Sequential Photosubstitution of Carbon Monoxide by (E)-Cyclooctene in Hexacarbonyltungsten: Structural Aspects, Multistep Photokinetics, and Quantum Yields

Article abstract of DOI:10.1021/om990300t

The photochemical conversion of W(CO)₆ (1) into a trans-W(CO)₄(η-olefin)₂ complex has been investigated using (E)-cyclooctene (eco) as a model olefin possessing extraordinary coordination properties. trans-W(CO)₄(η²-eco)₂ (4) is generated as an equimolar mixture of two diaetereoisomers (4a, S₄ symmetry; 4b, D₂ symmetry) which can be separated by fractional crystallization. The entire reaction sequence involves the intermediate formation of W(CO)₅(η₂-eco) (2) and cis-W(CO)₄(η₂-eco)₂ (3: two diastereoisomers, 3a and 3b, with apparent C_s and C₂ symmetry, respectively). Complexes 2 and 3, although difficult to isolate from the photochemical reaction mixture, are conveniently accessible via alternative thermal ligand exchange routes. The molecular structures of 2 and 4a in the crystal were determined by X-ray diffraction techniques. The olefin double bonds, with trans-orthogonal arrangement in 4a, are eclipsed to a OC-W-CO axis in either case. The course of the conversion of 1 into the olefin-substituted products was monitored by quantitative IR spectroscopy. Photokinetic equations developed for this study describe the concentrations of all four components as implicit functions of the amount of light absorbed by the system, of the quantum yields of the individual photoprocesses, and of the UV-vis absorbance coefficients of the compounds involved. Based on these functional relationships, the individual quantum yields at λ_exc = 365 nm (Φ₁₂ = 0.73, Φ₂₃ = 0.34, Φ₂₄ = 0.16, Φ₃₄ = 0.15) were evaluated from a series of experimental data sets by an iterative procedure which involves variation of the quantum yield input data until the best fit of the computed to the measured concentrations is achieved. Low-temperature matrix isolation techniques were employed to characterize the W(CO)₄(η²-eco) fragment (5) as a key intermediate in the photolysis of W(CO)₅(η²-eco) (2).

Full text of DOI:10.1021/om990300t

3278
Organometallics 1999, 18, 3278-3293
Sequ en tia l P h otosu bstitu tion of Ca r bon Mon oxid e by
(E)-Cycloocten e in Hexa ca r bon yltu n gsten : Str u ctu r a l
Asp ects, Mu ltistep P h otok in etics, a n d Qu a n tu m Yield s
Friedrich-Wilhelm Grevels,* J u¨rgen J acke, Werner E. Klotzbu¨cher,
Franz Mark,* Volker Skibbe, and Kurt Schaffner
Max-Planck-Institut fu¨r Strahlenchemie, Postfach 10 13 65,
D-45413 Mu¨lheim an der Ruhr, Germany
Klaus Angermund, Carl Kru¨ger, and Christian W. Lehmann
Max-Planck-Institut fu¨r Kohlenforschung, Kaiser-Wilhelm-Platz 1,
D-45470 Mu¨lheim an der Ruhr, Germany
Saim O¨ zkar*
Department of Chemistry, Middle East Technical University, 06531 Ankara, Turkey
Received April 23, 1999
The photochemical conversion of W(CO)6 (1) into a trans-W(CO)4(η2-olefin)2 complex has
been investigated using (E)-cyclooctene (eco) as a model olefin possessing extraordinary
coordination properties. trans-W(CO)4(η2-eco)2 (4) is generated as an equimolar mixture of
two diastereoisomers (4a , S4 symmetry; 4b, D2 symmetry) which can be separated by
fractional crystallization. The entire reaction sequence involves the intermediate formation
of W(CO)5(η2-eco) (2) and cis-W(CO)4(η2-eco)2 (3: two diastereoisomers, 3a and 3b , with
apparent Cs and C2 symmetry, respectively). Complexes 2 and 3, although difficult to isolate
from the photochemical reaction mixture, are conveniently accessible via alternative thermal
ligand exchange routes. The molecular structures of 2 and 4a in the crystal were determined
by X-ray diffraction techniques. The olefin double bonds, with trans-orthogonal arrangement
in 4a , are eclipsed to a OC-W-CO axis in either case. The course of the conversion of 1
into the olefin-substituted products was monitored by quantitative IR spectroscopy.
Photokinetic equations developed for this study describe the concentrations of all four
components as implicit functions of the amount of light absorbed by the system, of the
quantum yields of the individual photoprocesses, and of the UV-vis absorbance coefficients
of the compounds involved. Based on these functional relationships, the individual quantum
yields at λexc ) 365 nm (Φ12 ) 0.73, Φ23 ) 0.34, Φ24 ) 0.16, Φ34 ) 0.15) were evaluated from
a series of experimental data sets by an iterative procedure which involves variation of the
quantum yield input data until the best fit of the computed to the measured concentrations
is achieved. Low-temperature matrix isolation techniques were employed to characterize
the W(CO)4(η2-eco) fragment (5) as a key intermediate in the photolysis of W(CO)5(η2-eco)
(2).
In tr od u ction
cyclooalkenes,5-7 R,â-unsaturated esters,8 and tetracya-
noethene9 as the entering ligand. Upon continued ir-
radiation, most of the initially generated M(CO)5(η2-
olefin) complexes readily undergo secondary photo-
substitution with formation of the respective trans-
M(CO)4(η2-olefin)2 products. Apart from its synthetic
potential, this chemistry may also be relevant to the
carbonyltungsten-assisted isomerization of olefins.10,11
Olefin-substituted group 6 metal carbonyls are con-
veniently accessible from the parent M(CO)6 complexes
(M ) Cr, Mo, W) by means of photolytic CO detachment
in the presence of the appropriate olefin. This has been
demonstrated using ethene and linear alkenes,1-5
* To whom correspondence should be addressed. E-mail: grevels@
mpi-muelheim.mpg.de.
(1) Stolz, I. W.; Dobson, G. R.; Sheline, R. K. Inorg. Chem. 1963, 2,
1264-1267.
(6) Grevels, F.-W.; Skibbe, V. J . Chem. Soc., Chem. Commun. 1984,
681-683.
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7536-7537.
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E.; Byrn, S. R. J . Crystallogr. Spectrosc. Res. 1993, 23, 41-47.
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C. Z. Naturforsch. 1980, 35b, 1298-1309.
(3) Gregory, M. F.; J ackson, S. A.; Poliakoff, M.; Turner, J . J . J .
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J . J . J . Organomet. Chem. 1996, 509, 19-28.
