Allylic and allenylic carbenium ions, which have been reported in the
literature to be labile or not isolable at all, have no inherent insta
bility. Progressive substitution with ferrocenyl groups affords allyli
c cations, whose stability toward photochemical decay and nucleophilic
collapse increases with increasing number of ferrocenyl substituents,
if the allylic moiety is at least 2-fold-substituted with ferrocenes
in the 1,3-positions. The unmatched cation-stabilizing capacity of met
allocenes is illustrated in cyclopentadienyl( 1,3-diferrocenyl-1-ylium
pentalenyl)iron tetrafluoroborate, a compound which survives exposure
to sunlight in aqueous solution for days. In addition to full characte
rization for these allylium salts by the usual analytical and spectros
copic methods, X-ray analysis of cyclopentadienyl(1,3-diferrocenyl- 1-
yliumpentalenyl)iron tetrafluoroborate showed (a) deviations of the re
gular conformation of the ferrocenyl substituents in the 1,3-positions
that are similar to those observed in other simple metallocenyl-stabi
lized carbenium ions, indicative of electronic stabilization by intram
olecular charge delocalization, (b) these two ferrocenyl substituents
are structurally distorted in an unequal manner and seem to contribute
unequally in this charge delocalization, but the observed dissymmetri
c structure is most likely the result of crystal forces, and (c) the a
nnelated ferrocene of this formal pentalene system is undistorted, rul
ing out (together with results from solution NMR measurements) any sig
nificant conjugation; therefore, this cation should be envisaged as an
allylium system and not as an antiaromatic cyclopentadienyl cation. I
n the case of allenylium <-> propargylium ions, attachment of two ferr
ocenes results in labile or intermediate propargylium ions, which can
be trapped with triethylamine to afford ammonium allenes, formally Lew
is base complexed allenylium salts, with an interesting bent allene un
it as is evidenced by single-crystal structure analysis. Introduction
of a third ferrocene into the allenylium <-> propargylium system affor
ds 1,3,3-triferrocenylallen-1-ylium tetrafluoroborate, the first stabl
e and fully characterized allenylium compound, with only minor resonan
ce contributions from the propargylium structure, proven most clearly
by the observation of an extremely intense and acceptor-shifted cumule
nic stretching vibration of 2151 cm(-1) in the IR spectrum.