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

Complexes of arenes
in the whole molecule, assuming that the same total charge resides on the open as
on the closed ligand.
While nucleophiles nearly always attack terminal positions of even open
polyenes in cationic complexes, this is not always so for odd open systems.
Addition to the end of the 1] 1 -allyl group in [CpMo(CO)(NO)(IJ 3 -C 3 H 5 )] + is
observed, an 1]2-alkene complex being formed. The stereochemistry of this
reaction is rather complicated because the cation exists as exo and endo
geometrica! isomers. These isomers interconvert in solution to an equilibrium
mixture, but sufficiently slowly for their reactions tobe studied independently.lt
should also be noted that the molybdenum centre is chiral. as it carries four
different ligands in a roughly tetrahedral arrangement. It happens that nucleophiles
such as methoxide or NaBH 1 CN (effectively H-) attack each isomer
regiospecifically in such a way that only one of the possible diastereoisomers of
the alkene complex is formed. * This diastereoisomer is in fact the same o ne
starting either from the exo or from the endo isomer of the cation. It is also the
more stable one in which the non-bonded interactions between alkene and other
ligands are less (Fig. 1 0.8).
While nucleophiles generally add to terminal positions in allyl complexes, there
are a few examples where attack at the central carbon atom occurs. When CO and
NO in [CpMo(CO)(N0)(1] 3 -C 3 H 5 )]+ are replaced by 1J 5 -C 5 H 5 , the metal centre
becomes more electron-rich and the energy of the frontier LUMO is raised. The allyl
group may then behave more like an allyl anion than an allyl cation (Fig. 1 0.8). If
this is so, it is the central carbon atom which is most positive (or Ieast negative).
The three rules successfully correlate a large body of experimental data on the
site ofnucleophilic attack on cationic 18-electron complexes. They do not always
work; rule 3 in particular is not very well defined in relation to odd open polyenes.
Rule 2 seldom fails, but several exceptions to Rule 1 are now known. Rule 1. for
example breaks down in relation to the cation [Fe(IJ 5 -C6H7)(1J6-C6H6)] +. Sodium
borohydride and severa! carbon nucleophiles including KCN, NaCH(C02Me)2 and
PhCH2MgBr react with the cyclohexadienyl group ( odd open) in preference to the
benzene ligand (even closed). With borohydride the product is formed under
kinetic control and not as a result ofrearrangement via the predicted complex (IJ 5 -
C6H7)2Fe. Some reactions, in particular those which involve soft nucleophiles,
may not operate under charge control but rather are directed by the extent of
overlap between the frontier orbitals of the complex and those of the incoming
nucleophile. It has also been suggested that the preferred direction of attack from
the exo side remote from the metal centre is not caused mainly by steric effects as
*Diastereoisomers require two chiral centres. On coordination a monosubstituted alkene
H Nu
'-.._/
_c
loses its plane of symmetry. Alternatively the bonding can be viewed as Mo 1
'cH 2
showing a chiral centre at carbon. Racemic mixtures (RS) ofthe molybdenum cations were
used.
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