Chemistry Reference
In-Depth Information
co ordination
M + L
ML
dissociation
Scheme 1.3
The equilibrium between the coordinated and dissociated species may be driven towards the dissociated
side, to generate the more reactive complex, by the addition of a second metal ion (Scheme 1.4). The
purpose of the second ion is to absorb the dissociated ligand, thus driving the equilibrium according to
le Chatelier's principle. In the case of phosphine ligands, this can be done using copper(I) salts (see Sec-
tion 2.5.7). In the case of chloride ligands, this can be done, effectively irreversibly, but the addition of
silver(I) salts (see Section 6.2 and Section 11.3.1). In this latter situation, what is achieved is the exchange
of the chloride for a more labile anionic ligand, often triflate, but also perchlorate, tetrafluoroborate and
hexafluoroantimonate.
L
n
MPPh
3
+ CuX
L
n
M
+ Ph
3
PCuX
L
n
MCl
+
A
g
O
T
L
n
MOTf
+
A
g
C
l
Scheme 1.4
Dissociation may also be made easier by replacing a strongly bound ligand with a weakly bound ligand in
a separate step (Scheme 1.5). This can be achieved by destruction of a ligand. This is often done by oxidation
of a CO ligand to CO
2
using an amine oxide. The vacant site is then taken up by the more labile amine ligand.
Another strategy is to substitute a less-labile ligand for a more-labile ligand in a separate step, so that the
substrate avoids the more brutal conditions required for dissociation.
Me
3
NO
+ CO
2
L
n
M
CO
L
n
M
NMe
3
h
ν
, MeC
N
Mo(CO)
6
Mo(CO)
3
(NCMe)
3
Scheme 1.5
Exchange of ligands may also be employed to modify the reactivity of a complex. Substitution of CO by
NO
+
will make a complex much more electrophilic (see Scheme 10.19). Substitution of CO by PPh
3
will
achieve the opposite.
Complexation can also raise stereochemical issues. When a
-system is involved, the metal may attach to
either face (Scheme 1.6). Selectivity is often observed, and can be exploited. The selectivity may be due to
steric effects or neighbouring group effects.
32
(OC)
3
Cr
SiMe
3
SiMe
3
Cr(CO)
6
(OC)
3
Cr
OTHP
OTHP
Cr(CO)
6
Scheme 1.6