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10.1021/om990300t CCC: $18.00 © 1999 American Chemical Society
Publication on Web 07/24/1999

Photosubstitution of CO by (E)-Cyclooctene
Organometallics, Vol. 18, No. 17, 1999 3279
The existence of cis-M(CO)4(η2-olefin)2 complexes and
their possible involvement in the above photochemistry,
along with the respective cis vacant M(CO)4(η2-olefin)
fragments, has been addressed in theoretical studies,12-14
and in photochemical experiments employing continu-
ous irradiation at low temperatures in solid and liquid
media,3,5,15-17 and pulsed photolysis in the gas phase.18,19
In this way, several cis-M(CO)4(η2-olefin)2 complexes
could indeed be detected and identified on the basis of
some typical ν(CO) bands in the IR spectra, but no
further characterization has been reported. The only
member of this elusive class of compounds so far found
to be sufficiently stable for isolation and comprehensive
characterization is the one with (E)-cyclooctene (eco) as
the olefin ligand, dealt with in this paper.20 This
cycloalkene is superior to other olefins with respect to
its coordination ability. The particular stability of
transition metal-eco bonds21,22 apparently arises from
substantial relief of ring strain upon complexation.23 It
also facilitates the study of the M(CO)5(η2-olefin)-type
complex,6,20 which commonly is, at best, moderately
stable. Taking advantage of this property, we are now
in a position to investigate the multistep photochemical
conversion of W(CO)6 (1) into a trans-W(CO)4(η2-olefin)2
product (4) in full detail, including the evaluation of
quantum yields for the individual steps in a complete
photokinetic scheme with consideration of all mutual
internal light filter effects.
Spectra were recorded on the following instruments: NMR,
Bruker ARX 250 (62.9 MHz for 13C), Bruker WH 270 (270.1
1
MHz for H, and 67.9 MHz for 13C), and Bruker AM 400 (400.1
MHz for 1H, and 100.6 MHz for 13C); IR, Perkin-Elmer 1600
and Bruker IFS 66 (operating with 0.5 cm-1 for quantitative
measurements); UV-vis, Bruins Instruments Omega 10.
Elemental analyses were performed by Mikroanalytisches
Laboratorium Dornis und Kolbe, Mu¨lheim an der Ruhr.
R ea gen t s. Analytical grade and deuterated solvents
(Merck), (Z)-cyclooctene (zco; Merck), (+)-(1-phenyl)ethylamine
(Fluka, chiraselect quality), and (E,E)-1,4-diphenyl-1,3-buta-
diene (Aldrich) were used as received.
(E)-Cyclooctene was prepared according to literature pro-
cedures25,26 which were modified for large-scale production. (-)-
(E)-Cyclooctene was separated from the racemic material via
a platinum complex containing (+)-(1-phenyl)ethylamine, in
analogy with a published procedure,27 and extracted into
n-hexane. The enantiomeric purity (g99.5%) was checked by
analytical GC (HP 5890 instrument, 30 m capillary column,
50 °C, 0.8 bar H2 carrier gas) using a chiral stationary phase
(Restek Rt-âDEXsa).
W(CO)6 (1; Merck) was recrystallized from n-hexane [IR (n-
hexane): ν(CO) 1983.3 cm-1 (ꢀ ) 73435 L‚mol-1‚cm-1)].
W(CO)5(η2-(Z)-cyclooctene)7,28 [pale yellow crystals, mp 90 °C
(dec). IR (n-hexane): ν(CO) 2078.8 (m), 1960.1 (s), and 1946.6
1
(s) cm-1. H NMR (toluene-d8): δ 3.91 (2 H), 2.24 (2 H), 1.59
(2 H), 1.23 (4 H), and 0.99 (4 H). 13C NMR (toluene-d8): δ 201.3
(s, COax), 197.4 (s, COeq), 88.0 (d, 1J CH ) 160 Hz; -CHd), 32.5
1
1
(t, J CH ) 126 Hz; -CH2-), 30.7 (t, J CH ) 129 Hz; -CH2-),
and 25.9 (t, J CH ) 125 Hz; -CH2-)], W(CO)5[η2-bis(trimeth-
1
ylsilyl)ethyne)],29,30 and W(CO)4(η4-(E,E)-1,4-diphenyl-1,3-buta-
diene)31 [orange crystals, purified by successive crystallization
from toluene and n-pentane. IR (n-pentane): ν(CO) 2044.3 (m),
1973.1 (m), 1947.7 (s), and 1931.6 (s) cm-1] were prepared
according to the published procedures.
Exp er im en ta l Section
Gen er a l P r oced u r es. All reactions and manipulations
were carried out under argon and in argon-saturated solvents,
unless otherwise noted. Photochemical reactions on prepara-
tive scale were carried out in a water-cooled immersion-well
apparatus24 (solidex glass, λ > 280 nm) equipped with a Philips
HPK 125-W high-pressure mercury lamp.
W(CO)5(η2-(E)-cycloocten e) (2). A solution of (E)-cyclo-
octene (1.70 mL, 13.1 mmol) in n-hexane (10 mL) was added
to solid W(CO)5(η2-(Z)-cyclooctene) (0.70 g, 1.61 mmol). The
solution was stirred in the dark at room temperature for about
10 days (4 h at 60 °C gives the same result) until the starting
material was no longer observed in the IR spectrum. Upon
reducing the volume to ca. 2 mL and cooling the solution to
-78 °C, pale yellow crystals of 2 precipitated (0.66 g, 94%
yield); mp 69 °C (dec). IR (n-hexane): ν(CO) 2079.5 (3595),
1999.2 (vw), 1966.1 (20840), 1954.3 (9995), and 1947.4 cm-1
(ꢀ ) 22140 L‚mol-1‚cm-1). UV-vis (n-hexane): λmax ) 214
(41170), 239 (72160), 268 (8090), 302 (8780), and 353 nm (ꢀ )
(10) (a) Wrighton, M.; Hammond, G. S.; Gray, H. B. J . Am. Chem.
Soc. 1970, 92, 6068-6070. (b) Wrighton, M.; Hammond, G. S.; Gray,
H. B. J . Am. Chem. Soc. 1971, 93, 3285-3287. (c) Wrighton, M.;
Hammond, G. S.; Gray, H. B. J . Organomet. Chem. 1974, 70, 283-
301.
(11) Szymanska-Buzar, T.; J aroszewski, M.; Wilgocki, M.; Zio´lkows-
ki, J . J . J . Mol. Catal. A 1996, 112, 203-210.
(12) Daniel, C.; Veillard, A. Inorg. Chem. 1989, 28, 1170-1173.
