Nickel Polymerization Catalysts with Ylide Steering Ligands - Late Transition Metal Polymerization Catalysis - page 4

Tab. 1.2 (C=C)

vertical ionization potentials of conjugated ylides IE 2 or IE 1 and related -

hydrocarbons (IE 1 ).

IE 2 or IE 1 (Ylide) a) [eV] IE 1 ( -Hydrocarbon) [eV]

Me 3 P=CH- CH=CH 2 9.02 (IE 2 ) Ethylene: 10.51

Me 3 P= C 5 H 4 6.82 (IE 1 ) ! Butadiene: 9.10

Me 3 P=CH- C 6 H 5 8.32 (IE 2 ) Benzene: 9.25

a) Ionization from (C=C) localized orbitals.

species with ylide ligands sometimes in bridging or in chelating function. In
Fig. 1.1, A represents the fundamental monodentate

-complex. Its metal-carbon
bond can be described simply as the result of a two electron/two orbital stabilizing
interaction between the high lying carbon-centered ylide HOMO “n C - ” and a vacant
metal orbital. For a given ylide, the degree of complex stabilization depends on the
energy of the interacting metal orbital(s), which can be tuned chemically with the
other ligands attached to the metal. In B and C hydrocarbon -systems are attached

to the ylidic carbon which not only results in a delocalization of electron density but
also raises the energy levels of the -substituents with respect to the unperturbed

parent -hydrocarbon, i.e., for the ligands in B and C relative to ethylene and cis- bu-

tadiene (Tab. 1.2). In the exceptional case of cyclopentadienylidenephosphoranes
(R 3 PC 5 H 4 ) the extreme perturbation of the -substituent pushes the corresponding

cis -butadiene level even slightly above that of the ylide bond n C - -level.
The enhanced energetic availability of the substituent -electrons makes it clear

why strongly conjugated ylides may act as polyhapto -ligands when metal local-

ized orbitals of appropriate symmetry are accessible. In extreme cases the location
of highest reactivity may shift into a position remote from the onium center. This
“activation” of a substituent, attached to the ylidic carbon, is a general feature and
is not limited to CC- -systems. An impressive example is the photoelectron spec-

trum of acetylmethylene-trimethylphosphorane, where the low second ionization
potential IE 2 =8.2 eV originates essentially from an oxygen lone pair orbital [Eqs.

4

* K.A. Ostoja Starzewski, H. Blau, W. Malisch,
unpublished results