Page 220 - Arrow Pushing in Inorganic Chemistry A Logical Approach to the Chemistry of the Main Group Elements

THE HEAVIER PNICTOGENS
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acetal and thioacetal deprotections, and addition of trimethylsilyl cyanide to carbonyl
groups, among many other reactions.

5B.17 SUMMARY

Some of the main points of our discussion have been the following:

1. Trivalent pnictogen derivatives are typically nucleophilic. A good example is triph-
enylphosphine, a common nucleophile in organic chemistry.
2. Trivalent pnictogen compounds undergo oxidative addition to yield pentavalent, pen-
tacoordinate derivatives such as Ar PnX , where X is a halogen or alkoxide. Pentava-
3 2
lent, tetracoordinate derivatives are also common, especially for phosphorus.
3. Particularly notable for phosphorus are pentavalent cationic centers that are directly
bonded to one or more closed-shell anionic atoms such as oxygen (e.g., phosphine
oxides), nitrogen (phosphazenes), or carbon (phosphorus ylides).
+
–
4. The P –O linkage is extraordinarily strong, with a BDE of ∼544 kJ/mol. Not
surprisingly, a great deal of phosphorus chemistry is driven by the thermo-
+
–
dynamic imperative of forming one or more P –O linkages. Thus, much of
triphenylphosphine’s utility in organic synthesis stems from its propensity to form
+ –
triphenylphosphine oxide. Elsewhere in phosphorus chemistry, formation of P –O
units underpins such seemingly unrelated phenomena as the dehydrating action of
P O 10 and phosphoryl transfer by ATP.
4
+ – + –
5. Less stable than P –O linkages, P –N linkages nonetheless occur in a large family
of compounds—the phosphazenes. The most notable property of the nitrogen center
+ –
inaP –N linkage is its basicity. Thus, phosphazenes are used as neutral superbases
in organic chemistry, not infrequently as an alternative to lithium dialkylamides and
alkyllithiums.
+
–
6. The P –C linkage is stable enough to exist in isolable synthetic intermediates, the
phosphorus ylides, but is quite reactive and functions as essentially a carbanion. In
+
–
a typical application such as the Wittig reaction, a reactive P –C linkage is trans-
+
–
formed to a much more stable P –O linkage.
7. Pentavalent arsenic is well documented but prone to reduction to the trivalent state,
a phenomenon attributed to d orbital contraction.
8. The pentavalent state of antimony is more stable, compared with arsenic. While all
the PnF derivatives are strongly Lewis-acidic, SbF is particularly so and is used in
5
5
the preparation of superacids such as FSO H/SbF (magic acid).
3
5
9. Pentavalent bismuth compounds are fairly strong oxidants, because of the inert pair
effect, which renders them prone to reduction to the trivalent state. Their remarkable
lack of toxicity enhances their utility as “green” reagents.
FURTHER READING
In general, for more detailed information, we recommend the texts listed in Appendices 1 and 2. For
some of the more specialized topics, we recommend the following:
1. Emsley, J. The 13th Element: The Sordid Tale of Murder, Fire, and Phosphorus; Chemical Her-
itage Foundation: Philadelphia, 2002; 327 pp. A popular science book.