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

General survey
(c) COMPLEXES OF UNSATURATED HYDROCARBONS. The d-block transition
elements form an enormous range of complexes with unsaturated hydrocarbons.
This is the chief feature which distinguishes their organometallic chemistry from
that of the main groups. Virtually ali these compounds ha ve been prepared and
characterized sin ce 1951, the year in which ferrocene was reported. Some
examples of typical complexes are illustrated in Fig. 6.1. In these compounds the
metal-carbon bonding is essentially covalent in character. It arises by interaction
between the n-electron system of the organic ligand and the metal valence
orbitals, ns, np and (n-l)d. It is only with d-block transition elements that the d
orbitals have suitable energy for bond formation and also contain electrons
(Fig. 1.3). Moreover because of their shape, metal d orbitals are well suited to
interact with the n-molecular orbitals of unsaturated organic ligands. These
interactions can give rise to components of CJ-, n- and b-symmetries with respect
to the metal-ligand axis (p. 199 ).
Complex formation between unsaturated organic species and main group
Table 1.2. Summary of types of bonds in organometallic compounds
Ionic
The most electropositive elements,
including lanthanides.
Favoured where organic anion is
stable.
COVALENT
a-Bonding (single M-C bonds)
Essentially covalent M -C bonds occurs
with all except the most electropositive
Group IA and Group IIA elements.
Both main group and transition
elements. Compounds of transition
elements labile to thermal decomposition
unless low energy pathways blocked.
Bridge bonding, Clusters
Multicentre bonds involving
carbon. Carbon atom associated
with two or more metal atoms
in multicentre m.o.s.
Electropositive elements where
cation would be strongly
polarizing (Chapter 3).
Carboranes. Some transition
metal clusters, especially
carbonyls (Chapter Il).
(J- and n-Bonding (multiple M-C bonds)
In main groups multiple bonds often
labile to oligomerization or
polymerization e.g. R 2 C=SiR~. Some
exceptions e.g. ylides.
d-Block transition elements form
alkylidene and alkylidyne complexes
which contain formal double and triple
metal-carbon bonds (Chapter 7).
COMPLEXES WITH UNSATURATED ORGANIC LIGANDS
Chararteristic of d-block transition elements. Covalent interaction between 2p orbitals of
ligand and valence orbitals. (n-1 )d, ns and np of metal ( Chapter 6 ). Involvement of metal
d-orbitals important. 18-Electron configuration of metal commonly associated with kinetic
stability.
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