Chemistry Reference
In-Depth Information
Organometallic compounds also undergo reactions of coordinated ligands readily. A
simple example involves the susceptibility of coordinated carbon monoxide towards nu-
cleophilic attack. The [Mo(CO)
6
] complex reacts with methyllithium (6.48), with the new
ligand produced at one site also able to undergo additional reactions, not described here.
H
3
CO
1-
CO
C
-
OC
CO
CO
[Mo(CO)
6
] + LiCH
3
Li[Mo(CH
3
CO)(CO)
5
]
OC
CO
CO
Mo
(6.48)
Mo
OC
OC
CO
CO
A reaction with a similar outcome involves migration of one ligand to attack another adjacent
ligand (a 1,1-migratory insertion), promoted by the availability of another ligand to occupy
the site vacated by the first movement. In example (6.49) below, an initial R M C O
component of the molecule is converted to an M C(R) O component, leaving a vacant
coordination site, filled by an added phosphine ligand in this case.
H
3
CO
-
CH
3
C
-
OC
CO
CO
[Mn(CH
3
)(CO)
5
] + PR
3
[Mn(CH
3
CO)(CO)
4
(PR
3
)]
(6.49)
Mn
OC
PR
3
CO
Mo
OC
OC
CO
CO
The hydride ion (H
−
) is an efficient small ligand in organometallic chemistry. The first
transition metal hydrides were prepared using the Hieber base reaction, exemplified in
(6.50). The hydroxide adds to the carbon of one CO ligand to produce an intermediate that
rapidly loses carbon dioxide, leaving the hydride ion to occupy the coordination site.
[Fe(CO)
5
] +
-
OH [Fe(CO)
4
(COOH)]
#
[Fe(CO)
4
(H)] + CO
2
intermediate
#
1-
1-
-
COOH
H
-
CO
(6.50)
CO
CO
CO
OC
Fe
OC
Fe
OC
Fe
CO
CO
CO
CO
CO
CO
These are but a few examples of an array of reactions available to organometallic systems.
Clearly, the level of difficulty is greater in performing many organometallic reactions com-
pared with Werner-type coordination chemistry reactions through the special equipment
that must be employed because air and/or protic solvents can lead to unwanted reactions,
although the reactions themselves do not necessarily impose any greater inherent com-
plexity. The storage and handling of products may pose problems, however, due to their
reactivity, particularly in redox reactions. A detailed examination of their chemistry may
be pursued through advanced and/or specialist texts.
Concept Keys
The position of a metal in the Periodic Table has an impact on the type of chemistry
that element will undergo, and the complexes that can be readily synthesized.
The majority of complexes are prepared through reactions involving ligand substitution
in the coordination shell.
