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

Complexes of arenes
methyl iodide. The cyclohexadiene ligands are released from the anionic
complexes by oxidation in air.
The reaction of a nucleophile with a coordinated ligand may be viewed either
as an addition (as above) or as a substitution at carbon. In principle mechanisms
ranging from dissociative (D or ld, SN1) to interchange associative (1 •• SN2) are
possible. Kinetic measurements on reactions of this type ( especially the formation
of phosphonium salts by phosphines) reveal the rate law, rate=
k,[complex][PR 3 ] for the forward reaction. This probably suggests a synchronous
I.(SN2) process with inversion ofstereochemistry at carbon. A mechanism of
this type involving slippage of the metal atom to o ne end of the double bond on
approach of a nucleophile has been proposed for alkene complexes, on the hasis of
molecular orbital calculations.
Nu
-- __ c-e,
' 1
-~ 1 1 ',,
M
10.4.1 Choice of organic ligand for nucleophilic attack (Green's rules)
Three rules ha ve been proposed, based on much experimental data, which enable
the preferred site of attack to be predicted in a cationic complex which contains
more than one organic ligand. It is assumed that the nucleophile seeks out the
position of highest positive charge, and that no subsequent rearrangement of the
products occurs. That is to say, the reactions are assumed to be under kinetic
control. This is not always true; while additions of carbanions are often
irreversible, phosphines, amines or alkoxides may add reversibly.
Rule 1: Nucleophilic attack occurs preferentially at even coordinated polyenes
(Le. 1] 2 , IJ\ 1] 6 ... ) rather than at odd coordinated polyenyls (i.e. 1] 3 , 1] 5 , 1] 7 ... )
Rule 2: Nucleophilic addition to open coordinated polyenes/polyenyls (e.g. 1] 5 -
pentadienyl, 1] 5 -cyclohexadienyl) is preferred to addition to closed (cyclically
conjugated, e.g. 1] 5 -cyclopentadienyl) polyenes/polyenyls.
Rule 3: For even open polyenes (e.g. 1] 4 -butadiene) nucleophilic attack at the
terminal carbon atom is always preferred, for odd open polyenyls attack at the
terminal carbon atom occurs only if the attached metal containing group is
strongly electron-withdrawing.
In brief Rule 1. even before odd
Rule 2. open before closed
Rule 3. even open attacked at ends
Rule 1 can be understood in terms of a simple molecular orbital description.
The HOMO in an even polyene contains an electron pair. On interaction with a
metal-centred frontier orbital some formal transfer of electrons from ligand to
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