(13) Daniel, C.; Veillard, A. Nouv. J . Chim. 1986, 10, 83-90.
(14) Bachmann, C.; Demuynck, J .; Veillard, A. J . Am. Chem. Soc.
1978, 100, 2366-2369.
(15) Pope, K. R.; Wrighton, M. S. Inorg. Chem. 1985, 24, 2792-2796.
(16) Szymanska-Buzar, T.; J aroszewski, M.; Downs, A. J .; Greene,
T. M.; Morris, L. J . J . Organomet. Chem. 1997, 531, 207-216.
(17) Szymanska-Buzar, T.; Kern, K.; Stufkens, D. J . New J . Chem.
1998, 22, 1539-1544.
(18) Weiller, B. H.; Grant, E. R. J . Am. Chem. Soc. 1987, 109, 1252-
1253.
(19) Takeda, H.; J yo-o, M.; Ishikawa, Y.; Arai, S. J . Phys. Chem.
1995, 99, 4558-4565.
(20) Preliminary results have been communicated in: (a) Grevels,
F.-W.; J acke, J .; Klotzbu¨cher, W. E.; O¨ zkar, S.; Skibbe, V. Pure Appl.
Chem. 1988, 60, 1017-1024. (b) Grevels, F.-W. In Photoprocesses in
Transition Metal Complexes, Biosystems and other Molecules. Experi-
ments and Theory; Kochanski, E., Ed.; Kluwer: Dordrecht, 1992; pp
141-171.
(21) Angermund, K.; Grevels, F.-W.; Kru¨ger, C.; Skibbe, V. Angew.
Chem. 1984, 96, 911-913; Angew. Chem., Int. Ed. Engl. 1984, 23, 904-
905.
1
1340 L‚mol-1‚cm-1). H NMR (toluene-d8): δ 3.54 (2 H), 2.49
(2 H), 1.63 (4 H), 1.26 (2 H), 0.95 (2 H), 0.54 (2 H). 13C NMR
1
(toluene-d8): δ 201.71 (s, COax), 196.9 (s, COeq), 83.9 (d, J CH
1
) 160 Hz; -CHd), 41.3 (t, J CH ) 128 Hz; -CH2-), 37.7 (t,
1J CH ) 128 Hz; -CH2-), 28.5 (t, 1J CH ) 126 Hz; -CH2-). Anal.
Calcd for C13H14O5W (M ) 434.1): C, 35.97; H, 3.25; W, 42.35.
Found: C, 35.95; H, 3.50; W, 42.25.
cis-W(CO)4(η2-(E)-cycloocten e)2 (3). Meth od A. A solu-
tion of W(CO)5[η2-bis(trimethylsilyl)ethyne] (0.99 g, 2.0 mmol)
and excess (E)-cyclooctene (1.10 g, 10 mmol) in n-hexane (100
(25) Vedejs, E.; Snoble, K. A. J .; Fuchs, P. L. J . Org. Chem. 1973,
38, 1178-1183.
(26) Inoue, Y.; Tsuneishi, H.; Hakushi, T.; Tai, A. In Photochemical
Key Steps in Organic Synthesis, an Experimental Course Book; Mattay,
J ., Griesbeck, A., Eds.; VCH: Weinheim, 1994; p 207.
(27) Cope, A. C.; Ganellin, C. R.; J ohnson, H. W., J r.; Van Auken,
T. V.; Winkler, H. J . S. J . Am. Chem. Soc. 1963, 85, 3276-3279.
(28) Skibbe, V. Doctoral Dissertation; Universita¨t Duisburg/MPI fu¨r
Strahlenchemie, 1985.
(22) (a) Ball, R. G.; Kiel, G.-Y.; Takats, J .; Kru¨ger, C.; Raabe, E.;
Grevels, F.-W.; Moser, R. Organometallics 1987, 6, 2260-2261. (b)
Angermund, H.; Grevels, F.-W.; Moser, R.; Benn, R.; Kru¨ger, C.;
Roma˜o, M. J . Organometallics 1988, 7, 1994-2004. (c) Angermund,
H.; Bandyopadhyay, A. K.; Grevels, F.-W.; Mark, F. J . Am. Chem. Soc.
1989, 111, 4656-4661.
(23) Klassen, J . K.; Yang, G. K. Organometallics 1990, 9, 874-876.
(24) Grevels, F.-W.; Reuvers, J . G. A.; Takats, J . Inorg. Synth. 1986,
24, 176-180.
(29) J acke, J . Doctoral Dissertation; Universita¨t Duisburg/MPI fu¨r
Strahlenchemie, 1989.
(30) Grevels, F.-W.; J acke, J .; Kru¨ger, C. Manuscript in preparation.
(31) O¨ zkar, S.; Peynircioglu, N. B. Inorg. Chim. Acta 1986, 119, 127-
129.

3280 Organometallics, Vol. 18, No. 17, 1999
Grevels et al.
128 Hz; -CH2-), and 29.14 (t, J CH ) 124 Hz; -CH2-); (4b)
1
mL) was stirred for 24 h at ambient temperature in the dark.
After reducing the volume to 50 mL the solution was cooled
to dry ice temperature, whereupon yellow crystals of 3
precipitated (0.46 g, 45%); a second crop (0.17 g, 16%) was
obtained after reducing the volume of the mother liquor to 10
mL. The isolated samples are mixtures of two isomers, 3a
(major component) and 3b, as identified by 13C NMR spec-
troscopy. IR (n-hexane): ν(CO) ∼2037 (s), ∼1950 (vs), ∼1934
(vs), ∼1930 (m, sh), ∼1914 (w), ∼1894 (m) cm-1. UV-vis (n-
hexane): λmax ) 245 (42600), 288 (12900), 325 nm (sh, ꢀ ) 7300
1
1
δ 200.88 (s and d, J CW ) 123 Hz, CO), 58.05 (s and d, J CW
)
14 Hz, -CHd), 41.85 (-CH2-), 38.28 (-CH2-), and 29.16
(-CH2-). MS (the spectra of 4a and 4b are virtually identi-
cal): m/z ) 516 (M+), 488 and other fragment ions. Anal. Calcd
for C20H28O4W: C, 46.53; H, 5.47; W, 35.61. Found: (4a ) C,
46.78; H, 5.13; W, 35.84.
