Chemistry Reference
In-Depth Information
a
b
c
7.6
C
β
C
H
α
12.0
7.9
15.6
O
Fe
C
4.7
Fig. 11 Source function percentage contributions from atomic basins in the trimethylenemethane
(TMM) complex (Fe(
4
-C{CH
2
}
3
)-(CO)
3
) to the electron density at different reference points
(denoted by a
small yellow sphere
). These are (a) the Fe-C
a
bcp; (b) the Fe-C
b
midpoint (mp);
(c) the Fe-C
b
bcp for a slightly deformed geometry of the complex, where the Fe-C
a
-C
b
angle is
reduced from the optimized equilibrium value of 76.6
to 73
, while retaining the original C
3v
symmetry. This small geometry change yields to the formation of three new bond paths linking
iron to the C
b
carbons also (adapted from Fig. 7 and Fig. S11, with permission from [
72
],
Copyright 2006, American Chemical Society)
the molecular graph, would so imply a significant interaction of iron also with the
C
b
atoms and a consequent classification of the complex as a
4
one.
29
It was thus not surprising that a picture at odds with that provided by the bond
path criterion emerged from the (Fe,C) delocalization indices and the SF analysis
(both were only performed on the gas-phase theoretical densities). Instead of being
negligible, the
d
(Fe,C
b
) value, 0.571, is large and about 40% larger than the
d
(Fe,
C
a
) value, equal to 0.369. Similarly, the atomic SF contributions to the density at
the Fe-C
a
bcp exhibit a pattern which closely resembles that obtained when the rp
is taken at the Fe-C
b
midpoint (mp) (Fig.
11a, b
). Even more, the percentage
contribution from C
b
to the Fe-C
a
bcp density is larger than the corresponding
one from C
a
to the Fe-C
b
mp density. In both cases the sources are fairly deloca-
lized, with cumulative contributions from the three methylene groups being equal
to about 60% of the total rp density, to be compared with a contribution from the
central C atom of about 7-10% only. The sum of contributions from the iron atom
and the bonded C
a
atom is small and about 20% for both rp densities, while that
from the iron and the pertinent C
b
atom is also approximatively constant but
slightly larger, amounting to about 27%. Not insignificant is also the contribution
from the three carbonyl groups (about 20% for both rps). All these values imply a
delocalized picture for the metal-(
p
-hydrocarbyl) interaction with a significant
p
-interaction with the C-C bonds and a clear involvement of the methylenic H
atoms as well.
29
The (TMM) complex, analogously to what found in other metal
p
-hydrocarbyl complexes,
represents the uncommon case of the absence of a bond path where chemical intuition would
expect a bond, which is an opposite situation to the more common recovery of many bond paths
where chemists would not draw any bond line.
