Treatment of fluorenone, 2,3-diphenylindenone, tetraphenylcyclopentadienone, or 2,5-diethyl-3,4-diphenylcyclopentadienone with ((trimethylsilyl)ethynyl)lithium gives, after
hydrolysis, the analogous alkynol; subsequent addition of dicobalt carbonyl and then
fluoroboric acid yields the corresponding fluorenyl, indenyl, or cyclopentadienyl cation
stabilized by complexation to a tricarbonylcobalt moiety. Variable-temperature NMR data
on these cluster cations, and on their bis(diphenylphosphino)methane derivatives, reveal
that the barrier to migration of the cationic center between cobalt cluster vertices increases
in the order fluorenyl < indenyl < cyclopentadienyl and suggest that the cations with more
antiaromatic character have the greatest need for charge delocalization onto the metal center.
Replacement of a Co(CO)3 cationic fragment by an Fe(CO)3 unit yields the mixed-metal
species [((fluorenyl)CCSiMe3)FeCo(CO)6] (22) and [((2,3-diphenylindenyl)CCSiMe3)FeCo(CO)6] (27). In these structural models for the cationic complexes, the Fe−C(9) distance
in 22 is 2.626(11) Å, while in the indenyl system 27 the Fe−C(1) distance is 2.347(7) Å,
again indicating that the 8π indenyl cation interacts more strongly with the metal center
than does the 12π fluorenyl cation.