Qu a n tu m Yield s. Light absorption was measured by
means of a modified version of an electronically integrating
actinometry device,32 which compensates for incomplete ab-
sorption of light in the sample cell. The instrument was
calibrated by ferrioxalate actinometry.33 Irradiation at 365 nm
of 3.0 mL aliquots of stock solutions of complexes 1 (ca. 1.5
mM), 2 (ca. 1.5 mM), or 3 [ca. 1.3 mM, ratio 3a :3b ≈ 3:1,
generated by method B from W(CO)4(η4-(E,E)-1,4-diphenyl-1,3-
butadiene) and (E)-cyclooctene] in n-hexane containing (E)-
cyclooctene (g10-fold excess) was carried out at 25 ( 1 °C in
quartz cuvettes (d ) 1 cm) by using a Hanovia 1000 W Hg-
Xe lamp connected to a Schoeffel Instruments GM 252 grating
monochromator. The light intensity at 365 nm was on the
order of 2 × 10-6 einstein‚min-1 absorbed by the 3.0 mL
samples.
1
L‚mol-1‚cm-1). H NMR (benzene-d6): δ 3.47/3.42 (4 H), 2.56
(4 H), 1.61-1.78 (8 H), 1.03-1.44 (8H), 0.45-0.76 (4 H). 13C
NMR (toluene-d8, 253 K): (3a ) δ 206.83 (COeq), 201.55 (COax),
201.33 (COax), 81.63 (-CHd), 40.17 (-CH2-), 37.59 (-CH2-
), 28.60 (-CH2-); (3b) δ 208.28 (s, COeq), 204.95 (s, COax),
86.69 (-CHd), 39.68 (-CH2-), 37.33 (-CH2-), 28.43 (-CH2-
). Anal. Calcd for C20H28O4W (M ) 516.4): C, 46.53; H, 5.47;
W, 35.61. Found: C, 46.48; H, 5.60; W, 35.29.
Meth od B. A solution of W(CO)4(η4-(E,E)-1,4-diphenyl-1,3-
butadiene) (0.05 g, 0.1 mmol) and (E)-cyclooctene (0.07 mL,
0.5 mmol) in n-hexane (5 mL) was stirred at ambient temper-
ature in the dark, whereupon the solution changed its color
from orange to yellow. Quantitative conversion into 3 occurs
within 30 min, as monitored by IR spectroscopy. After removal
of (E,E)-1,4-diphenyl-1,3-butadiene, which precipitated upon
cooling the solution to -40 °C, yellow crystals of 3 (ratio 3a :
3b ≈ 1:1, by 13C NMR) were isolated at -78 °C.
Quantitative IR spectroscopy (Bruker IFS 66 spectrometer
operating with 0.5 cm-1 resolution; IR cell with CaF2 windows,
d ) 509 µm), with computer-assisted subtraction of spectra,
was employed to determine the concentrations of the involved
carbonyltungsten complexes on the basis of the ν(CO) absor-
bance data reported above. The study includes 10 individual
runs starting with 1 (with conversions up to 80%; cf. Figure
9),34 12 individual runs starting with 2 (with conversions up
to 70%; cf. Figure 8),34 and 8 individual runs starting with 3
(with conversions up to 50%; cf. Figure 4).34 The following UV-
vis molar absorbance data at 365 nm were used to account for
mutual internal light filtering. 1: ꢀ ) 305 L‚mol-1‚cm-1. 2: ꢀ
) 1010 L‚mol-1‚cm-1. 3: ꢀ ) 1540 L‚mol-1‚cm-1. 4: ꢀ ) 170
The analogous reaction of W(CO)4(η4-(E,E)-1,4-diphenyl-1,3-
butadiene) (1.5 mM) with (-)-(E)-cyclooctene (10-fold excess)
in n-hexane was monitored by means of quantitative IR
spectroscopy in order to record the ν(CO) pattern of cis-W(CO)4-
[η2-(-)-eco]2 (3b′′), which is one of the enantiomers of 3b:
2035.8 (4885), 1948.6 (13460), 1930.7 (2600), 1914.2 (2515),
and 1894.1 cm-1 (ꢀ ) 6505 L‚mol-1‚cm-1).
Computer-assisted subtraction of the 3b ′′ spectrum from
quantitative spectra of 3a +3b mixtures, generated from
W(CO)4(η4-(E,E)-1,4-diphenyl-1,3-butadiene) (1.5 mM) and ra-
cemic (E)-cyclooctene (10-fold excess), yielded the ν(CO) pat-
tern of 3a : 2037.3 (4930), 1950.0 (1760), 1933.4 (ꢀ ) 19315
L‚mol-1‚cm-1), and 1929 (sh) cm-1. The 3a :3b ratio in such
solutions was found to change with time from ca. 50:50 (initial
ratio) to ca. 75:25 (equilibrium ratio), while the total concen-
tration remained constant.
L‚mol-1‚cm-1
.
Low -Tem p er a tu r e Ma tr ix P h otoch em istr y. The equip-
ment has been described previously,35 including the devices
for controlling the temperature (10-12 K), flow of matrix gas
(1.5-2 mmol‚h-1), and metal complex deposition rate, the
setup for monochromatic and broad-band irradiation, and the
IR (Perkin-Elmer 580) and UV-vis (Perkin-Elmer 320) spec-
trometers. The evaporation temperature (29 °C) of complex 2
was adjusted (quartz microbalance) to achieve a guest/host
ratio of e1:1000.
tr a n s-W(CO)4(η2-(E)-cyclooct en e)2 (4). A solution of
W(CO)6 (1; 1.06 g, 3.00 mmol) and (E)-cyclooctene (2.8 mL, 22
mmol) in n-pentane (300 mL) was irradiated while a stream
of argon was passed through the solution. The irradiation was
continued until the IR bands of 1 and of the intermediate
products 2 and 3 had largely disappeared (2 h). The slightly
turbid solution was filtered and evaporated to dryness in
vacuo. The residue was an equimolar mixture of two isomeric
products, 4a and 4b, as recognized by 13C{1H} NMR spectros-
copy. Recrystallization from toluene (35 mL; dissolved at 20
°C, cooled to -25 °C) yielded colorless crystals of pure 4a (0.52
g, 34%; mp 192-195 °C, dec, sealed capillary). After reducing
to 15 mL the mother liquor was cooled to -78 °C, whereupon
colorless crystals of 4a +4b (70:30 ratio, by 13C{1H} NMR; 0.21
g, 14%) precipitated. After removal of toluene from the
remaining solution, the remnant was dissolved in n-hexane
and filtered over silica gel. Crystallization from n-hexane (15
mL; dissolved at 20 °C, cooled to -78 °C) yielded colorless
crystals of pure 4b (0.53 g, 34%; mp 176 °C, dec, sealed
capillary). IR (n-hexane; the spectra of 4a and 4b are virtually
identical): ν(CO) 1983.8 (vw) and 1949.9 cm-1 (ꢀ ) 34450
L‚mol-1‚cm-1). UV-vis (n-hexane, the spectra of 4a and 4b
are virtually identical): λmax ) 243 (53560) and 293 nm (ꢀ )
X-r a y Diffr a ction Str u ctu r e An a lyses. Crystals of 2 and
4a , recrystallized from n-pentane, were mounted in glass
capillaries under argon. Details of the X-ray crystal structure
analyses are given in Table 1. The structures were solved by
the heavy atom method (SHELXS-86).36 Refinement was by
full matrix least-squares methods (GFMLX highly modified
version of ORFLS),37 where the function ∑w(∆F)2 was mini-
mized with w ) 1/σ(F)2 and ∆F ) |Fo| - |Fc| for 4a and
SHELXL-97 where the function ∑w(Fo2 - Fc2)2 was minimized
for 2. H atom positions were found and included but con-
strained in the final refinement stage for both compounds.
