3.11 The Unusual Structures of Some Group 13 Compounds
An unusual feature of the Lewis structure of the colorless gas boron trifluoride,
BF3 (29), is that the boron atom has an incomplete octet: its valence shell consists
of only six electrons. We might expect the boron atom to complete its octet by
sharing more electrons with fluorine, as depicted in (30), but fluorine has such a
high ionization energy that it is unlikely to exist with a positive formal charge.
Experimental evidence, such as the short B—F bond lengths, suggests that the true
structure of BF3 is a resonance hybrid of both types of Lewis structures, with the
singly bonded structure making the major contribution.
The boron atom in BF3 can complete its octet if an additional atom or ion with
a lone pair of electrons forms a bond by providing both electrons. A bond in which
both electrons come from one of the atoms is called a coordinate covalent bond.
For example, the tetrafluoroborate anion, BF4
(31), forms when boron trifluoride
is passed over a metal fluoride. In this anion, the formation of a coordinate covalent