Page 63 - Arrow Pushing in Inorganic Chemistry A Logical Approach to the Chemistry of the Main Group Elements

1.25 ELEMENTS OF BONDING IN HYPERVALENT COMPOUNDS 43

Antibonding

Nonbonding

Bonding
+ −
-Molecular orbitals of the allyl system. From left to right, the electron occupancies are
Figure 1.8
for the cation, radical and anion, which have two, three, and four -electrons, respectively.

Arguments of a similar nature also provide a qualitative explanation of the bonding in
SF ,BrF ,IF ,IF , and the various xenon fluorides. This book is not an appropriate place
6 3 5 7
for a discussion of each of these cases. The key point is that, for the purposes of arrow
pushing, all the element–fluorine bonds may be viewed as normal two-electron bonds.
We are now in a position to discuss the question of multiple bonds between higher-valent
main-group elements and typical multivalent ligands such as O and N. For this, consider
the following molecules/ions:
−
H PO , SO , SO , SNF , I O , ClO , and XeO 4
3
4
2
5
4
2
3
3
Drawn traditionally, the structural formulas all have a hypervalent central atom and one or
more multiple bonds:
O O N
P S S S
HO F
HO OH O O O O F F
H 3 PO 4 SO 2 SO 3 SNF 3
O O
O O O
I I − Cl Xe
O O
O O O O O O
−
I 2 O 5 ClO 4 XeO 4
The structural formulas immediately raise questions about the nature of the multiple bonds
in them. Specifically, are there -bonds involved? The answer to this question is: to a
degree, yes, but not in the conventional sense. Thus, in SNF ,any S–N -bonding involves
3
lone pairs on the nitrogen and S–F -antibonding orbitals. This is a subtle issue that, in
our opinion, should not preoccupy us, nor for that matter dominate day-to-day mechanistic
reasoning. Consequently, we prefer the ionic formulations shown below, which also
happen to be nonhypervalent (i.e., the central atom in each case has an octet of electrons):