(32) Amrein, W.; Gloor, J .; Schaffner, K. Chimia 1974, 28, 185-
188.
(33) (a) Hatchard, C. G.; Parker, C. A. Proc. R. Soc. London A 1956,
235, 518-536. (b) Murov, S. L. Handbook of Photochemistry; Dekker:
New York, 1973; p 119.
(34) The concentrations and the amount of absorbed light are
normalized to the initial concentration of the starting material 1
(Figure 9), 2 (Figure 8), or 3 (Figure 4) by setting c˜i ) ci/c01(or2)(or3) and
t
τ ) (1/c01(or2)(or3))∫ Qabs dt.
(35) (a) Gerha0rtz, W.; Grevels, F.-W.; Klotzbu¨cher, W. E. Organo-
metallics 1987, 6, 1850-1856. (b) Klotzbu¨cher, W. E. Cryogenics 1983,
23, 554-556.
1
2560 L‚mol-1‚cm-1). H NMR (toluene-d8): (4a ) δ 2.64 (4 H),
2.57 (4 H), 1.83 (8 H), 1.44 (4 H), and 0.99 (8 H). 13C/13C{1H}
NMR (toluene-d8): (4a ) δ 200.93 (s, CO), 57.76 (d, 1J CH ) 151
(36) Sheldrick, G. M. Acta Crystallogr. A 1990, 46, 467-473.
(37) Busing, W. R.; Martin, K. O.; Levy, H. A. ORFLS, Report ORNL-
TM-305; Oak Ridge National Laboratory: TN, 1962.
Hz; -CHd), 41.94 (t, 1J CH ) 125 Hz; -CH2-), 38.24 (t, 1J CH
)

Photosubstitution of CO by (E)-Cyclooctene
Organometallics, Vol. 18, No. 17, 1999 3281
Ta ble 1. Deta ils of th e X-r a y Cr ysta l Str u ctu r e An a lyses for th e Com p lexes 2 a n d 4a
2
4a
formula
mol wt, g‚mol-1
cryst color
cryst syst
space group [no.]
a, Å
C
13H14O5W
C20H28O4W
516.3
434.1
pale yellow
monoclinic
P21/c [14]
12.3481(8)
8.3768(5)
13.4821(9)
95.469(2)
1388.21(15)
4
2.077
83.33
0.71073
824
colorless
monoclinic
C2/c [15]
13.460(2)
14.382(2)
12.412(2)
122.50(1)
2026.6
b, Å
c, Å
â, deg
V, Å3
Z
4
D
calcd, g‚cm-3
1.69
µ, cm-1
58.4
Mo KR radiation, λ, Å
0.71069
1016
F(000), e
diffractometer
scan mode
Siemens Smart-CCD
ω
Enraf-Nonius CAD4
ω-2θ
[(sin θ)/λ]max, Å-1
T, K
0.70
0.70
100
293
cryst size, mm
abs corr
0.13 × 0.10 × 0.06
empirical
0.57, 0.93
12089 ((h, (k, +l)
4763
empirical
0.859, 1.404
5853 ((h, (k, +l)
2984
(min., max.)
no. of measd reflcns
no. of indep reflcns
no. of obsd reflcns (I > 2σ(I))
Rav
3610
2491
0.04
172
0.02
109
no. of refined params
R1
0.0355
0.024
wR2/ Rw [w ) 1/σ2(Fo)]
resid electron dens, e Å-3
0.0799
0.024
2.371
0.66
Ta ble 2. Selected Bon d Len gth s (Å) a n d An gles (d eg) for W(CO)5(η2-eco) (2) a n d tr a n s-W(CO)4(η2-eco)2 (4)
(Isom er 4a )
2
4a
Bond Lengthsa
W-C(1)
2.047(5)
2.069(5)
2.050(5)
2.031(4)
2.011(5)
1.141(6)
1.136(6)
1.134(6)
1.148(6)
1.150(6)
2.451(5)
2.425(4)
W-C(1)
W-C(2)
2.026(5)
2.049(4)
W-C(2)
W-C(3)
W-C(4)
W-C(5)
O(1)-C(1)
O(2)-C(2)
O(3)-C(3)
O(4)-C(4)
O(5)-C(5)
W-C(11)
W-C(12)
O(1)-C(1)
O(2)-C(2)
1.150(6)
1.134(6)
W-C(22)
2.328(3)
W-C(12)
C(22)-C(22)*
C(12)-C(12)*
2.327(3)
1.412(6)
1.424(5)
C(11)-C(12)
1.384(6)
Bond and Torsional Anglesa
C(12)-W-C(11)
33.0(2)
C(22)*-W-C(22)
35.3(1)
35.6(1)
123.7
123.4
86.1
C(12)*-W-C(12)
C(18)-C(11)-C(12)-C(13)
-131.4
C(13)*-C(12)*-C(12)-C(13)
C(23)-C(22)-C(22)*-C(23)*
C(22)*-C(22)-W-C(2)
C(12)*-C(12)-W-C(1)
C(22)*-C(22)-W-C(1)
C(2)-W-C(12)-C(12)*
Plane(1)b-Plane(2)b
C(2)-W-C(11)-C(12)
C(1)-W-C(11)-C(12)
C(4)-W-C(11)-C(12)
C(3)-W-C(11)-C(12)
92.5
-89.7
91.7
0.0
175.2
178.7
87.7
177.9(3)
176.8(3)
180.0
O(1)-C(1)-W
O(2)-C(2)-W
O(3)-C(3)-W
O(4)-C(4)-W
O(5)-C(5)-W
C(2)-W-C(1)
C(3)-W-C(4)
C(3)-W-C(2)
C(4)-W-C(2)
C(5)-W-C(2)
C(5)-W-C(3)
179.3(4)
177.6(4)
178.4(4)
178.1(4)
176.5(4)
177.0(2)
175.4(2)
88.3(2)
O(2)-C(2)-W
O(1)-C(1)-W
C(1)*-W-C(1)
C(2)*-W-C(2)
C(2)*-W-C(1)
C(2)-W-C(1)
176.6(1)
176.2(1)
91.2(2)
88.9(2)
92.8(2)
91.1(2)
90.1(2)
a
b
Esd’s are given in parentheses. Plane(1) ) W-C(12)-C(12)*, Plane(2) ) W-C(22)-C(22)*.
