_P.-Powell-auth.-Principles-of-Organometallic-Chemistry-Springer-Netherlands-1988

5
Some transition metal
chemistry relevant to
organometallic chemistry
5.1 The 18-electron rule
The character of bonds between transition metal centres and attached groups or
ligands can span the whole range from electrostatic to covalent. This is reflected
in the approximations which have been used to discuss the coordinate bond. On
the one hand crystal field theory starts from the assumption that the interactions
between transition metal ions and ligands are entirely electrostatic. While this
approach is useful in explaining magnetic and spectroscopic properties of aquo
and ammine complexes, for example, especially when allowance is made for some
covalency, it is far from successful in treating the essentially covalent compounds
formed by ligands such as carbon monoxide, unsaturated hydrocarbons or alkyl
and aryl phosphines.
In 1927 Sidgwick suggested that a coordinate bond arises from the donation of
a pair of electrons from the ligand to the central metal atom. In a nob le gas atom
all the valence orbitals ns, np, (n- l)d are filled, and a stable chemically inert
entity results. Similarly, by filling all the valence orbitals of a transition metal
centre by donation of an appropriate number of electrons from the ligands,
unreactive complexes arise. Eighteen electrons are required from the metal and its
associated ligands to attain this noble gas configuration. This is the 'effective
atomic number (EAN)' or 18-electron rule. A consequence of the rule is that a
metal should have a coordination number appropriate to its oxidation state. As
we shall see below, this does apply in the case of metal carbonyls, but clearly does
not hold for complexes such as the hexaaquo ions M(H 2 0)~+ (n = 2, 3) or
ammines M(NH 3 n + . The 18-electron rule is of great value, however, in
rationalizing and predicting the stoichiometries and structures of essentially
covalent complexes, such as those formed by organic ligands.
The 18-electron rule is essentially a statement about the 'kinetic stability' of
complexes. In this it resembles the octet rule in the chemistry of elements of the
First Short Period, including carbon. In 18-electron compounds full use is ma de of
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