Chemistry Reference
In-Depth Information
H
6
)(C
4
H
6
)(C
3
H
5
)Mo
+
PF
6
-
Example 3: (C
6
+ Mo
2+
+
+
Mo
Oxidation state = +2
Method 1
Method 2
6
-C
6
H
6
=
6
6
4
3
6
-1
18
6
4
4
4
__
18
Ligands:
η
Ligands:
η
-C
6
H
6
=
4
-C
4
H
6
=
η
4
-C
4
H
6
=
η
η
η
3
-allyl =
Metal: Mo =
Charge = +1
Total =
3
-allyl =
Metal: Mo(+2) =
Total =
Figure 1.17
Example 4: NaCo(CO)
4
Co(CO)
4
Co + 4 CO
Oxidation state = -1
Method 1
Ligands: 4 x CO = 4 x 2 =
Metal: Co =
Charge = -1
To t a l =
Method 2
8
9
+1
18
Ligands: 4 x CO = 4 x 2 =
Metal: Co(-1) =
Total =
8
10
__
18
Figure 1.18
1.4 Mechanism and the Basic Reaction Steps
To adapt the well-known phrase of Lord Rutherford, organic chemistry without mechanism is just stamp
collecting. Mechanisms have been proposed for all of the major reactions catalysed or mediated by transition
metals and used in organic synthesis. In many cases, the proposed mechanisms are supported by sound and
thorough studies. In other cases, this is not so. The reader should approach any publishedmechanism (including
those in this topic) with caution. Unless the mechanism is backed up by the proper experiments such as kinetics
and isotopic labeling, it should be regarded as speculative and fully open to reinterpretation. Nevertheless,
thinking about mechanisms is one of the most valuable activities and an excellent source of new ideas.
Mechanisms for reactions catalysed or mediated by transition metals are multistep. While the overall result
can be complex and bewildering, the individual steps are taken from a quite small and relatively simple list.
Some of these are common to “classical” organic chemistry; others are specific to the transition metals. While
the basic reactions such as nucleophilic and electrophilic attack do operate, the presence of transition metals
means that another set of basic reaction steps also operate. Combinations of these steps give us the overall
reactions that we use.
1.4.1 Coordination and Dissociation
The most fundamental step is the simple coordination and dissociation of ligands (Scheme 1.3). This is
important because a stable complex cannot coordinate the substrate, but must first dissociate a ligand.
Although some ligands are sufficiently labile to dissociate under mild conditions, in other cases it is necessary
to use heat or light to achieve this. Often, reaction conditions are dictated by this initial dissociation.
