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

10
Complexes of arenes
10.1 Bis(arene)complexes
The commonest six-electron ligands are benzene and substituted benzenes.
Ligands of this type are termed 'arenes'. They form complexes with most of the
transition elements, although the variety of derivatives is somewhat smaller than
for the cyclopentadienyls. Bis(benzene)complexes (1} 6 -C 6 H 6 ) 2 M are known for M
=Ti, V, Nb, Cr, Mo and W and an even wider range of elements including V, Cr.
Pe and Co form bis(mesitylene) and bis(hexamethylbenzene)derivatives. Cations
such as Mn(C 6 Me 6 ); and Fe(C 6H 6 )~+ are also common.
Bis(benzene )chromium has the 18-electron configuration and is thus isoelectronic
with ferrocene. It forms brown-black crystals. m.p. 281 oc and is
thermally rather stable, although it is readily oxidized in air. In the crystal the
molecule has a sandwich structure in which the rings are eclipsed (D 6 h symmetry)
(p. 204). The barrier to rotation ofthe rings about the ring-metal axis, however. is
very low as in ferrocene.
10.1.1 Preparation-Fischer's method
The first general method for the preparation of bis( arene )complexes was reported
by Fischer and Hafner in 19 54. It involves heating the anhydrous metal halide
with the arene and aluminium powder, which acts as a reducing agent, and
aluminium trichloride as halide ion acceptor. Because an arene and AlCl 3 are
involved, the procedure is sometimes called Fischer's 'reducing Friedel-Crafts'
reaction, but the name is somewhat misleading as no electrophilic substitution of
the arene occurs. The reaction exemplifies well the general strategy for preparing
complexes ofmetals in low formal oxidation states-Metal salt+ reducing agent
+ ligand-noted in Chapter 5 for metal carbonyls. The addition of a catalytic
quantity ofmesitylene, which readily forms a complex and also readily exchanges
with benzene, enables lower temperatures and shorter reaction times to be used.
The first product in these reductions is the tetrachloroaluminate ofthe complex
cation:
311