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

Organometallic compounds of the transition elements
The 18-electron rule is often of value in predicting likely structures for their
complexes. 1, 3-Butadiene, for example forms the iron carbonyls (C 4 H 6 )Fe(C0) 4
and (C 4 H 6 )Fe(C0) 3 • In the former the butadiene is 1] 2 , linked to iron through only
one of the double bonds, whereas in the latter it is 1] 4 bonded.
Fe
(C0)4
2e
8e
8e
18e
C 4 H6 4e
Fe Se
3CO 6 e
18e
Cyclooctatetraene is an extremely versatile ligand in this respect. Some
examples of the different modes of bonding it exhibits are illustrated in Fig. 6.1.
The room temperature 1 Hn.m.r. spectrum of (C 8 H 8 )Fe(C0) 3 shows only one
signal, but the symmetrical1] 8 ligand this suggests would give an electron count of
22. X-ray diffraction, however, reveals a 1] 4 structure consistent with the 18-
electron rule. The explanation is that the Fe(C0) 3 group is rapidly switching its
position between the four double bonds, so that an averaged n.m.r. signal is
observed.
Most of the examples in Fig. 6.1 obey the 18-electron rule although some
exceptions are shown. The platinum compound has a 16-electron configuration,
common for many planar complexes late in the transition series (p. 174).
Complexes of the early transition elements also often depart from the rule; the
metal valence orbitals are relatively high in energy (especially 4p) leading to
greater ionic character in the bonding.
For some compounds it is possible to write down two or more structures which
obey the 18-electron rule. A decision between these can be made only in the light
of further evidence. The structure of C 8 H 8 Fe(C0) 3 could be written with an 1] 4 -
cyclooctatetraene ligand (as determined by X-ray diffraction) or with an 1] 2 , 1] 2 -
chelating ligand (as found in C 5 H 5 CoC 8 H 8 ). In fact both isomers of the
cyclooctadiene analogue, (1] 4 -1, 3-C 8 H 12 )Fe(C0) 3 and (1] 2 :1] 2 -1. 5-C 8 H 12 )Fe(C0) 3
are known, although the latter isomerizes quite easily to the former. Dipolar
structures can usually be excluded. Ready isomerization between the 1] 6 and 1] 5
forms of some .fluorenyl complexes, however, is observed.
ţ}
+
Fe
(C0)3
Extreme dipotar
representotion of the
bonding in C8H8 Fe(COl3
( not correct)
194
Cr
(C0) 3
fJ6
lsomers of [fluorenyl Cr(COlJ~
fJ5