Modern inorganic chemistry

HYDROGEN 115

phosphine PH 3 is weakly basic, hydrogen sulphide H 2 S is weakly

acidic and hydrogen chloride HC1 markedly acidic. We should note

that these descriptions^4 basic' and 'acidic' refer to solutions in water;

a gaseous hydrogen halide does not display acidity (p. 87).

COMPLEX HYDRIDES

A non-metal or weakly electropositive metal X in Group III of the

periodic table would be expected to form a covalent volatile hydride

XH 3. In fact, the simplest hydride of boron is B 2 H 6 and aluminium

hydride is a polymer (A1H 3 )B.

The structure of diborane B 2 H 6 is considered later (p. 145). Here

we may note that kBH 3 ' and kA!H 3 ' will be acceptor molecules since

there are only six valency electrons around the B or Al atom and a

vacant orbital exists. Both in fact can accept the electron pair from

a hydride ion thus:

BH 3 4- H~ -^BH4

"borane" tetrahydridohorate or

horohydride

AiH 3 + H- -» AIH;

"alane' tetrahydroaluminate or

aluminohydride

Salts containing these ions can be prepared for example. b> the

reaction

4LiH + A1C1 3 -^U LiAlH 4 + 3LiCl

LiAlH 4 , lithium tetrahydridoaluminate (lithium aluminium hyd-
ride', so-called) is an excellent reducing agent in ether solution for
both organic and inorganic compounds; it may be used to prepare
covalent hydrides SiH 4 , PH 3 * from the corresponding chlorides in
ether, for example

SiCl 4 + LiAlH 4 -> LiCl + A1C1 3 + SiH 4

silicon silane

tetrachloride

The tetrahydridoborate ion, as 'sodium borohydride' NaBH 4 is
soluble in water and is similarly an excellent reducing agent in this
solvent. (Lithium tetrahydridoaluminate cannot be used in water,
with which it reacts violently to give hydrogen.)

This method produces an endothermic hydride by indirect means.