Chemistry Reference
In-Depth Information
between M atoms in the binuclear 3
d
metal carbonyls. Full details of this study
may be found in [
14
], while only the main results are reviewed here. As shown in
Table
9
, each complex exhibits an M-M bcp, even the one with M
Pd and a zero
formal M-M bond order. Furthermore, the
r
b
,S(M)andS%(M)valuesaremuch
larger than those found for the M-M bond in the saturated binuclear 3
d
metal
carbonyls or in the unsaturated binuclear Co carbonyls series with corresponding
M-M formal bond orders. The Mo atom determines more than 40% of the density
at the Mo-Mo bcp in Mo.4.4.4, analogously to what observed for the bonded
atoms in a standard covalent single bond between second-row atoms, and defi-
nitely more than the contribution (27.9%) the Co atom yields to the Co-Co bcp
density in Co
2
(CO)
5
, despite both systems do share an equal M-M formal bond
order of four. The sum of the S% contributions from the two M atoms in Rh.4.1.4
almost reaches 60, while that for the binuclear metal carbonyls having the same
formal bond order of one oscillates around zero and is even largely negative in
Mn.10.1.0.
The notably larger S%(M) contribution to the M-M bcp density in the M
2
(for-
mamidinate)
4
(M
¼
4
d
transition metal) compounds relative to that in the saturated/
unsaturated binuclear Co carbonyl series with corresponding M-M formal bond
orders fits with the documented enhancement in the M-M bond strength down a
group, arising from the greater spatial extension of
d
orbitals in heavier atoms
[
103
]. For the bulk metals, the M-M bonds in the
d
-block are strongest in the 4
d
and
5
d
series, and this feature is known to carry over into their compounds [
103
]. In nice
compliance with the largely enhanced S%(M) values, also the metal-metal delo-
calization indices for the M
2
(formamidinate)
4
series are much greater than for the
cobalt carbonyls series (Table
9
). For Mo.4.4.4,
d
(Mo,Mo
0
) amounts to 2.930, a
value about three times as large as that found [
d
(Co,Co
0
)
¼
0.976] for the binuclear
Co compound sharing the same formal bond order of four, while Rh.4.1.4, having a
formal bond order of 1, shows a
d
(Rh,Rh
0
) value very close to one to be compared
with an average value of about 0.4 for the two Co
2
(CO)
8
isomers with analogous
formal bond order. It is worth noting that the
d
(M,M
0
) values are ordered as the
formal bond orders for the M
2
(formamidinate)
4
series, whereas the corresponding
S%(M) values do not seem to clearly distinguish the compound with formal bond
order of four (M
¼
¼
Mo) from the two systems with a value of three for such order
(M
Ru) from
Rh.4.1.1 having one as formal bond order. However, for these specific cases the
observed
d
differences amount on average to about 0.3 and are therefore much
lower than the difference of one between the corresponding formal bond orders
(Table
9
). One may thus safely conclude that the trends of the S%(M) and
d
(M,M
0
)
values almost agree also within this M
2
(formamidinate)
4
series. Both trends concur
with a largely enhanced covalent character (electron sharing) with respect to the
two investigated 3
d
metal series and with a similar covalency for the compounds
having formal bond orders of 3 and 4 or of 1 and 2 in the M
2
(formamidinate)
4
series. Note that this apparently anomalous behavior, with respect to the formal
bond orders, for both the
d
(M,M
0
) and S%(M) values fits extremely well with the
corresponding known anomalies [
85
] of the M-M bond lengths in the series.
¼
Nb and Tc) and the compound with formal bond order of two ( M
¼
