_P.-Powell-auth.-Principles-of-Organometallic-Chemistry-Springer-Netherlands-1988

4.9.6 Pentaalkyl and pentaaryl derivatives
Organic derivatives of Group V
The search for pentaalkyl and -aryl derivatives of the elements of Group V
provides a fascinating story, especially because some rather important discoveries
have been made along the way. It began in 1862 when Cahours claimed to have
prepared pentamethylarsenic from tetramethylarsonium iodide and dimethylzinc.
In 1896 Lachman attempted to make pentamethylnitrogen by a similar
route; at that time chemists' imaginations were not restricted by such ideas as
thc octet rule. Schlenk and Holz did isolate a red compound from the reaction
Me 4 N+r- + Ph/~ Na ~ -+ Me 4 N+CPh; + Nal
which has since been shown to bc ionic. In general, however, only amines and
hydrocarbons were obtained.
Undaunted, G. Wittig tried again during the 19 30s and · 40s to make R 5 N,
taking the strongly nucleophilic reagent phenyllithium instead of the resonance
stabilized species Ph 3 C. An intermediate ylide Me,N-CH 2 was formed which
was trapped as a zwitterion with benzophenone.
+
[Me,N-CH,]CI
+ -
Phl.i + - PhlCO
-----> Me ,N -CH 2 ---------->
- PhH. UCI
+ HX . -
ylide
Me,NCH 2 C(Cr)Ph 1 ____:_, [Me,N-CH 2 C(OH)PhJX
Zwitterion
Even if nitrogen is unlikely to form pentacoordinate derivatives, phosphorus is
a much more promising candidate. 'Octet expansion' is common; PF 5 , for
example, has a covalent trigonal bipyramidal structure in both the gaseous and
condensed states. Me 4 P+r-, however. reacts with phenyllithium exactly analogously
to Me 4 N+r-. This is because the carbon--hydrogen bonds in the methyl
groups are too acidic to let the nucleophile attack phosphorus. By taking
Ph 4 P+r-, which has significantly less acidic C-H functions. pentaphenylphosphorus,
m.p. 124oC dec .. was isolated as a colourless crystalline solid, which
is stable in air at room temperature. Subsequently pentaphenyl derivatives of all
the heavier Group VB elements were prepared by similar routes. While Ph 5 Sb
(m.p. 170°C) is somewhat more stable thermally than the phosphorus compound,
possibly because the molecule is less congested sterically, Ph,Bi can be
kept for only a few days under nitrogen at room temperature. It forms a violet
crystalline powder. Some routes to the pentaphenyl compounds are
summarized in Fig. 4.11.
An X-ray diffraction study of pentaphenylphosphorus reveals a slightly
distorted trigonal bipyramidal structure; P-Caxiai' 1.987 A; P-Ceq' 1.850A,
cax -P-Cax• l 76.9 A. Pentaphenylarsenic is isomorphous, indicating a similar
structure. The antimony compound, however. is one ofthe very few Main Group
derivatives to have square pyramidal geometry, both in the solid state and in
solution in dichloromethane. The p-tolyl derivative, (4-MeC"H 4 ) 5 Sb. is trigonal
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