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

Bis( cyclopentadienyl )complexes
coordination positions and fulfils a role similar to the rather inert alkyl or aryl
groups in organic chemistry.
Related to cyclopentadienyl are the open chain pentadienyl group and also the
cyclohexadienyl and cycloheptadienyl ligands in which the IJ 5 system is
contained in a larger ring. In general the chemistry of such ligands has not been
explored to the same extent as that of cyclopentadienyl, but their complexes
usually show greater reactivity and some significant differences (p. 301)
(Fig. 9.2).
9.2 Bis(cyclopentadienyl)complexes- the metallocenes
Bis(cyclopentadienyl)complexes have been isolated for the elements of the First
Transition Series from vanadium to nickel inclusive and also for ruthenium and
osmium. The most general method of preparation is to treat an anhydrous halide
of the metal with sodium cyclopentadienide in tetrahydrofuran under an
atmosphere of nitrogen or argon.
MX 2 + 2NaC,H,
THF
-----> M(C,H,) 2 + 2NaX (X= CI, Br)
The dihalides of Mn, Fe, Co and Ni are readily available. Trihalides of vanadium
and chromium can be used, but an excess of NaC 5 H 5 is then required as a
reducing agent. Alternatively MX 1 can be reduced in situ to MX 2 using zinc or
magnesium before NaC,H, is added.
Cyclopentadiene can be deprotonated by dimethylamine in the presence of
anhydrous TiCl 4 , FeCl 2 or NiCI 2 to yield Cp 2 TiCI 2 , (p. 290), Cp 2 Fe (ferrocene) or
Cp 2 Ni (nickelocene) respectively. Ferrocene has even been prepared in a rapidly
stirred two phase system (benzene/water) from FeCl 2 • 4H 2 0, cyclopentadiene and
potassium hydroxide using a crown ether, 18-crown-6, as a phase-transfer
catalyst.
The metallocenes are coloured crystalline solids, all of which melt at about
173 °C. They are ali thermally robust and sublime readily under reduced pressure.
Ferrocene is remarkably resistant to air, moisture and heat. surviving even up to
470°C. Ali the other metallocenes listed in Table 9.1 are oxidized in air,
manganocene and cobaltocene being especially vulnerable. Manganocene is also
very easily hydrolysed, making it a rather difficult substance to handle.
9.2.1 Structures
The sandwich structure of ferrocene was suggested in 1952, soon after its
discovery, by G. Wilkinson and independently by E.O. Fischer*, and confirmed
*These two chemists were responsible for much of the early work on sandwich
compounds. They were jointly awarded the Nobel Prize for Chemistry in 1973.
279