Chemistry Reference
In-Depth Information
Farrugia et al. [
83
] also investigated a number of cases where the geometrical
distortion in the metal-
n
-hydrocarbyl interaction is sufficiently large to yield a
formal reduction in the hapticity. In such cases, it was pleasingly found that both the
delocalization indices and the SF patterns clearly discriminate the formally bonded
from the formally nonbonded M-C
ring
interactions.
One minor point remains to be clarified concerning this otherwise excellent
paper by Farrugia et al. [
83
]. When discussing the (
E
)-{(
5
-C
5
H
4
)CF
5
-
¼
CF(
5
-C
5
H
5
)
2
Fe
2
system, it is mentioned that the weak C-H
C
5
H
4
)}(
F interaction
provides an interesting case, with “characteristics and highly delocalized” sources
and with the three interacting basins all acting as very large “sinks” for the density,
while the other F atom provides the single largest source. We are not interested here
in the discussion of this specific SF pattern, but concerned with the physical
interpretation that was given for atoms acting as “sinks.” Farrugia et al. indeed
note that “when a basin acts as a sink, the kinetic energy dominates over the
potential energy density when averaged over that basin.” Clearly this is not true,
since for the very definition of QTAIM basins, the kinetic energy density and the
potential energy densities, when averaged over
, fulfill the atomic virial theorem,
and the ratio of their averaged magnitudes has to be always equal at equilibrium to
the virial ratio of 1:2 as it is for the global system. As shown in (5), an atom yields a
negative source if LG dominates over LV when averaged over its basin. Although
LG and LV are related to G and V, they clearly differ from them because of their
Green's or influence function term (Sect.
2.1
). It is this term which causes an atom
to act as a source or as a sink, depending on the position of the rp, as shown earlier
in this chapter (Sect.
3.3
) for the H atom involved in weak or moderate strength
OH
O
O bonds.
The chemistry of
2
coordination of a ligand
E
-H
s
-bond complexes formed by
bond (
E
C, Si, H, B, Sn or Ge) to a TM center has been the subject of intense
interest over the past three decades, as these systems provide an insight into the
E
-H activation of
E
-H bonds by TM centers [
113
]. Silane
¼
-bond complexes,
which were the first to be isolated and recognized back in 1969 [
114
], presently
represent the second largest class of
s
s
-bond complexes after molecular hydrogen
systems and serve also as a model for their more ephemeral alkane
s
-bond cousins
and for C-H activation [
115
]. Recently, Mc Grady et al. [
84
] have investigated the
nature of metal silane
-bond interaction in a number of key systems by a range of
experimental and computational techniques, including an SF analysis, which is
briefly reviewed below. In particular, their study focused on three simple Schubert-
type [
116
] complexes [Cp
0
Mn(CO)
2
(
s
2
-HSiXY
2
)], with
X
¼
H, F, and Cl, respec-
tively, and
Y
being Ph for
X
¼
H, F or being Cl for
X
¼
Cl (the molecular scheme
and structure for
X
Ph is shown in Fig.
13a, b
). This series of complexes
may be viewed as “snapshots” at various stages along the reaction coordinate for
oxidative addition of the Si-H bond to the metal, although the determination of the
actual reaction stage in a given complex turns out to be difficult because X-ray
diffraction fails to locate the H atoms with sufficient accuracy. Earlier topological
analyses of the charge density [
111
] and photoelectron spectra ([
117
] and note 17 of
[
84
]) have indeed classified the compound with
X
¼
F,
Y
¼
¼
Y
¼
Cl as a nearly complete
