Journal of Organometallic Chemistry 89 
Elsevier Sequoia SA., Lausanne - Printed in The Netherlands 
PHOTOELECTRON SPECTRA AND MOLECULAR PROPERTIES 
XV*. THE EFFECTS OF oc- AND f3-SIL YL SUBSTITUENTS ON 1t-SYSTEMS 
P. MOLLERE and H. BOCK 
I nstilll! for Anorganische Chemie II der U niversitiit, 6 Frankfurt} Main 70. Llldwig-Relm-Strasse 14 (Germany) 
G. BECKER and G. FRITZ 
Institutfor Anorganische Chemie der Universitiit. Karlsruhe (Germany) 
(Received May 19th. 1972) 
SUMMARY 
The photoelectron spectra of substituted ethylenes. H 2 C=CHX (X=H, CH3 , 
SiH3 • CH2CH3 • CH2SiH3 ) are discussed in the light of results of CNDO-type 
molecular orbital calculations. The characteristic shifts of the first bands are explained 
in terms of a qualitative model for the substituent effects of oc- and f3-silyl groups upon 
n-systems. 
INTRODUCTION 
The dichotomous donor-acceptor effects of silyl substituents upon n-systems 
has long been a topic of interest2·3 •4 •5 , and the ethylene moiety-the simplest of 
n-systems-is well suited to their investigation. Trimethylsilyl-substituted ethylenes 
have already been thoroughly investigated by various other techniques5a•f , and the 
photoelectron (PE) spectra of two such derivatives have been reported recently6. In 
order to eliminate extraneous influences and to facilitate the understanding of specific 
effects, the photoelectron spectra of five simpler ethylene derivatives have been ~tudied 
with the aid of CNDO/2 and modified CNDOj2 MO calculations. 
~::C=C::::~ (X=H, CH3 , SiH3 , CH2 CH3 , CH2 SiH3 ) 
It shouid be noted at this point, that PE spectroscopy simply allows the 
measurement of energy differences between electronic states, and it is only via 
Koopman's theorem that these energies can be associated with factorized molecular 
orbitals 7, The perturbation of the latter may be further partitioned into various effects 
(inductive, conjugative. etc.), and these in turn may exhibit some dependence on 
geometry (conformation). Although this simplification of the true molecular states is 
contrived, it does provide an instructive, if artificial, model for substituent effects, the 
component contributions of which may be derived from the proper comparisons 
within series of compounds. 
* For part XIV see ref. L 
J. Organometal. Chern., 46 (1972) 
90 P. MOLLERE. H. BOCK. G. BECKER. G. FRITZ 
PHOTOELECTRON SPECTRA 
The PE spectra of the five ethylene derivatives are given in Fig. 18. If all 
occupied valence basis set MO's were within the 6.00-21.21 eV range, the number of 
bands expected would be six (X=H), nine (X=CH3 , SiH3 ) and twelve (X=CH2CH3 , 
CH2 SiH3 ) respectively. However, those MO's consisting largely of carbon 2s and 
silicon 3s orbitals are generally beyond the range of the He(I) source; the situation 
here is further complicated by the overlapping of spectral bands and ambiguous 
intensity ratios. Nevertheless, the molecular orbitals of prime concern, namely the 
highest occupied 1t-Ievels, certainly correspond to the well isolated first peaks of the 
five spectra; these assignments are conHrmed for the three hydrocarbons by the fine 
., 
go 
., 
'" u 
., 
0. 
U 
U 
t x = H 
J V 
'\ 1\ w ~ lot 
.. f\... r-
I IX.= CH3 
~ \ 1/\ Jl ~ U IV ~ r-
I~ x = 51.H3 1/\ I I r.... U f\ (\r-... , ......... 
x = CH CHa 
8 10 12 14 16 18 20 
IE (eVI 
Fig. 1. Photoelectron spectra of five ethylene derivatives. H 2 C=CHX. 
