STRUCTURE AND BONDING 41
'donor-acceptor bonding' is very marked in Group HI, for when
elements in this group form three covalent bonds by sharing, they
have only six outer electrons. Consider for example the trichlorides
of boron and aluminium :
Cl Cl Cl Cl\B/ and \\KCl ClBoth these molecules exist in the gaseous state and both are trigonal
planar as indicated by reference to Table 2.8. However, in each, a
further covalent bond can be formed, in which both electrons of the
shared pair are provided by one atom, not one from each as in
normal covalent bonding. For example, monomeric aluminium
chloride and ammonia form a stable compound :
H:CI: H ci
xx**.. \ /
H x N x Al : Cl : i.e. H— N
XX ..
H : Cl:H C1In this molecule, the aluminium receives a pair of electrons from
the nitrogen atom. The nitrogen atom is referred to as a donor atom
and the aluminium as an acceptor atom. Once the bond is formed
it is identical to the covalent bond of previous examples ; it differs
only in its origin. It is called a co-ordinate or dative bond, and can be
- expressed either as H 3 N->A1C1 3 or H 3 N— A1C1 3. In the latter
formula the positive and negative charges are not ionic charges ; they
are merely formal charges to show that in forming the co-ordinate
link, the nitrogen lost a half share in its original electron pair which
is now shared with the aluminium, the latter having gained a half
share in the electron pair.
The formation of a fourth covalent bond by the aluminium atom
results in spatial rearrangement from the trigonal planar, for three
bonding electron pairs, to tetrahedral, for four bonding electron
pairs.
Other compounds containing lone pairs of electrons readily form
co-ordinate links and in each case a change in spatial configuration
accompanies the bond formation. The oxygen atom in dimethyl
ether, CH 3 — O — CH 3 , has two lone pairs of electrons and is able to
donate one pair to, for example, boron trichloride :
- expressed either as H 3 N->A1C1 3 or H 3 N— A1C1 3. In the latter