Modern inorganic chemistry

GROUPV 227

atom or ion can "back-donate' electrons from its d orbitals into the

vacant d orbitals of the phosphorus -this is not possible with

ammonia.

DIPHOSPHANE,? 2 H 4

This can be extracted from impure phosphine prepared by the action

of sodium hydroxide on phosphorus. Unlike hydrazine, it has no

basic properties. It is a powerful reducing agent and burns spontane-

ously in air, this reaction explaining why impure phosphine con-

taining traces of diphosphane ignites spontaneously in air.

Hydrides of arsenic and antimony

Arsine, AsH 3 , and stibine, SbH 3 , are formed when arsenic and

antimony compounds respectively are reduced by a process in

which hydrogen is evolved. They are colourless, unpleasant smelling,

poisonous gases. Stibine is less stable than arsine but both decom-

pose readily on heating to form the element and hydrogen. Both

arsine and stibine are covalent compounds and they have little

power to donate electrons; although the arsonium ion, AsH^, is

known, this forms no stable compounds. The donor ability of arsine

is enhanced when the hydrogen atoms are replaced by methyl

groups (cf. phosphine, p. 226).

DIARSANE, AS 2 H 4

Arsenic (but not antimony) forms a second hydride. This is extremely

unstable, decomposing at very low temperatures. Replacement of

the hydrogen atoms by methyl groups gives the more stable sub-

stance tetramethyldiarsane, cacodyl (CH 3 ) 2 As—As(CH 3 ) 2 , a truly

foul-smelling liquid.

BISMUTHINE

Very small quantities of bismuthine are obtained when a bismuth-
magnesium alloy, Bi 2 Mg 3 , is dissolved in hydrochloric acid. As
would be expected, it is extremely unstable, decomposing at room
temperature to bismuth and hydrogen. Alkyl and aryl derivatives.
for example trimethylbismuthine, Bi(CH 3 ) 3 , are mote stable.