Chemistry Reference
In-Depth Information
the metal-metal (M-M) bond and the metal-ligands (M-L) interactions in polynu-
clear transition or alkaline-earth metal compounds [
14
,
77
,
80
-
82
], and also the
M-L interactions in mononuclear metal complexes involving
p
-bonded unsaturated
hydrocarbyl ligands [
72
,
83
]or
-bonded ligands [
84
].
Topological approaches to the description of chemical bonding have revealed
how many traditional bonding paradigms become no longer appropriate when
applied to organometallics [
85
-
87
]. Moreover, it has become evident that the
well-consolidated bonding classification schemes derived from the various topo-
logical approaches also need to be critically analyzed and even revised when
metal-metal (M-M) bonds or metal-ligand (M-L) interactions are concerned [
2
,
66
,
85
,
86
]. Problems involve in a first instance the choice of a suitable criterion to
establish which atoms are actually bonded to one another - eventually leading to the
so-called molecular structure - and, second, the characterization and classification
of the resulting chemical bonds.
When translated to common “chemical thinking,” the very successful and
“universal” bond path criterion [
7
], which defines whether two atoms are bonded
to one another, appears, in the case of organometallics, not completely free of
limitations [
2
,
60
,
66
,
72
]. For these systems, continuous rather than discontinuous
bonding indicators and descriptions seem to be perhaps more appropriate. Very soft
potential energy surfaces - hence often very flat electron densities - characterize
the M-M and M-L interactions in organometallics, so that the resulting structural
diagrams exhibit an enormous sensitivity to computational or experimental details
[
2
,
66
,
88
]. And, as a consequence, the presence or lack of these interactions, when
judged solely by the bond path criterion, may in some instance be a rather subtle
and controversial issue. On top of this, structure diagrams of organometallics are
often at or very close [
66
,
72
,
80
,
83
,
88
,
89
] to bifurcation or conflict catastrophe
points so that their structures typically travel from one structural region to another
one just for very small displacements along one of their softer vibrational modes.
As a challenging example, a system, whose structure diagram evolves for a number
of reasons through a conflict mechanism by moving through points of the nuclear
configuration space in the immediate neighborhood of the conflict catastrophe
point, will exhibit two alternative pair of atoms competing for a bond path, despite
the electron sharing for these two pairs of atoms -measured by the delocalization
index - will be almost physically indistinguishable. However, according to the
bond path criterion and the electron density topology, one pair will be termed as
“bonded” and the other one as “not bonded.” Even when the structure is not so
topologically unstable, interpretation problems may arise. Gatti and Lasi [
14
]
showed, for instance, that the electron sharing between the metal atoms in unsup-
ported binuclear metal carbonyls may be comparable or even smaller than in the
corresponding bridged carbonyl compounds, notwithstanding a bond path is found
to connect the metal atoms in the former and not in the latter compounds. This
observation is obviously tied to the intrinsic inability of the bond path criterion to
directly detect multicenter bonding [
60
]. Bonding through the bridging ligands and
direct M-M bonds are usually alternative and competitive options using the bond
s
