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

The first three periodic groups
o-
R
R HgX
&:.x
R
~H
HgX
Fig. 3.23 Fluxional behaviour of CpHgX (X= CI, Br, Me, Cp).
The remarkable variety in structures is continued in the compounds of Zn, Cd
and Hg. In the vapour CpZnMe has the same structure as CpBeMe. In the solid
state the units associate into zigzag chains bridged by cyclopentadienylligands.
The 1 H n.m.r. spectra of Cp 2 Hg, CpHgCl, CpHgBr and CpHgMe each show only
a single line at room tempera ture. Of these examples, a frozen, limiting spectrum
has been obtained only for CpHgCI. at - 113 °C. This shows conclusively that
the cyclopentadienyl group is bonded IJ 1 • Using 13 C n.m.r. the rearrangement
has been shown to take place by a series of 1, 2-shifts ( cf. p. 300), with an activation
energy of only 32 kJmol- 1 (Fig. 3.23).
· Treatment of InCl 3 with excess of sodium cyclopentadienide in THF, followed
by sublimation affords Cpin rather than Cp 3 In, which is formed in very low yield
as a by-product. Cp 3 In can be prepared using LiCp and InCl 3 , but it is rather
unstable. The indium atom in Cpin has a non-bonding pair of electrons and hen ce
forms a 1:1 adduct with boron trifluoride. (cf. p. 292). In the vapour it is monomeric,
and has C,v symmetry. In the solid, however, it polymerizes into chains via
bridging cyclopentadienyl groups.
Cyclopentadienylthallium is precipitated when cyclopentadiene is added to
aqueous thallium(I) sulphate in the presence of sodium hydroxide. It is stable to
air and moisture although like many organothallium compounds, it is slightly
light sensitive. It is a useful reagent for exchanging the cyclopentadienyl group for
chloride, especially in transition metal chemistry. The reaction is usually carried
out in an inert polar medium such as dichloromethane. Insoluble TlCl is
precipitated, leaving the desired cyclopentadienyl complex in solution. e.g.
2TlCp + HgCl 2 --+ Cp 2 Hg + 2TlCl. CpTl is monomeric in the vapour, but,like Cpln,
is associated in the solid state.
Cyclopentadienyl complexes are formed by most elements, including the very
electropositive alkali metals and alkaline earths. While the bonding of the C,H,
unit to d-block transition elements is considered to be largely covalent in
character, involving the d-orbitals of the metal (p. 205), compounds such as
Cp 2 Mg and Cpin ha ve often been described as 'ionic'. It does seem plausible,
however, that even here there is significant covalent interaction between the
carbon 2p orbitals ofthe ligand and the valence orbitals (ns, np and possibly nd) of
the main group element. This is most reasonable in cases such as CpTl in which
the carbanionic nature of the cyclopentadienyl group is relatively low.
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