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

9.2.2 Bonding in metallocenes
Bis( cyclopentadienyl)complexes
In the literature bonding in metallocenes has usually been discussed on the basis
of D 5 d symmetry, even though the eclipsed conformation (D,h symmetry) is
slightly more stable. This has the advantage that the symmetry labels of related
orbitals of (C,H 5 ) 2Fe and of (C 6 H 6 ) 2 Cr(D 6 h) are the same (Fig. 6.10. p. 205). The
main interactions in which the metal d orbitals are perturbed are as follows:
Ligand n 1 + dz' (a 1 g, a-symmetry), ligand 11: 3 + dx'-r'• dxy (e 2 g' <5-symmetry) and
ligand n 2 + dzx• dyz (e~g' n-symmetry). These orbitals are filled sequentially, as
shown in Table 9.1, as the number of electrons increases from V to Ni. Here is a
series of organometallic compounds with electron numbers ranging from 15-20.
The eighteen electron compound, ferrocene, is certainly the most robust and inert
but with the reasonably strong donor characteristics of C 5 H 5 other configurations
arise for elements ofthe First Transition Series. This is in contrast with carbonyls
which contain the strongly n-accepting ligand CO. All the metallocenes apart
from ferrocene, ruthenocene and osmocene are paramagnetic, the ground state
electron configurations being indicated in the Table. While vanadaocene,
chromocene and ferrocene are low spin complexes, manganocene has five
unpaired electrons in the ground state, which is therefore high spin. The high spin
and low spin states (the latter has configuration (e 2 gP (e 1 g) 2 in manganocene
are very close in energy and in 1, 1 '-dimethylmanganocene both forms can be
observed in equilibrium. for example in the UV photoelectron spectrum, over a
wide temperature range.
The metal-carbon bond distances at first sight seem to vary in a rather
random fashion. If, however. the value for Mn(C,Me,) 2 , which like its neighbours
bas a low-spin ground state, is taken there is a general contraction from V to Fe
following the contraction in atomic radius. The longer distances in cobaltocene
and nickelocene reflect the presence of electrons in antibonding (e 18 )* orbitals.
Perhaps the most striking thing about the thermochemical data is the
observation that these extremely heat resistant materials, ferrocene in particular.
are so strongly endothermic with respect to the elements. The mean bond
dissociation energies parallel the binding energies of the metals as discussed in
Chapter 1. The iron-ring bond in ferrocene ( 352 kJ mol- 1 ) is about as strong as a
carbon-carbon single bond (ca. 360kJmol- 1 ).
9.2.3 Chemistry of ferrocene
It was the remarkable inertness of ferrocene towards air, water and thermal
decomposition as well as its novel structure which caught the imagination of
many chemists in the 19 SOs. Nevertheless. ferrocene undergoes many reactions
leading to numerous derivatives.
(a) OXIDATION. Despite its 18-electron configuration, ferrocene is oxidized in
solution even by fairly mild reagents such as iodine. copper(II) or iron(III) to give
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