Chemistry Reference
In-Depth Information
adopted in the crystal phase by the
-hydrocarbyl complexes to depend on kinetic
as well as thermodynamic factors [
83
,
107
,
108
]. For instance, in the case of
metallocenes such as ferrocene, (
p
5
C
5
H
5
)
2
Fe, examples of staggered, eclipsed,
and in-between ring geometries may be found in the Cambridge Structural Data-
base [
109
] for this complex included as a guest in the lattice. Even in the gas phase,
the search for the ground state minimum geometry of metallocenes is often criti-
cally dependent on the level of computation - the adequate treatment of electron
correlation being a stringent condition for their correct geometry determination
[
110
]. Because of the characteristic potential energy surface flatness for the metal-
(
p
-hydrocarbyl) interactions, fewer M-C bond paths than expected on the basis of
the formal hapticity are normally recovered, and their actual number is neither
usually predictable nor easily explainable [
83
].
28
It is thus not surprising that based
on a number of theoretical studies on metal-(
p
-hydrocarbyl) complexes [
111
,
112
],
Richard Bader et al. were led to conclude that “the bonding of a metal atom to an
unsaturated ring is not well represented in terms of a set of individual bond paths,
but rather by a bonded cone of density
is best viewed
as involving an interaction with the delocalized density of the entire ring perimeter,
a picture that is conceptually similar to that conventionally used to denote the
interaction of a metal with an unsaturated ring in chemical structure diagrams”
[
111
]. Does the SF description reflect such a delocalized picture of bonding in metal
p
...
” so that such bonding “
...
-hydrocarbyl complexes? How does it relate with the bond path view or with the
nature of the interatomic electron sharing indicated by the corresponding delocali-
zation indices?
Through a sequence of two outstanding papers, Farrugia et al. [
72
,
83
] have
given illuminating answers to all these questions. In the earlier one, a combined
experimental and theoretical charge density study of the trimethylenemethane
(TMM) complex (Fe(
4
-C{CH
2
}
3
)-(CO)
3
) was performed [
72
]. The complex pro-
vides an example of a delocalized
-hydrocarbyl system where the unsaturated C-C
bonds are arranged in a stellated fashion rather than in a ring or chain. Agreement
between the theoretical and experimental topologies was excellent. Curiously
enough, both density topologies found only one bond path between the TMM
ligand and the Fe atom, from the C
a
atom, whereas none of such paths was
recovered between the metal and any of the three C
b
atoms (Fig.
11a, b
).
According to the bond path criterion, the complex should be described as a
p
1
-
complex, but normal-mode analysis, ESCA and photoelectron spectra, NMR bar-
rier to rotation of the TMM ligand in the complex, and qualitative analysis of the
Kohn-Sham orbitals seem to all suggest a significant
-interaction with the C-C
bonds ([
72
] and references therein). The available physicochemical evidences, save
p
28
It was noted, however, that the virial graphs, at variance with the molecular graphs, do reflect in
general the formal hapticity in metal
p
-hydrocarbyl complexes [
83
]. Virial graphs are the analo-
gous of molecular graphs for
r
V replacing
rr
[
6
]. Because of their general homeomorphism,
these two vector fields usually exhibit topologically equivalent graphs, but this may not hold true
when the equilibrium structure is close to a catastrophe point for at least one of the two fields.
