Chemistry Reference
In-Depth Information
complexes. This relates to the usually low oxidation state of the target compounds, that
makes many of them air-sensitive (requiring the use of an inert atmosphere), the distinctive
types of ligands involved, and the tendency for these compounds to be insoluble in or to
react with water (leading to nonaqueous solvents being required). Solvents such as diethyl
ether, tetrahydrofuran or toluene are more likely to be employed in this field. It is also com-
mon to employ sealed glass reaction vessels flushed with nitrogen or argon gas or else an
inert atmosphere 'glove box', which is a large glass-fronted container fitted with portholes
and rubber gloves that allow work to be carried out separated from the atmosphere in an
inert gas (such as dinitrogen or helium) environment.
Historically, organometallic compounds have been known for at least as long as Werner-
type complexes, with coordinated ethene first reported by Zeise in Denmark in 1827, and
metal carbonyls like Ni(CO)
4
prepared by Mond in France in the 1890s, although it is
true that the vast number of examples date from the 1950s and beyond. The two gross
classes of ligands met in organometallic chemistry are: simple
-bonded type, such as
M
CH
3
, that behave in many ways like a conventional metal-donor bond; and multi-centred
-bonded systems such as occur with an alkene that binds symmetrically side-on and
involves its
-electrons in the linkage to the metal centre. This has been discussed earlier in
Chapter 2.5.
As an aid to understanding the outcomes of organometallic reactions and in synthesis,
there is a convenient way in which the stability of a compound can be predicted, called the
18-electron rule. In the light elements of the p block, we traditionally invoke an octet (or
8-electron) rule to probe stability. This assumes that an s and three p valence orbitals are
used in bonding, and allows us to understand why CH
4
is stable whereas CH
5
is not. In
d-block chemistry, it is possible to use what is in that case an 18-electron rule (using an s,
three p and five d orbitals) to help predict stability of a complex. This is used mostly for
organometallic complexes, and has value since it limits the number of combinations of metal
and ligand that lead to the desired electron count. This concept was explained in Chapter
2 and will not be developed further here, but is invariably met in more specialized and
advanced courses. It works best for low-valent metals with small neutral high-field ligands
like carbonyls, but is less effective for high-valent metal ions involving weak-field ligands,
and thus is not usually invoked for traditional Werner-type coordination complexes.
Metal carbonyls represent a key class of organometallic compounds, and are often a
starting point for other chemistry. They tend to be monomers, dimers or small oligomers,
such as Ni(CO)
4
and Mn
2
(CO)
10
, the latter involving a formal metal-metal bond. Most
metal carbonyls are made by reduction of simple salts or oxides in the presence of CO, or
direct reaction of CO with finely-divided metals at elevated pressure. Examples appear in
(6.43) and (6.44).
25
o
C
1 atmosphere
OC
CO
(6.43)
Ni + 4 CO [Ni(CO)
4
]
Ni
CO
OC
(6.44)
200
o
C
100 atmospheres
OC
CO
CO
CO
Re
2
O
7
+ 17 CO [Re
2
(CO)
10
] + 7 CO
2
OC
Re
Re
CO
OC
OC
CO
CO
The carbonyl ligands are able to be substituted by some other ligands, or else the coor-
dination sphere can be expanded by addition reactions with other compounds. It is also
straightforward to prepare compounds that incorporate carbonyl and other ligands such as
