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the nuclei of the heavier atoms involved in the HB. It thus appears that the
properties at the HB critical point and in the region close to it contain the least
information on the electron density polarization due to intermolecular interac-
tion (and also to molecular formation, if IAM density is taken as reference).
This observation clearly raises serious doubts about the use of the bcp properties
only when discussing intermolecular interactions in crystals. Changes in atomic
properties and related molecular properties, e.g., the molecular dipole, may be
much more significant than changes in the HBs bcp densities [
61
,
62
]. The
analysis of the differences in the local source contributions to the HB critical
point density using the crystal density or one of the model densities introduced
earlier (IAM and superposition of the molecular densities) enables one to single
out those remote molecular regions which mostly contribute to determining the
small density changes at the HB critical point. Gatti and Bertini [
13
]discuss
such changes by examining the difference LS profiles along the juxtaposition of
the D-H and H
O) bond paths in the urea crystal. Conversely,
analysis of these same LS difference profiles, but referred to rps located in those
regions showing the largest variations in the crystal
L
(r) distribution with
respect to the model densities, might reveal those molecular regions which
contribute more in determining such maximal changes. The interaction density
hasalwaysbeconsideredasaratherelusive quantity, and a number of studies
have discussed [
63
,
64
] whether it could be amenable to experimental determi-
nation. The combined
A(D
¼
N, A
¼
LS profile studies sketched above should permit to
individuate the causes and magnify the density changes leading to the interac-
tion density. The preliminary investigation reported in [
13
] certainly deserves to
be deepened.
D
3.4 SF Description of Metal-Metal and Metal-Ligand Bonds
in Organometallics
3.4.1 Why Exploiting the SF for Organometallics?
The relationships between the geometrical and electronic structure of transition
metal complexes and the description of how metal atoms get bonded to one another
or to the ligands have been the source of lively and debated discussions in literature
through the years and are still the subject of a growing number of ongoing theoreti-
cal and experimental investigations [
2
,
14
,
60
,
65
-
79
]. The ligands customarily
provide the necessary glue for the energy stabilization of transition metal com-
plexes, and the study of the interplay and/or competition between metal-metal and
metal-ligand bonding is an interesting and challenging subject.
For a number of reasons detailed below, the SF has enjoyed an increasing
attention as a useful tool to be exploited in this area, and especially so when used
in combination with other techniques. SF applications have in particular concerned
