Page 65 - Arrow Pushing in Inorganic Chemistry A Logical Approach to the Chemistry of the Main Group Elements

1.26 THE CONVENTION 45

+
+ − S x x x
− S Cl x x O xx x Cl x
x x x
x x
O x x xx x
Cl Cl x
Thionyl chloride
−
− x x O x x x x
O
2+
2+ − x x S x x
− S Cl x x O x x Cl x x
x x
O x x Cl x x
Cl x x x x
Sulfuryl chloride
Applying the above definition of valence (= no. of bonds + FC), we find

V(SOCl )= 3 + 1 = 4
2
V(SO Cl )= 4 + 2 = 6
2 2
where the symbol V(SOCl ) refers to the valence of sulfur in SOCl . Thus, the valence of
2 2
sulfur in SOCl or SO Cl does not depend on the Lewis structures we choose to draw, that
2 2 2
is, whether we use double bonds or not.
Your inorganic textbook, in all likelihood, adopts the traditional structural formulas,
involving a greater number of multiple bonds. As mentioned, for the purpose of arrow
pushing, this should not make a difference, at least not in the vast majority of cases. Both
are equivalent methods for electron bookkeeping. A valid criticism of the new style is that it
results in unusually high FCs on certain atoms. But then FCs and OSs rarely provide a real-
istic description of the electrostatic character of atoms. Also, note that the unusually high
charges are always balanced by opposite charges on adjacent atoms. On balance, therefore,
we have decided to adopt the new style in this book.
2−
As a final example of a hypervalent species, let us consider SiF 6 , whose Lewis struc-
ture is shown in Figure 1.6. As shown, silicon uses all four of its valence electrons in
2−
bonding in SiF 6 and is therefore tetravalent. Indeed, the species may be represented by
−
a “no-bond resonance form” consisting of SiF and two closed-shell F anions; the lat-
4
ter do not contribute to the valence of the silicon. This is a general point: coordination of
closed-shell Lewis bases has no effect on the valence of an atom. For SiF 6 2− , we can also
determine the valence of silicon more mechanically:
2−
V(Si in SiF )= number of bonds + formal charge = 6 +(−2)= 4
6
Thus, although the silicon in SiF 2− has a perfectly normal valence of 4, the fact that it has
6
12 valence electrons around itself in the Lewis structure of SiF 6 2− makes it hypervalent.
The term “hypervalent” thus does not indicate an unusually high valence.

1.26 THE CONVENTION

Given that multiple valence states are a common feature of p-block elements, it’s
useful to familiarize oneself with the convention, which is useful for describing