Modern inorganic chemistry

THE ELEMENTS OF GROUP III 153

The melting and boiling points of the aluminium halides, in

contrast to the boron compounds, are irregular. It might reasonably

be expected that aluminium, being a more metallic element than

boron, would form an ionic fluoride and indeed the fact that it

remains solid until 1564 K. when it sublimes, would tend to confirm

this, although it should not be concluded that the fluoride is, there-

fore, wholly ionic. The crystal structure is such that each aluminium

has a coordination number of six, being surrounded by six fluoride

ions.

All the other aluminium halides are covalently bonded with

aluminium showing a coordination number of four towards these

larger halogen atoms. The four halogen atoms arrange themselves

approximately tetrahedrally around the aluminium and dimeric

molecules are produced with the configuration given below:

XXX

\ / \ /

Al AK (X = halogen atom)

X^ \ / ^X

/v

These molecules exist in the solid halides, explaining the low melting

points of these halides, and also in the vapour phase at temperatures

not too far above the boiling point. At higher temperatures, how-

ever, dissociation into trigonal planar monomers, analogous to the

boron halides, occurs.

The monomers are electron pair acceptors, and donor molecules

are often able to split the dimeric halide molecules to form adducts;

thus, whilst the dimeric halides persist in solvents such as benzene,

donor solvents such as pyridine and ether appear to contain mono-

mers since adduct formation occurs. Aluminium halides, with the

one exception of the fluoride, resemble the corresponding boron

halides in that they are readily hydrolysed by water.

Fluorides

Boron trifluoride is a colourless, reactive gas which can be prepared
by heating boron trioxide and fluorspar with concentrated sulphuric
acid,
B 2 O 3 + 3CaF 2 + 3H 2 SO 4 -> 2BF 3 4- 3CaSO 4 + 3H 2 O
or by the direct combination of the elements. The gas must be
collected and kept under rigorously dry conditions; it fumes in moist
air and reacts vigorously with water forming boric acid and tetra-
fluoroboric acid, H + BFr.