Chemistry Reference
In-Depth Information
1
Photoelectron spectroscopic data place this class of isolable energy-rich com-
pounds on the very top of an energy scale of ligands that do not have a net
charge. The molecular property “IE
1
” is a quantitative description for the ener-
getic availability of an outermost valence electron and thus a prominent feature of
reactivity.
Within a molecular orbital approximation, the electron is ejected from the high-
est occupied molecular orbital (HOMO). Molecular orbital calculations at various
levels of sophistication describe the highest occupied MOs of most ylides as being
strongly localized on the ylidic carbon. Exceptions to this are found for example
in cyclopentadienide derivatives, where the orbital of corresponding symmetry is
the HOMO-1 (IE
2
). In terms of reactivity, the low first ionization potentials of
ylides reflect high oxidizability, high proton affinity, and basicity. UV photoelec-
tron spectra in conjunction with detailed molecular orbital calculations for each
individual ylide structure have made possible a rationalization of the different sub-
stituent and heteroatom effects.
Tab. 1.1
Gas-phase UV-PES vertical ionization potentials IE
1
(eV) of phosphorus and arsenic
ylides (n
c
-
) and related phosphines (n
p
).
a)
R=Me
R=Ph
R
3
PCHCHCHMe
n
c
-
6.02
5.95
R
3
PCHCHCHPh
n
C
-
6.20
R
3
PCHCHCH
2
n
C
-
6.20
R
3
PCHPh
n
C
-
6.19
6.01
R
3
PCHMe
n
C
-
6.15
R
3
PCH
2
n
C
-
6.81
6.62
R
3
PCHSiMe
3
n
C
-
6.81
6.71
R
3
PCH(SiMe
3
)
2
n
C
-
6.92
R
3
AsCH
2
n
C
-
6.72
R
3
AsCHSiMe
3
n
C
-
6.56
R
3
AsC(SiMe
3
)
2
n
C
-
6.66
R
3
PCp
C=C
/n
C
-
6.82/7.02
6.66/6.91
R
3
PC(CN)
2
n
C
-
7.63
R
3
P
n
P
8.60
7.80
a)
Abbreviations: Me=methyl, Ph=phenyl, Cp=cyclopentadienyl; “n
C
” designates ionization from an
orbital with largest coefficient on the ylide carbon; “n
P
” designates ionization from the phos-
phorus lone pair orbital.
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