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motif, where each strongly Jahn-Teller distorted Cu(II) ion is ligated in its primary
coordination sphere to three O atoms and one N atom in a square planar arrange-
ment and with Cu-
X
(
X
O, N) distances all below 2.05
˚
. One axial coordination
site is occupied by an oxygen atom of a pendant L
anion from an adjacent complex
in the lattice, while the other coordination site could potentially be considered as
occupied by an additional O atom from the L
anion already O-bonded to Cu(II) in
the primary coordination sphere. For this second axial site, the angle subtended at
the Cu(II) center is, however, much smaller than 90
. The Cu
¼
O axial interactions
all lie in the 2.45-2.70
˚
range, suggesting their weakly coordinating nature.
Graphical representations of the S% contributions for the various bcps in the
examined complexes visibly and markedly distinguish the strong interaction of
the Cu(II) ion with the ligators in the first coordination sphere from the much
weaker ones in the secondary sphere (see Fig. 9 of [
81
]). Those for the former
interactions are fairly localized and denoted by similar contributions from the
bonded Cu and
X
N, O atoms, summing up to about 65-70% of the bcp density
and typical of strong metal-ligand interactions. Instead, those in the secondary
sphere involve quite delocalized sources which extend over large parts of the
complex and with S% contribution from the Cu atom opposite in sign to that of
all atoms of the weakly linked ligand. The strikingly different portrait of sources in
the two coordination spheres denote the shared nature of the metal-ligand interac-
tions in the former and their largely polarized, non-shared nature in the latter.
The two Cu(II) centers, lying about 3
˚
far apart, are known to be strongly
antiferromagnetically coupled [
81
] but the lack of a Cu
¼
Cu bcp and bond path
suggests the absence of any direct metal-metal interaction in the complex. The
molecular graph rather provides an experimental support for an exchange pathway
via the bridging O-atoms, as also clearly suggested by the plot of the spin density in
the plane of the Cu
2
(
Cu midpoint, that
is at the rcp of this unit, the S% contributions appear fairly delocalized, with the
four atoms in the ring determining about 40% of the density at the rcp and with
the largest contributions being those from each Cu atom, amounting to about 13%.
The small sources from the metal atoms agree with the insignificant value of 0.02
found for the
d
(Cu,Cu
0
) delocalization index and with the lack of a shared direct
interaction between the two metal atoms. The
d
(Cu,
X
;
X
m
-O)
2
unit. When the rp is taken at the Cu
¼
N, O) values are instead
close to 0.5, corroborating the large difference observed for the SF portraits of the
Cu
Cu and Cu-
X
interactions.
In their work on the dinuclear borylene complex [{Cp(CO)
2
Mn}
2
(
m
-B
t
Bu)]
(Cp
C
5
H
5
), Flierler et al. [
77
] have applied, among other tools, also the SF
analysis to discuss the nature of the Mn-Mn bonding in a system that represents a
model compound for such kind of investigations in bridged and nonbridged organ-
ometallic complexes. The 18-electron rule would predict a bond between the
two Mn atoms, as would be suggested also by their short internuclear distance of
2.78
˚
and by the lack of any evidence for unpaired electrons in the complex.
However, no bond path was recovered between the two Mn atoms, neither using the
electron density derived from the low-temperature high-resolution X-ray diffraction
¼
