Chemistry Reference
In-Depth Information
The empirical relationships observed among
d
(M,M
0
) and the S% contributions
from the bonded metals M to the electron density at their intervening bcp have a
particular relevance in view of the prominent role that the localization/delocaliza-
tion index descriptors play in characterizing the M-M bonding in organometallic
materials [
66
]. The possibility to get S% values directly from experiment adds an
obvious importance to such a parallelism.
Even when the M-M bond is lacking and the M-M midpoint is used as an rp for
evaluating the SF contributions, low
d
(M, M
0
) values seem to come along with low
S%(M) magnitudes. Implications of such an observation for the mechanism of
indirect M-M bonding through 3c-2e interactions with the bridging ligands have
been pointed out. In spite of not too unlike S%(M) magnitudes for bonded and
unbonded M-M systems, the occurrence or lack of a M-M bcp is, however, clearly
signaled by the quite distinct features of the LS profiles along the M-M internuclear
axis in the two cases.
Four very recent studies [
77
,
80
-
82
] have applied for the first time the SF
analysis to the experimental electron densities of a number of binuclear metal
complexes. A comparison with SF data derived from theoretical approaches was
also provided in a number of cases. The main outcomes from such studies are
briefly reviewed below, as for the insight drawn from the SF tool.
Overgaard et al. [
80
] carried out accurate diffraction experiments on a binuclear
Co complex, composed of two Co(CO)
3
dimers bridged by an alkyne bearing one
hydrogen and one cyclohexanol group. The charge density of the complex, which
crystallizes with two molecules in the asymmetric unit, was determined using
multipolar refinement of single-crystal X-ray diffraction data collected either with
a synchrotron source at very low
T
(25 K) or using a conventional source at an
intermediate
T
(100 K). The study thus offers several interesting potential opportu-
nities. It allows for the comparison of the SF description in identical molecules
using a single data set or the comparison of the SF description for each of the two
molecules in the asymmetric unit using two data sets, obtained from different X-ray
sources and collection temperatures. In addition, ab initio complete active space
and DFT gas-phase calculations performed on one of the two molecules in the
asymmetric unit permit a comparison between theory and experiment. Both agree as
for the lack of a Co-Co bcp in the complex, whereas it proved impossible [
80
,
104
]
to locate in the experimental density
24
all of the four expected and theoretically
found Co-C(alkyne) interactions. The S%(Co) contributions to the density at the
Co-Co mp were small for both molecules and for all examined densities and in line
with the values found for the binuclear Co complexes with similar Co-Co distances
[
14
]. The S%(Co) values for the two molecules, whose structures may almost be
perfectly overlaid and differ only in the relative orientations of the cyclohexanol
moiety, are very much alike and slightly negative (about
18%/
16%) in the case
24
It was indeed demonstrated that the extreme flatness of
r
in the CoC
2
triangles prevents the
consistent location of the expected ring and bond CP in this region because their hypothetical
experimental density differences would be surely much smaller than the typical multipole model
residual density in the area.
