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

Five electron ligands
slowly oxidized in air, although the solutions are more sensitive. The compound is
cleaved by halogens to yield mononuclear derivatives:
xl
[CpFe(C0) 2 ), ----+ CpFe(C0) 2 X (X= Br, I)
HCl/0 2
---~ CpFe(C0) 2 Cl
AgBF 4 /L
------+ [CpFe(C0) 2 L]+ BF; (L=CO, PR,. alkene)
Silver tetrafluoroborate in the presence of ligands oxidizes it to cationic species
(p. 240).
A typical reaction of many cyclopentadienylmetal carbonyls is reduction to
anionic species (Fig. 9.11 ). Sodium amalgam is often used as the reducing agent.
The liquid sodium-potassium alloy can also be used. A solution of the starting
material in tetrahydrofuran is stirred with the amalgam under nitrogen and the
spent amalgam run off through a stopcock at the base of the flask when the
reduction is complete. The solutions of the anions are very air sensitive. The
anions themselves are strong nucleophiles which undergo numerous reactions.
This chemistry is exemplified with reference to CpFe(CO);. The behaviour of
other mononuclear species, especially CpM(CO); (M = Mo, W) shows many
parallels.
Cis and trans geometrica! isomers of [CpFe(C0) 2 L have been prepared. The
stable form of the molecule has the trans structure shown in Fig. 9.12, buton
crystallization at low temperature a cis isomer can be isolated. In this molecule the
Fe 2 C 2 group is folded about the Fe-Fe axis, giving an angle of 164 o between the
two Fe-C-Fe planes. In the trans isomer, however, the Fe 2 C 2 atoms are
coplanar. Observations of the proton and 13 C n.m.r. spectra of [CpFe(C0) 2 ), in
solution over a range of temperature reveal rapid exchange of carbonyl groups
between bridging and terminal positions (Fig. 9.13). In the case of the trans
isomer this is sufficient to cause equilibration of the carbonyl groups even at low
temperatures (see Newman projection). At - 65°C this isomer gives only one
averaged carbonyl signal in the 13 C spectrum. The activation energy for the
exchange is about 30k}mol- 1 • The cis isomer, however, affords two signals
which arise from COb and cot respectively. Only two ofthe carbonyl groups (COb)
lie in the conformation required to form the two bridges, that is, trans to each
other. Rotation about the Fe-Fe bond in the unbridged structure can make the
cob and cot groups equivalent, but this is detected by the n.m.r. experiment only
above -3 5°C. The activation energy for the rotation ( 4 7 k} mol- 1 ) is greater
than that for the bridge opening. In agreement with this mechanism, the same
value has been obtained for the cis-trans isomerization from 1 H n.m.r.
measurements.
The iron-iron distance in the cis and also in the trans isomer is quite long,
about 2.53 Â, compared with 2.48 Â in metallic iron. X-ray diffraction reveals
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