ORTEP drawings38 of the molecular structures are displayed
in Figure 1 (complex 4a ) and Figure 5 (complex 2), and selected
bond distances and angles are collectively listed in Table 2.
More comprehensive collections of structure data and complete
lists of atomic coordinates and thermal parameters are avail-
able as Supporting Information for 2 (Tables S-1, S-2, S-3) and
4a (Tables S-4, S-5, S-6). X-ray diffraction studies of W(CO)6
39
(38) J ohnson, C. K. ORTEP II, Report ORNL-5138; Oak Ridge
National Laboratory: TN, 1976.

3282 Organometallics, Vol. 18, No. 17, 1999
Grevels et al.
Sch em e 1
F igu r e 1. Structure of trans-W(CO)4(η2-eco)2 (isomer 4a ,
S4 symmetry) in the crystal.
and trans-W(CO)4(η2-zco)240 were carried out in connection with
the work described in this paper and, independently, by other
authors7,41 as well. Our results (bond lengths and angles,
atomic coordinates, and thermal parameters) are available as
Supporting Information for W(CO)6 (1) (Figure S-1; Tables S-7,
S-8, S-9) and trans-W(CO)4(η2-zco)2 (Figure S-2; Tables S-10,
S-11, S-12).
structure of the W(CO)4 skeleton, which implies that
in the presumed octahedral coordination geometry the
olefin ligands are in mutual trans-positions.
In general, bis-olefin complexes can exist in different
diastereoisomeric forms if the olefin is prochiral (two
enantiotopic sites) or chiral. Accordingly, two diastere-
oisomers of complex 4 are expected from the reaction of
1 with racemic (E)-cyclooctene. One of them, 4a , con-
tains both enantiomers of (E)-cyclooctene in the same
complex molecule, while the other one, 4b, is a racemate
of two enantiomeric complexes each carrying two mol-
ecules of the same (E)-cyclooctene enantiomer.
Resu lts a n d Discu ssion
The photochemical conversion of W(CO)6 (1) in the
presence of excess (E)-cyclooctene (eco) into trans-
W(CO)4(η2-eco)2 (4), monitored by IR spectroscopy (vide
infra), involves two intermediate products, which are
identified as W(CO)5(η2-eco) (2) and cis-W(CO)4(η2-eco)2
(3) (Scheme 1). Although difficult to isolate from the
photochemical reaction mixtures at any stage of the
conversion, compounds 2 and 3 are conveniently acces-
sible via alternative thermal ligand exchange routes.
tr a n s-Tetr a ca r bon ylbis(η2-(E)-cycloocten e)tu n g-
sten (4). Extended irradiation of 1 in the presence of
(E)-cyclooctene ultimately results in the nearly complete
conversion into trans-W(CO)4(η2-eco)2 (4) (Scheme 1).
The appearance of one single strong ν(CO) band in the
IR spectrum of 4 is indicative of a square-planar
(39) Crystal data of W(CO)6 (1): C6O6W, colorless crystals, molecular
weight 351.9 g‚mol-1, crystal size 0.13 × 0.29 × 0.36 mm, a ) 11.965-
(3) Å, b ) 11.392(4) Å, c ) 6.465(1) Å, V ) 881.3(3) Å3, T ) 293 K, dcalc
) 2.65 g‚cm-3, µ ) 133.81 cm-1, Z ) 4, orthorhombic, space group
Pnma (no. 62), Enraf-Nonius-CAD4 diffractometer, ω-2θ scan mode,
λ ) 0.71069 Å, empirical absorption correction (min. 0.777, max. 1.288),
10522 measured reflections ((h,(k,(l), [(sin θ)/λ]max 0.70 Å-1, 1322
independent and 1012 observed reflections [I g 2σ(I)], 67 refined
parameters, heavy atom method, R1 ) 0.027, Rw ) 0.035 [w ) 1/σ2-
(Fo)], residual electron density 1.69 e Å-3. The molecular structure in
the crystal is essentially octahedral, although slightly differing W-C
(2.020-2.055 Å) and C-O distances (1.14-1.16 Å) are found.
(40) Crystal data of trans-W(CO)4(η2-(Z)-cyclooctene)2: C20H28O4W,
colorless crystals, molecular weight 516.3 g‚mol-1, crystal size 0.15 ×
0.36 × 0.37 mm, a ) 7.504(1) Å, b ) 12.652(2) Å, c ) 21.445(5) Å, â )
95.48(1)°, V ) 2026.7(5) Å3, T ) 293 K, dcalc ) 1.69 g‚cm-3, µ ) 58.4
cm-1, Z ) 4, monoclinic, space group P21/c (no.14), Enraf-Nonius-CAD4
diffractometer, ω-2θ scan mode, λ ) 0.71069 Å, empirical absorption
correction (min. 0.931, max. 1.000), 4567 measured reflections
((h,+k,+l), [(sin θ)/λ]max 0.66 Å-1, 4567 independent and 3242 observed
reflections [I g 2σ(I)], 226 refined parameter, heavy atom method, H
atom positions were calculated, R1 ) 0.028, Rw ) 0.035 [w ) 1/σ2 (Fo)],
residual electron density 0.72 e Å-3. The molecular structure in the
crystal essentially shows a square-planar arrangement of the four
carbonyl ligands around the metal center [W-C distances: 2.041(5),
2.044(5), 2.031(6), and 2.066(6) Å] and trans-orthogonal orientation of
the CdC double bonds of the two olefin ligands [W-C distances: 2.351-
(6), 2.355(6), 2.360(5), and 2.360(6) Å] which are eclipsed to the
respective OC-W-CO axes.
The crude product is indeed a nearly equimolar
mixture of two isomers which were separated from each
other by fractional crystallization with a total yield on
the order of 80%. The two isomers give virtually
identical ν(CO) spectra, but are clearly distinguishable
by 13C NMR spectroscopy. However, a definite assign-
ment is not possible on this basis, since they both exhibit
the same number of resonances, namely, one line in the
carbonyl region and four signals for the (E)-cyclooctene
ligand.