J. Organornetal. Chern., 46 (1972) 
PHOTOELECTRON SPECTRA AND MOLECULAR PROPERTIES. XV 91 
TABLE 1 
ASSIGNMENT OF THE IONIZATION ENERGIES OF THE MONOSUBSTITUTED ETHYLENES 
BASED ON MOLECULAR ORBITAL CALCULATIONS" 
HzC==CHCH3 H2 C=CHSiH3 H2 C=CHCH2CH3 H2C=CHCHzSiH3 
Orbital IE Orbital IE Orbital IE Orbital IE 
type (eV) type (eV) type (eV) type (eV) 
2 a" 9.86 2 aU 10.37 3 a" 9.72 3 a" 9.49 
7 a' 12.22 70' 11.46 9a 11.75 90' 11.72 
6 a' 13.16 60' 12.24 8a 12.24 g a' 11.92 
5a' 14.40 1 a" 12.7 20" 12.52 2a" 12.74 
1 a" 14.7 5 a' 13.80 7a 13.05 70' 13.67 
4a' 15.94 4a' 15.22 6a 13.68 6a' 14.59 
3 a' 18.2 3d 17.15 Sa' 15:83 1 a"? 15.61 
1 a" 18.1 5 a' ? 17.2 
4a' 18.3 
a The symmetry assignments are for the point group C ... 
structure exhibited (X=H, V= 1230 (ref. 9); X=CH 3 , V= 1370; X=CH2 CH3 , V= 
1370 em-I). There being no other direct evidence, tentative assignments for the 
remaining PE bands have been made solely on the basis of the calculated orbital 
sequences. (Fig. 1 and Table L) 
DISCUSSION 
The initial band of each PE spectrum, unequivocally assignable to the highest 
occupied n-orbital, is seen to undergo characteristic shifts; the magnitudes of the flrst 
ionization energies (lEI) conform to the following sequence: X=CH2SiH3 < 
CH2CH3 < CH3 ~ SiR 3 < H. When treated properly (Vide infra), the results of the 
MO calculations reproduce this sequence surprisingly well. (Fig. 3.) 
In the case of the IX-substituted ethylenes, the introduction of a methyl or silyl 
group produces a destabilization of the occupied ethylenic n-orbital, which, in terms 
of substituent effects, arises from two factors-an inductive effect and a symmetry-
allowed antibonding admixture of the out-of-plane C-H and Si-H q-bonds (hyper-
conjugation). In view of the valence state ionization potentials (VSIP) (C 2s, 19.54 eV; 
C 2p, 11.20 eV; Si 3s, 14.76 eV; Si 3p, 7.90 eVpo, the inductive donor strength of the 
H 3Si group would be expected to exceed that of the H3C substituent. Likewise. the 
lower VSIP's of silicon should guarantee more effective a/n-mixingll in the silyl 
derivative. Nevertheless, the destabilization in vinylsilane is only a fraction of that in 
propylene. This observation can be explained in terms of the counter-influence of 
vacant Si orbitals of n-symmetry (3d and/or 4p) which afford a mechanism for de-
localizing and concomitantly stabilizing the ethylene x-electrons. This description 
is in keeping with the results of the calculations which are qualitatively depicted in 
Fig. 2. Moreover, corroborative evidence for the n-delocalization is provided by the 
spectra, which show that while the n-band of propylene displays well resolved flne 
structure, that of vinylsilane is broad and featureless. 
In the case of I-butene (X=CH2CH3 ) and allylsilane (X=CH2SiH3 ), the 
J. Organometal. Chem., 46 (1972) 
92 P. MOLLERE. H. BOCK, G. BECKER, G. FRITZ 
" n 'r X 
" . 
" I, 
:. I, 51 3d ! _,f if 
:: j ; ~~:; Ii / 
... :/#7 /~/4J·J .. I! ___ ~~ H-/ - H-! i~fL~-
, , "'-------' 
c-c I I 
f .~ 
I 
~___ I 
.\~J...J /A-if 
,~-----------~-------------~/ 
Fig. 2. Qualitative MO scheme for the perturbation of the ethylenic :rr-orbital by methyl and silyl substi-
tuents. 
situation is complicated by the significant differences in the n-orbital energies to be 
expected in the three distinctive conformations I, II, III. Weidner and Schweig6 , who 
considered only conformations II and III for allyltrimethylsilane, chose to interpret 
(I) (U) errn 
M=C 
Etotal 937.7699 937.8407 937.7~29 (eV) 
M=Si 
Etotal 855.6994 855.2013 655.0325 (eV) 
their results solely in terms of II on the basis of a calculated 3-5 kcaljmole difference 
in total energies. While conformation (I) may be :reasonably excluded in light of sterle 
considerations in the instance of the trimethylsilyl derivative, the calculated total 
energies of the simpler allylsilane and I-butene favour a free rotation model. The 
energies of the n-orbitals within such a model can be roughly approximated by the 
simple, statistically weighted average given in eqn. 1. 
