Chemistry Reference
In-Depth Information
Evaluation of the S% contributions from the Mg at the Mg-Mg bcp reveals that
the two Mg atoms determine about 30% of the bcp density, which is a small
amount if compared to that of a normal shared covalent interaction like C-C
having a
d
(C,C
0
) value close to one, but large if compared to M-M bonding in the
3
d
dinuclear metal carbonyls and of similar size as that found for the insaturated
dinuclear Co carbonyl with formal bond order of two. A comparison with
corresponding values for model systems featuring homonuclear bonding between
neutral and variably charged third row atoms would be helpful to place on a
relative scale the observed S%(Mg) value. Also important would be to know the
d
(Mg,Mg
0
) from a gas-phase calculation of the dimer. Precious further insight on
bonding could then be obtained by evaluation of the S%(Mg) contributions for a
model density made by the sum of the electron densities of the two noninteracting
monomers, placed as in Mg(I) dimer complex. Operating this way, the effect of
Mg-Mg bonding formation could be singled out and evaluated as we did previ-
ously for Mn
2
(CO)
10
[
14
].
Overgaard et al. also discuss the LS profile along the Mg-Mg bond path showing
“an intermediate-sized drop near the bcp (Fig. S7 in the supporting information of
[
82
])” and noting that this drop should disappear with the onset of chemical
bonding. This observation is probably not particularly meaningful in its own, as it
should be supplemented by an LS profile study on a comparative scale, for instance
on the Mg(I) dimer complex and on the corresponding noninteracting dimer, as
performed for Mn
2
(CO)
10
[
14
]. The remark is not to be meant as a criticism, but just
as a suggestion for further work on this important, newly discovered metal-metal
interaction.
3.4.3 Metal-Ligand (M-L) Interactions in Single Metal
-Hydrocarbyl
p
Complexes and in Metal-Silane
-Complexes
s
This section deals with the application of the SF approach to the interesting cases
of (a) the delocalized interactions between transition metal (TM) atoms and open
or close-conjugated
p
s
-systems [
72
,
83
], and (b) the nature of metal silane
-bond
2
coordination of a ligand Si-H bond to a
TM center, to provide an insight into the mechanism of Si-H bond activation
within these complexes [
84
]. As we will show, it is the fairly delocalized and
nonclassical character of all such interactions that makes SF a quite attractive tool
for their study.
The nature of the chemical bond between transition metals and
interactions in complexes formed by
-hydrocarbyl
ligands has been the subject of several MO investigations, and it is in general
considered to be well understood within this specific model approach (see [
83
] and
references therein). Conversely, the well-known fluxional mobility of ligands in
such systems challenges the usual QTAIM description of bonding in terms of two-
center interactions and of corresponding lines of maximum accumulation of density
(bond paths) between bonded pair of nuclei. Indeed, the very low barrier commonly
observed in the gas phase for the rotation of ligands makes the actual structure
p