X-ray crystallography reveals (Figure 1) that the less
soluble isomer has the structure 4a (S4 molecular
symmetry), which leaves structure 4b (D2 molecular
symmetry) for the other isomer. The trans-orthogonal
orientation of the two olefins, eclipsed to the respective
OC-W-CO axes of the square-planar W(CO)4 moiety,
is in accord with theoretical predictions made for the
14
related complex trans-Mo(CO)4(η2-ethene)2 and was
also observed for the analogous complex of (Z)-cy-
clooctene, trans-W(CO)4(η2-zco)2.7,40 In the latter com-
pound the W-C(olefin) distances are slightly longer, by
ca. 0.03 Å, than those in 4a (Table 2), thus underlining
the better coordination ability of (E)-cyclooctene.
(41) Heinemann, F.; Schmidt, H.; Peters, K.; Thiery, D. Z. Kristal-
logr. 1992, 198, 123-124. The structure of W(CO)6 (1) in the crystal
is essentially octahedral, although it shows slightly varying W-C
(2.018-2.032 Å) and C-O (1.130-1.158 Å) distances, as calculated
from the published atomic coordinates.

Photosubstitution of CO by (E)-Cyclooctene
Organometallics, Vol. 18, No. 17, 1999 3283
Sch em e 2
the number of bands and the variations, from sample
to sample, in the relative intensities indicate that the
spectra represent mixtures of two species.
By analogy with the trans-W(CO)4(η2-eco)2 complexes
4a and 4b, two different products can also be expected
for cis-W(CO)4(η2-eco)2 (3), viz., a meso-compound (3a )
of composition cis-W(CO)4[η2-(+)-eco][η2-(-)-eco] and a
racemate (3b ) of the two enantiomers cis-W(CO)4[η2-
(+)-eco]2 (3b′) and cis-W(CO)4[η2-(-)-eco]2 (3b′′).
Sch em e 3
In accordance with this notion, the 13C NMR spectra
of various samples of 3 showed indeed two sets of
signals. The two isomers are clearly distinguishable on
the basis of the number of carbonyl resonances: 3a , with
(virtual) Cs symmetry, exhibits three lines with 2:1:1
relative intensities, while two lines with equal intensi-
ties are observed for 3b with (virtual) C2 symmetry.
Two sets of four lines each are observed for the (E)-
cyclooctene ligands in the 13C{1H} NMR spectrum of the
3a +3b mixture, thus confirming the position of the
olefins in chemically equivalent environments and,
moreover, indicating that the rotation about the metal-
olefin bond axes in either of the two compounds is rapid
on the NMR time scale. The olefinic carbon atoms
exhibit a coordination shift42 to higher field, ∆δ ) 52.3
for 3a and 47.2 for 3b. This is much smaller than that
observed for the trans-W(CO)4(η2-eco)2 complexes 4a and
4b (∆δ ) 76.2 and 75.9, respectively). The difference of
ca. 25 ppm is similar to the values previously reported
for various other olefins in related pairs of compounds43
in which an olefin ligand is either trans-orthogonal to
a second olefin ligand or trans to a CO group. These
observations not only confirm the structural assignment
of cis-W(CO)4(η2-eco)2 (3a and 3b) but also further
substantiate the relevance of coordination shift data as
a general criterion in recognizing the position of an
olefinic unit relative to other ligands in octahedral
complexes.43
F igu r e 2. IR spectra of cis-W(CO)4(η2-eco)2 (3) in the ν-
(CO) region, recorded in n-hexane: (A) spectrum of a
mixture of 3a (meso form) and 3b (racemate); (B) spectrum
of cis-W(CO)4[η2-(-)eco]2 (enantiomer 3b′′) [2035.8 (A),
1948.6 (A and B, overlapping), 1930.7, 1914.2, and 1894.1
(B) cm-1] generated using pure (-)-(E)-cyclooctene; (C)
spectrum of 3a [2037.3, 1950.0, 1933.4, and 1929 (sh) cm-1
]
obtained from the spectrum of the 3a +3b mixture by
computer-assisted removal of the whole spectrum of 3b′′.
The vertical dashed line indicates where the strong absorp-
tion of trans-W(CO)4(η2-eco)2 (4) would appear (1949.9
cm-1).
cis-Tet r a ca r b on ylb is(η2-(E)-cyclooct en e)t u n g-
sten (3). Complex 3 is difficult to isolate from the
reaction mixtures photochemically generated according
to Scheme 1. However, analytically pure samples are
accessible via independent thermal routes involving
either the reaction of a W(CO)5(η2-alkyne) complex29
with (E)-cyclooctene (Scheme 2) or the displacement of
the diene in a W(CO)4(η4-diene) complex31 by (E)-
cyclooctene (Scheme 3).
The IR spectra of various samples of 3 exhibit up to
six discernible absorptions in the CO stretching vibra-
tional region. An illustrative spectrum, with frequencies
extending over a range of about 140 cm-1, is displayed
in Figure 2A. It resembles the type of ν(CO) pattern
previously reported for several in situ photogenerated
cis-W(CO)4(η2-alkene)2 complexes.3,5,15,16,18,19 However,
Despite the exceptional coordination ability of (E)-
cyclooctene, cis-W(CO)4(η2-eco)2 (3) decomposes slowly
in solution at ambient temperature, unless an excess
of free (E)-cyclooctene is present. The solid material can
nevertheless be handled at ambient temperature, al-
though cooling is recommended for long-term storage.
To find a sound basis for monitoring the concentration
of cis-W(CO)4-(η2-eco)2 (3) as an intermediate in the
photochemical formation of trans-W(CO)4(η2-eco)2 (4)
(Scheme 1) by means of quantitative IR spectroscopy,
(42) 13C NMR chemical shifts of free (E)-cyclooctene: δ ) 133.91,
36.03, 35.88, and 29.46 (in toluene-d8).
(43) Grevels, F.-W.; J acke, J .; Betz, P.; Kru¨ger, C.; Tsay, Y.-H.
Organometallics 1989, 8, 293-298.