( ) _ B",(I)+2s",(II)+s",(III) e", ave - 4 (1) 
J. Organometal. Chern., 46 (1972) 
PHOTOELECTRON SPECTRA AND MOLECULAR PROPERTIES. XV 93 
15.0 
14.0 
x = SiH3 • 
12.0 
11.0 
o CNDO/2 
• MOdified CNOo/2 
10.0 11.0 12.0 
Measured ionization energies 
Fig. 3. Correlation of calculated eigenvalues with measured ionization energies of the two highest occupied 
orbitals in the monosubstituted ethylenes. H 2 C=CHX. 
As indicated in Fig. 3, the results obtained by this method are in good agreement with 
the measured vertical ionization energies*. 
With the aid of the following simple diagrams, the influences governing the 
energies of the first n-orbitals may be assessed; 
* Figure 3 clearly illustrates one of the primary differences between the two methods of calculation: 
the reparametrization in the modified CNDOj2 technique tends to destabilize :n:-type orbitals. In addition, 
it should be noted that while the normal CNDO/2 results appear to provide better numerical correlations 
with the experimental data, they also reverse the order of the two highest occupied orbitals in vinyl- and 
allylsilane; the modified CNDOj2 calculations correctly predict this :n:/a-ordering, while the remaining 
orbital sequences are similar for both methods. 
J. Organometal. Chem~ 46 (1972) 
94 P. MOLLERE, H. BOCK, G. BECKER, G. FRITZ 
d/,-r - mixing 
~~~"? 
all) 
Participation of Si 3d orbitals 
L The major oin-mixing in conformations (I) and (III) involves the C-H bonds 
of the methylene unit, and is likely to provide comparable degrees of destabilization 
in I-butene and allylsilane. 
2. The (T/n-mixing in (II) consists primarily of the anti-bonding admixture of 
the CH2-M single bond (i.e., CH2-CH3 or CHz-SiH3 ) into the ethylenic n-bond, 
where the higher-energy carbon-silicon bond (AVSIP) should yield a considerably 
greater destabilizing contribution. 
3. Conformations (1) and (III) allow a certain amount of d-orbital participation 
in allylsilane, the effect of which should be to mitigate the destabilizing effects of 
(T/n-mixing. 
Not only is this interpretation consistent with the CNDOj2 calculations, but 
it is also supported by the PE spectra: despite the much larger destabilizing inductive 
and hyperconjugative effects to be anticipated in allylsilane on the basis of VSIP's, 
the highest occupied molecular orbital is only moderately destabilized relative to that 
in 1-butene; furthermore, the first band of the spectrum ofthe former is very broad 
and devoid of fine structure, just as was the case in the spectrum of vinylsilane. 
A distinct disadvantage inherent in monosubstituted ethylenes as a model 
system is their low symmetry: the only possible classification of orbitals is the dis-
tinction between n and (J (a" and a'in Cs ), and even this becomes unclear for I-butene 
and allylsilane in conformation II (e l ). The association of particular molecular 
orlJitals with specified units of the molecules becomes especially difficult (Fig. 4), and 
th~ applicability of a simple perturbation model is thereby severely limited. It is seen 
in Fig. 4, however, that the highest occupied (T-or:bital of each the 4 substituted ethyl-
enes contains major contributions from the specific valence bonds-the H 2CCH-C 
and H 2 CCH-Si bonds in propylene, 1-butene, and vinylsilane, and the CH2-Si bond 
in allylsiIane. Assuming that the second ionization energies (IE2 ) of the corresponding 
spectra are essentigtlly due to the expUlsions of electrons more or less localized in these 
bonds, they are then found to display an intelligible pattern. The second ionization 
energy of allylsilane is greater than that for vinylsilane, due to the stabilization of the 
carbon-silicon bond of the former stemming from its interaction with the 2pn orbitals 
of the ethylene moiety. In contrast, 1E2 in 1-butene is smaller than 1E2 in propylene 
since the highest occupied (T-orbital ofthe former corresponds mainly to a bond which 
cannot be directly involved in oin-mixing, but which is destabilized by the inductive 
effect of the terminal methyl group. The correlation of the calculated and measured 
J. Organomecal. Chern., 46 (1972) 
PHOTOELECTRON SPECTRA AND MOLECULAR PROPERTIES. XV 95 
second ionization energies (treated exactly as were the lEI values, cf. eqn. 1) is also 
given in Fig. 3. 
