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

Complexes of arenes
if"••d•
R"xo
Mn
LiR
H"xo
if"'""
Mn
(C0) 3
ATTACK
AT
RING
(C0) 3
-cojPR 3 /
hv
~R~­
~rk
H
~
,,Mn'\..
oc' 1 C=O
co 1
NHR
~
Mn(PR3)
(C0) 2
Mn
(C0) 3
ATTACK AT
CARBONYL
LIGAND
ATTACK AT
METAL
Fig. 10.4 Various sites for nucleophilic attack in (17 6 -C 6 H 6 )Mn(CO),.
While trialkyl(aryl)phosphines in the dark add to the ring of (C 6 H 6 )Mn(CO):,
in the presence of light substitution of a carbonyl ligand occurs, probably by
dissociative photolysis. A third possible site of attack is ata carbonylligand. This
occurs with hard nucleophiles such as amines and alkoxides and is reversible. The
reactivity of carbonyl groups correlates with the v(CO) stretching frequency. The
higher this frequency the less back donation into n* orbitals has occurred and
hence the greater the susceptibility to nucleophilic attack at carbon.
17 5 -Cyclohexadienyltricarbonylmanganese complexes are reconverted into the
17 6 -arene cation by the triphenylmethyl cation only if they have an exo-H
substituent. The bulky reagent is unable to remove an endo-H on account of steric
hindrance. Advantage may be taken, however, of a rearrangement which occurs,
probably via an 17 6 -arene(hydride) intermediate. The new areneMn(CO): cation
can now be attacked by a second nucleophile to give a mixture of disubstituted
cyclohexadienyl complexes.
322