Chemistry Reference
In-Depth Information
the Co-Co mp determine more negative LS contributions to the mp density in the
bridged than in the unbridged isomer, whereas the opposite holds true at the
extremes of the LS profiles shown in Fig.
10
. Indeed, far from the mp, the region
of charge depletion related to the atomic
M-
shell is a bit more locally depleted in the
unbridged system, so determining slightly more negative sources contributions
from this region and for this isomer at the Co-Co bcp.
All in all, combined information from the integrated and local form of the source
function discloses notable and interesting differences in the direct and indirect
schemes of metal-metal bonding, differences which may be hidden or even lead
to apparently contradictory results when descriptors like the electron density at the
M-M mp, the S(M) or S%(M) values, and the
d
(M,M
0
) values are examined
separately.
The [Co
2
(CO)
x
,
x
¼
7-5] series covers the case of unsaturated binuclear 3
d
metal
carbonyls, having the same metal atom and where the number of CO ligands
decreases with increasing formal bond order from two to four through the series
(Table
9
). By including also the two saturated Co
2
(CO)
8
isomers, we can review a
set of systems having a formal bond order extending from one to four. All systems
exhibit a Co-Co bcp, except the bridged ones with
x
7 and 8 (Co.7.2.1 and
Co.8.1.2). The presence of bridging ligands does not instead inhibit the formation of
a Co-Co bcp for the two systems with highest bond order and shortest bond length,
Co.6.3.2 and Co.5.4.1.
21
The S%(Co) for the density at the Co-Co bcp increases from
¼
4.4% in Co.8.1.0
to a maximum of 27.9% in Co.5.4.1, on passing from a formal bond order value of
one up to four. The S%(Co) value regularly increases through the series with the
decrease in the net positive charge on the Co atom and of the number of CO ligands
(Table
9
). The SF contribution from the two Co atoms exceeds that from the ligands
only for the highest formal bond order (55.8% and 44.2%), whereas for a formal
bond order of two or even three, the contribution from the ligands largely dominates
[89% on average for the two considered Co
2
(CO)
7
systems and 63% for Co.6.3.2].
The S% from the two bonded atoms in Co.5.4.1
is still far even from that of a
standard single covalent bond, despite a formal bond order of four
. The peculiar
nature of the M-M interaction in the 3
d
binuclear metal carbonyls anticipated by
the SF analysis is corroborated, also for the unsaturated series, by the Co-Co
delocalization indices. The number of shared pairs of electrons does not reach
one even for the system with a formal bond order of four where it amounts to 0.976
pairs only, instead of four. Fine details as for the almost paralleling trends of
S%(Co) and
d
(Co, Co
0
) values through the series are discussed in [
14
]. Here, we
emphasize the interesting result that a description of Co-Co bonding, thoroughly at
variance with that provided by the 18-electron rule, can be already obtained using
a tool which is based only on the electron density and on the electron density
Laplacian.
21
Note that also in the case of the bridged Co
2
(CO)
8
isomer a Co-Co bcp occurs when the Co-Co
distance is decreased by about 0.2
˚
with respect to equilibrium [
60
].