From Fig. 4, it can also be seen that a second n-orbital should occur within the 
PE spectra of the substituted ethylenes. The ionization energies of these MO's would 
be of interest, inasmuch as n-type back-bonding involving Si 3d and/or 4p orbitals 
could be expected to be minimized at deeper energy levels. However, the uncertainty 
involved in assigning the remaining portions of the spectra does not warrant their 
further consideration. 
L;t--i;}R /~-----!~ L-~n----&-----l-y L-l-----!:-----(ny 
/?--------? ~------n? ~----n-~--;r7 /-:~------F5V 
"",--~?f? /--\----5~ ~---y ~------?<--y 
/-~ /-l----i~ /-~------!:----+-y L-:!--n--~------t-y 
Fig. 4. Diagrams ofthe five highest occupied orbitals in the monosubstituted ethylenes, based on SCF-M 0 
calculations. 
In conclusion, it should be stated that the energy shifts of the first two bands 
of the PE spectra of the ethylene derivatives H 2 C=CHX (reproducible by CNDO/2 
calculations) allow the development of a qualitative perturbation model for substituent 
effects. The well known arguments concerning the donor-acceptor influences in 
IX-silyl derivatives of n-systems are once more substantiated. The properties of the 
p-silyl derivatives, which, have been variously explained in terms of inductive effectsS, 
o/n-mixingll, and 1,3-p-?dn interactions, appear to require consideration of all three 
for proper understanding. We emphasize again, however, that the separation of these 
several effects is only a model-a means for gaining some insight into the nature ofthe 
states whose energies are the only measureable quantities. 
EXPERIMENTAL 
Calculations 
SCF-MO calculations of two types were obtained for all molecules: CNDOj 
212 -13 and modified CNDOj2, the latter having been reparametrized according to 
the method of Jaffe and Del Bene 14. Both types of calculations included Si 3d orbitals 
in the basis set. 
J. Organometal. Chem., 46 (1972) 
96 P. MOLLERE. H. BOCK, G. BECKER. G. FRITZ 
Spectra 
The spectrum of ethylene was taken from the literatureS. All other spectra were 
recorded with a Perkin-Elmer PS-16, employing a 127" electrostatic deflection-type 
analyzer, and were calibrated ·with argon. The samples of vinyl- and allyl silane were 
introduced into the spectrometer without exposing them to air. 
Synthesis 
The preparations of vinyI- and allylsilane were carried out by esta hUshed 
methods 1 5. 
Vinylsilane was obtained in 90 % yield via the LiAIH4 reduction of vinyl-
trichlorosilane in n-butyl ether16• The reduction was followed by fractional conden-
sation, in which the product was collected by trapping ,vith a cold bath at - 136° . 
Purity was ascertained by molecular-weight determination (found: 58.5; calcd.: 58.1), 
as well as by NMR and IR spectroscopy! 7. 
Allyisilane was prepared via the synthesis and subsequent reduction of allyl-
trichlorosilane. Allylmagnesium chloride reacted in ether with a 2.5-fold excess of 
SiCl4 to give allyltrichlorosilane in 28% yield!8.!9. This was then reduced with 
LiAIH4 in n-butyI ether19 to form allyisilane, which was isolated by fractional 
condensation and collected at -116°. The product exhibited the correct NMR 
spectrum and molecular weight (found: 72.6; calcd.: 72.2). 
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J. Organometal. Chem~ 46 (1972)