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

Complexes of arenes
gas phase (5.01 eV cf. Na 5.14eV). In solution it is readily oxidized (polarographic
half-wave potential E 112 = -0.71 V vs. standard calomel electrode).
Consequently it has been described as an 'electron-rich' compound. This arises by
virtue of the essentially donor character of the arene ligands.
One of the arene ligands in Mo(C 6 H 6 ) 2 is easily displaced, for example by
phosphines. The products Mo(C 6 H 6 )(PR 3 ) 3 are strong bases which can be
protonated once or twice by acids. The addition of allyl chloride (3-chloropropene)
with elimination of one benzene ligand gives a chloro-bridged allyl dimer.
These and related reactions provide an entry into mono-arene molybdenum
chemistry (Fig. 10.3).
10.2 Arene(tricarbonyl)chromium complexes
The arene(tricarbonyl)chromium complexes exemplified by (C 6 H 6 )Cr(C0) 3 belong
to the series of compounds based on the M(C0) 3 group, (C 7 H 7 )V(C0) 3
through to (C 3 H 5 )Co(C0) 3 • They were first prepared in 1958 by Nicholls and
Whiting who heated Cr(C0) 6 with the arene under reflux, if necessary in a high
boiling solvent. One difficulty with the preparation is that the hexacarbonyl
sublimes out of the mixture and hence is removed from the reaction vessel into the
condenser. An ingenious apparatus was devised by Strohmeier to overcome this,
in which the sublimed carbonyl is washed back using condensed solvent.
Recently, however, it has been shown that simple heating ofCr(C0) 6 and arene in
a mixture of di-n-butyl ether and tetrahydrofuran gives excellent yields, the
volatile tetrahydrofuran serving to return any sublimed carbonyl.
Benzenetricarbonylchromium forms yellow crystals, m.p. 163°C which are
stable in air, although its solutions are slowly oxidized, especially in light. In fact a
good method of removing the Cr(C0) 3 group from the arene is to photolyse the
complex in air or oxygen. In the solid state the molecules adopt a staggered
conformation. On the basis ofX-ray and neutron diffraction it has been suggested
that the bond Iengths in the benzene ring alternate slightly. The ring is planar but
the hydrogen atoms are bent out of the plane towards the metal by 1. 6 o. In this
way the C(2p) orbitals point more towards the metal, giving better overlap.
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