Page 176 - Arrow Pushing in Inorganic Chemistry A Logical Approach to the Chemistry of the Main Group Elements

5B

The Heavier Pnictogens

Lady Astor: If you were my husband, I’d put arsenic in your coffee.
Churchill: Madam, if I were your husband, I’d drink it!

The chemical properties of the group 15 elements or pnictogens (for which we have
occasionally used the symbol Pn) span a wide range but perhaps not quite to the same
extent as groups 13 and 14. The chemistry of nitrogen and phosphorus thus do not differ as
sharply as that of carbon and silicon. To illustrate, both N and P are classic nonmetals and
form molecular acidic oxides. As with other p-block groups, metallic character increases
down the group; As and Sb are considered metalloids and Bi is a metal. Thus, As O and
4 6
Sb O are molecular species, isostructural with P O ;Bi O , by contrast, consists of an
4 6 4 6 2 3
ionic lattice of Bi 3+ and O 2– ions. Some additional group trends are as follows:
• As for other “first-row” elements (B–F), the coordination number of nitrogen cannot
exceed 4, even though a valence of 5 is quite common for nitrogen (e.g., NH + and
4
–
NO ). This is believed to be simply a reflection of nitrogen’s small atomic size. By
3
contrast, coordination numbers of 5 and 6 are common for all the other group 15
elements; for Bi, even 9-coordination has been documented.
• Unlike nitrogen, the heavier pnictogens do not form true multiple bonds with ease.
+
–
Thus, molecules such as POCl are better viewed as Cl P –O , as opposed to
3
3
Cl P=O. In this newer view, the shortness of the P–O linkage (1.58 Å) is attributed
3
to its ionic rather than double-bond character.
• For p-block elements, phosphorus exhibits above-average catenating properties. Thus,
although linear, single-bonded chains tend to be unstable, elemental phosphorus forms
a variety of single-bonded cage structures.
• Valences of 3 and 5 are well established for all the elements, although the relative
stability of the tri- and penta-valent states varies considerably among the elements.
The propensity to adopt pentavalent states is probably the strongest for phosphorus,
+
–
especially when molecules containing the highly stable P –O unit are involved. The
−
−
+
+
bond dissociation energy (BDE) for R P −O (R P −O → R P + O) averages a
3
3
3
Arrow Pushing in Inorganic Chemistry: A Logical Approach to the Chemistry of the Main-Group Elements,
First Edition. Abhik Ghosh and Steffen Berg.
© 2014 John Wiley & Sons, Inc. Published 2014 by John Wiley & Sons, Inc.
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