Dictionary of Chemistry [6th Ed.]

borane(B 2 H 6 ): see formula. Other bo-

ranes include B 4 H 10 , B 5 H 9 , B 5 H 11 ,

B 6 H 10 , and B 10 H 4. The larger borane

molecules have open or closed poly-

hedra of boron atoms. In addition,

there is a wide range of borane deriv-

atives containing atoms of other el-

ements, such as carbon and

phosphorus. Borohydride ions of the

type B 6 H 6 2–also exist. Boranes and

borohydride ions are classiÜed ac-

cording to their structure. Those

with a complete polyhedron are said

to have a closo-structure. Those in

which the polyhedron is incomplete

by loss of one vertex have a nido-

structure (from the Greek for ‘nest’).

Those with open structures by re-

moval of two or more vertices have

an arachno structure (from the Greek

for ‘spider’). See also wade’s rules.

borateAny of a wide range of ionic

compounds that have negative ions

containing boron and oxygen (see

formulae). Lithium borate, for exam-

ple, contains the simple anion

B(OH) 4 –. Most borates, however, are

inorganic polymers with rings,

chains, or other networks based on

the planar BO 3 group or the tetra-

hedral BO 3 (OH) group. ‘Hydrated’ bo-

rates are ones containing –OH

groups; many examples occur natu-

rally. Anhydrous borates, which con-

tain BO 3 groups, can be made by

melting together boric acid and

metal oxides.

borax(disodium tetraborate-10-

water)A colourless monoclinic solid,

Na 2 B 4 O 7 .10H 2 O, soluble in water and

very slightly soluble in ethanol; mon-

oclinic; r.d. 1.73; loses 8H 2 O at 75°C;

loses 10H 2 O at 320°C. The formula

gives a misleading impression of the

structure. The compound contains

the ion [B 4 O 5 (OH) 4 ]2–(see borate).

Attempts to recrystallize this com-

pound above 60.8°C yield the penta-

hydrate. The main sources are the

borate minerals kernite (Na 2 B 4 O 7.

4H 2 O) and tincal(Na 2 B 4 O 7 .10H 2 O).

The ores are puriÜed by carefully

controlled dissolution and recrystal-

lization. On treatment with mineral

acids borax gives boric acid.

Borax is a very important sub-

stance in the glass and ceramics in-

dustries as a raw material for making

borosilicates. It is also important as a

metallurgicalÛux because of the abil-

borate 78

b

O– OO–

BB

OO

B

O–

B 3 O^36 – as in Na 3 B 3 O 6

B

O–

O B

O–

O B

O–

(BO 2 )nn– as in CaB 2 O 4

HO B

O

O

B–

OH

O

B OH

B–

O

OH

O

[B 4 O 5 (OH) 4 ]^2 –

as in borax Na 2 B 4 O 7 .10H 2 O

Borate