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

The main Groups IV and V
HX(Sbl
Ph M•x-
4 X~Sb,Bil
~Li
orX2
d
Li+MPh- ::;::/i'hLi
(As.~), Sb,(Bi ~J
Fig. 4.11 Pentaphenyls of Group VB.
Ph M+ BPh-
4 ' 4
~Li
/ (;;,·~b,Bil
bipyramidal in the solid. Proton n.m.r. spectra ofits solution show equivalence of
methyl groups even at low temperatures which indicates rapid interchange of
axial and equatorial substituents, presumably by pseudorotation. There is better
evidence for such dynamic processes from n.m.r. studies of spirocycles. Here the
barriers to rearrangement can be measured; they lie between 50-80 kJ moi- 1 .
Me
The only simple pentaalkyls of phosphorus which have yet been prepared
contain CF 3 groups, which assume axial positions in the most stable conformer.
Me 5 As, m.p. - 6°C, is thermally unstable, decomposing at lOOoC to Me 3 As,
methane and ethene as the major volatile products. The antimony analogue,
b.p. 126-7°C, obtained from methyllithium and Me 3 SbBr 2 or Me 4 SbBr, is much
more robust. Infrared and Raman spectra support a trigonal bipyramidal
structure (D 3 h). Acids cleave one methyl group to give a wide range of products.
Me
Me,Sb+HX-.Me 4 SbX+CH 4
(X=halogen, CN, RC0 2 -, OR)
4. 9. 7 Ylides
(a) PHOSPHORUS YLIDES-THE WITTING REACTION: Triphenylphosphonium
salts Ph 3 P+CHRR'.X-, which possess a-C-H functionality react with strong
bases such as phenyl or n-butyllithium to form orange solutions which contain
phosphorus ylides e.g.
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