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

Organotransition metal chemistry
excess
11 --­ ',Ni-\\
\\~
Tngonal planor, 16e
Colourless needles
7.3.2 Some cationic alkene complexes of iron
The chemistry of some cationic alkene derivatives of iron illustrates how sever al
different methods can sometimes be used to prepare the same compound, how the
reactions of an organic molecule can be modified on coordination and how
advantages of this can be taken in organic synthesis.
The dimer [CpFe(C0) 2 ] 2 (p. 295) is the starting material for these studies. The
11 5 -C 5 H 5 and CO ligands are spectators in the reactions to be described, so it is
convenient to write 11 5 -C 5 H 5 Fe(C0) 2 as Fp. The dimer is thus written Fp-Fp or
Fp 2 • Fp 2 is reduced by sodium amalgam in THF to a nucleophilic anion Fp- which
attacks alkyl halides:
(protonation)
Cotionic ~ 2 -aikene
complex
( formally fJ-hydride
abstraction)
Each ofthe isomeric 11 1 -alkyl complexes reacts with Ph 3 C+BF; by abstraction of a
hydrogen to iron to form the same cationic 11 2 -propene complex. The conversion
of a neutral11"-complex to a cationic 11n+ 1 -species is a property of the Ph 3 C+
reagent (p. 323). It occurs when the complex cation is more stable than the
triphenylmethyl cation. The propene complex is also obtained by protonation of
the 11 1 -propenyl derivative. Protonation of an uncoordinated double bond {3- to a
metal-carbon linkage also provides a general method of converting neutral
11"- into cationic 11n+ 1 species. Note the use of the weakly coordinating ion
BF;. PF-; or ClO: can also be used, although complexes containing the latter are
240