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2c separate bonds. Exact-exchange (hybrid functionals) seems to clearly favor the
former situation. A given bonding scheme is assigned by Goetz et al. [
78
] on the
basis of the number of
2
r
maxima in the valence shell charge concentration
(VSCC) [
6
] of B or by inspecting the synaptic character of the ELF valence basins;
3c bonding is defined through the presence of one charge concentration maximum
directed toward the M-M axis and of only one trisynaptic valence ELF basin,
while the 2c bonding scheme is typified by the occurrence of one -
r
2
r
maximum
along each Mn-B direction or by the presence of two additional disynaptic ELF
basins, besides the trisynaptic one. Note that the “borylene” compound discussed
by Flierler et al. [
77
] is rather classified as a substituted borane, on the basis of the
VSCC portrait of the B atom in the experimental Laplacian distribution. It thus
appears that the precise assignment to one of the two bonding situations is quite
challenging, since even small changes in the DFT functional or in the choice of
multipole model expansion may cause the shift from one bonding scheme to the
other. This is the typical case where distinction between two alternative bonding
descriptions is given in terms of local properties, which may possibly undergo
abrupt changes if the examined systems are characterized by distributions close to
bifurcation catastrophe points (in this case of the -
r
2
r
and ELF scalar fields). We
thus propose that more insight on the borylene/borane debate could perhaps come
from comparing the profiles of the SF contribution from the B atom to the electron
density along the B-Mn and the B-(Mn-Mn mp) lines for any pair of complexes
which are classified as substituted borane and borylene, respectively, in terms of
thenumberoftheirboronVSCC-
r
2
r
maxima. Examination of such profiles
should allow to establish and quantify how diverse are in reality the bonding
situations in the two systems. If a complex is close to a bifurcation point in its
Laplacian distribution, so that tiny changes in the DFT or multipole model may
shift it from the “borane” to the “borylene” assignment, one easily anticipates that
very small changes in both the Mn-B andB-(Mn-Mnmp)profileswillbe
manifest between the two kinds of bonding schemes. Conversely, if the -
r
2
r
maximum (a) is (are) quite evident, both profiles should show clearly distinct
features in the two bonding cases. Also informative (and quite less computation-
ally expensive) could be the comparison of the LS profiles along the same Mn-B
and B-(MN-Mn mp) lines and taking the Mn-B bcp and the Mn-Mn mp as
reference points, respectively. SF studies of such type have, however, not yet been
performed.
We conclude this section by mentioning a recent communication by Overgaard
et al. [
82
] on an ongoing experimental electron density study of the Mg-Mg
bonding character in Mg(I) dimer complexes, containing anion-stabilized Mg
2
2+
entities. Discovery [
105
] of thermally stable and relatively strong (~ 45 Kcal
mol
1
)Mg-Mg
r
-bond interactions in these complexes is unprecedented and
shows that the remarkable case of decamethyldizincocene [
106
], which possesses
a Zn(I)-Zn(I) single bond, can also be extended to the group 2 metal Mg. In view
of the novelty and impact of Mg-Mg bonding, Overgaard et al. [
82
]analyzeda
number of topological features for such bond, which shows a corresponding bcp
with very small
r
b
and
s
2
r
r
b
values amounting to about 0.015 and 0.010 au.
