226 GROUPV
PropertiesPhosphine is a colourless gas at room temperature, boiling point
183K, with an unpleasant odour; it is extremely poisonous. Like
ammonia, phosphine has an essentially tetrahedral structure with
one position occupied by a lone pair of electrons. Phosphorus, how-
ever, is a larger atom than nitrogen and the lone pair of electrons on
the phosphorus are much less 'concentrated' in space. Thus phosphine
has a very much smaller dipole moment than ammonia. Hence
phosphine is not associated (like ammonia) in the liquid state (see
data in Table 9.2) and it is only sparingly soluble in water.
Towards a simple Lewis base, for example the proton, phosphine
is a poorer electron donor than ammonia, the larger phosphorus
atom being less able to form a stable covalent bond with the acceptor
atom or molecule. Phosphine is, therefore, a much weaker base*
than ammonia and there is no series of phosphonium salts corre-
sponding to the ammonium salts; but phosphonium halides,
PH 4 X (X = Cl, Br, I) can be prepared by the direct combination of
phosphine with the appropriate hydrogen halide. These compounds
are much more easily dissociated than ammonium halides, the
most stable being the iodide, but even this dissociates at 333 K:
PH 4 I - PH 3 + HI
The other halides dissociate at lower temperatures and, if put into
water, all are decomposed, the proton transferring to water which
is a better electron pair donor:
PH 4 X + H 2 O -> PH 3 + H 3 O+ + X
Phosphine has a much lower thermal stability than ammonia and
sparking decomposes it to red phosphorus and hydrogen, 2 volumes
of phosphine giving 3 volumes of hydrogen. Not unexpectedly,
therefore, phosphine is a more powerful reducing agent than am-
monia. If passed into a solution of a salt of copper, silver or gold
the metal phosphide is obtained but this decomposes to give the
metal on standing or more quickly on boiling. Pure phosphine
ignites in air at 423 K and burns to phosphoric(V) acid :
PH 3 + 2O 2 -> H 3 PO 4
Replacement of the hydrogen atoms by methyl groups to give
trimethylphosphine (CH 3 ) 3 P, makes it a stronger base (as
[(CH 3 ) 3 PH]OH), and improves the donor power of the phosphorus
as it does with nitrogen. Towards some transition metal atoms or
ions, trimethylphosphine is a stronger ligand than ammonia, i.e.
forms more stable complexes. This is because the transition metal
*A Lowry-Bronsted base in this context.