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

Allyl and diene complexes
only with very reactive aroma tic compounds such as indole or phenol. Nitro- and
halogeno- derivatives cannot be obtained directly by electrophilic substitution as
the starting complex is too susceptible to oxidation.
Oxidation of 1} 4 -C 4 H 4 Fe(C0) 3 with aqueous iron(III) or cerium(IV) releases the
transient cyclobutadiene which either reacts with itself or which can be trapped
by a dienophile in a Diels-Alder reaction:
~ aqueous
Fe Ce <nll
(C0) 3
a Dewar benzene derivative
The sandwich complex which would complete the isoelectronic series (1J6-
C6H6)2Cr, (1J 5 -C 5 H 5 ) 2 Fe .. is bis(cyclobutadiene)nickel, (IJ 4 -C 4 H 4 ) 2 Ni. The tetraphenyl
derivative has been prepared by the following route.
PhfPh
Ph
NiBr2
Ph 2
Ph
+ ::~~:
Ph
8.2.3 Trimethylenemethane complexes
The reactive species, trimethylenemethane, has been observed only by photolysis
of an adduct of allene and diazomethane in a matrix of hexafluorobenzene at
-185°C. The e.s.r. spectrum shows that it has two unpaired electrons (a triplet
ground state) which is consistent with a planar structure with 3-fold symmetry
(D 3 h). This species can, like cyclobutadiene, be stabilized by coordination to
tricarbonyliron. The parent compound, a diamagnetic pale yellow solid, m.p.
28°C can be obtained by the two routes shown.
1 1
1 1
"'/' __ :'
/ '
..... + N2
The symmetrical structure of the trimethylenemethane ligand is indicated in
solution by the 13 C n.m.r. spectrum which shows three resonances: 211.6
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