extremely toxic gas with a distinctive smell often described as

similar to rotten fish or garlic; commercial samples usually ignite spontaneously in air because

of trace $\text{P}_2\text{H}_4$ impurity, burning with a bright flame (this spontaneous

flammability is exploited in the "Holmes signal", a traditional maritime marker). Unlike

ammonia, phosphine is only weakly water-soluble and is a much weaker base -- consistent with …