3284 Organometallics, Vol. 18, No. 17, 1999
Grevels et al.
efforts were made to disentangle the overlapping ν(CO)
bands in the changeable IR spectra of 3 (Figure 2A) and
to identify the individual patterns associated with the
two isomers, 3a and 3b. However, none of the attempts
to separate these compounds by fractional crystalliza-
tion scored a lasting success, such that no reliable
quantitative IR spectra could be recorded. This is due
to the interconversion of 3a and 3b, albeit slow, in
solution at ambient temperature which, logically, in-
volves the exchange of coordinated (+)- and (-)-(E)-
cyclooctene with the respective other enantiomer of the
free olefin added to the solution for the purpose of
preventing the complex from decomposition (Scheme 4).
indicates a different influence of the olefin ligands on
the carbonyl groups. It seems plausible to attribute this
to a different mutual orientation of the CdC units,
enforced by some steric constraints arising from the
face-to-face arrangement of the two enantiomers of (E)-
cylooctene. Similar considerations seem applicable to
postulate a conformer of 3b which may be present in
low equilibrium concentration and, thus, would give rise
to the unassigned minor absorptions in Figure 2B.
Sch em e 4
Once the individual ν(CO) spectra of 3a and 3b were
available, solutions of 3 could be analyzed on a quan-
titative level. In this way it was found that the reaction
in Scheme 3 initially generates the two isomers in
equimolar amounts. Upon standing at ambient temper-
ature, the ratio gradually changes in favor of 3a and
approaches, after several hours, an equilibrium value
of 3:1.
It has been suggested5 that cis- and trans-M(CO)4-
(η2-alkene)2 complexes coexist in a thermal equilibrium
which favors the thermodynamically more stable trans
isomers. Our observations made with (E)-cyclooctene as
the olefin ligand do not support this suggestion, al-
though it cannot entirely be ruled out for other, less
strongly bound olefins. As a matter of fact, all purified
samples of cis- and trans-W(CO)4(η2-eco)2 (3 and 4) were
free of the respective other isomer, as proven by NMR
spectroscopy. Particularly worth emphasizing, the IR ν-
(CO) pattern of cis-W(CO)4[η2-(-)-eco]2 (Figure 2B)
remained virtually unchanged with time. An increase
in the absorption band at 1950 cm-1 at the expense of
the other spectral features, which would have been
indicative of the formation of trans-W(CO)4[η2-(-)-eco]2,
was not observed, even after many hours at ambient
temperature.
The obvious strategy to circumvent this problem is
to preclude the formation of the meso-compound 3a by
using only one enantiomer of (E)-cyclooctene in the
synthesis according to Scheme 3. Thus, W(CO)4(η4-(E,E)-
1,4-diphenyl-1,3-butadiene)31 was quantitatively con-
verted into cis-W(CO)4[η2-(-)-eco]2 (3b′′) by the reaction
with a 10-fold excess of (-)-(E)-cyclooctene (g99.5%
enantiomeric purity). The resulting IR spectrum exhib-
its three prominent absorptions in the CO stretching
vibrational region (Figure 2B), in close similarity with
the ν(CO) pattern [A1, A1 and B1 (overlapping), and B2]
of the related compounds M(CO)4(η2:2-diene) (M ) Mo,
W; diene ) 1,5-cyclooctadiene,44 norbornadiene45), where
the two CdC units naturally are in mutual cis position
and oriented parallel to each other. This latter struc-
tural feature is reasonably adapted to 3b, along with
the assignment of the ν(CO) modes [A, A and B
(overlapping), and B].
The isomerization of cis- to trans-W(CO)4(η2-eco)2 (3
f 4) is exclusively a photochemical process. Quantum
yield measurements were performed, in the presence
of a 10-fold excess of (E)-cyclooctene, by irradiating at
λ ) 365 nm into the long-wavelength tail end of the
electronic absorption spectrum of 3 (Figure 3). At this
wavelength, the electronic absorption of 4 (Figure 3) is
low (ꢀ ) 170 L‚mol-1‚cm-1) in comparison with that of
3 (ꢀ ) 1540 L‚mol-1‚cm-1) so that internal light filtering
can be ignored in the evaluation of the quantum yield
as long as the conversion is not too high. At up to 40-
50% conversion, plots of the concentrations of 3 and 4
versus the total amount of absorbed light give indeed
virtually straight lines (Figure 4), the slopes of which
represent the respective quantum yields.
It has to be noted that the irradiation of 3 not only
gives rise to the cis f trans isomerization with forma-
tion of 4 but also disturbs the equilibrium of the two
isomers of the cis complex, 3a h 3b. In other words,
complex 3 is not represented by a uniform ν(CO)
intensity pattern which could directly be used to deter-
mine its concentration in the reaction mixtures by
The ν(CO) pattern of the meso-compound 3a (Figure
2C) is readily obtained from the spectrum of 3a +3b
(Figure 2A) by computer-assisted removal of the spec-
trum associated with 3b. Again consistent with the
proposed cis-M(CO)4L2 structure, it shows three well-
resolved bands and a shoulder at the low-frequency side
of the strongest absorption. However, the distinctly
different shape of the spectrum, compared with 3b,
(44) Kayran, C.; Kozanoglu, F.; O¨ zkar, S.; Saldamli, S.; Tekkaya,
A.; Kreiter, C. G. Inorg. Chim. Acta 1999, 284, 229-236.
(45) Darensbourg, D. J .; Nelson, H. H., III; Murphy, M. A. J . Am.
Chem. Soc. 1977, 99, 896-903.

Photosubstitution of CO by (E)-Cyclooctene
Organometallics, Vol. 18, No. 17, 1999 3285
F igu r e 5. Structure of W(CO)5(η2-eco) (2) in the crystal.
the higher value as the cis f trans isomerization
quantum yield, Φ34 ) 0.15.
F igu r e 3. UV-vis absorption spectra of W(CO)6 (1),
W(CO)5(η2-eco) (2), cis-W(CO)4(η2-eco)2 (3), and trans-
W(CO)4(η2-eco)2 (4), recorded in n-hexane.
The reverse process, the trans f cis photoisomeriza-
tion of W(CO)4(η2-olefin)2, has recently been observed
in low-temperature media,16,17 but is negligible under
the conditions of our experiments. This is evident from
control experiments involving the irradiation of 4 in the
presence of excess (E)-cyclooctene at λ ) 365, which
indicate an upper limit for the quantum yield Φ43 on
the order of 0.01.
P en ta ca r bon yl(η2-(E)-cycloocten e)tu n gsten (2).
Thermal ligand exchange using W(CO)5(η2-zco)7,28 as a
source of the W(CO)5 unit is a convenient route to
complex 2 (Scheme 5), providing nearly quantitative
yield.
Sch em e 5
F igu r e 4. Photochemical conversion of cis-W(CO)4(η2-eco)2
(3, O) into trans-W(CO)4(η2-eco)2 (4, 4) upon irradiation (λ
) 365 nm) in n-hexane solution containing (E)-cyclooctene.
The slope of c˜3 ) f(τ) yields Φ34 ) 0.15. The concentrations
(c˜i) and the absorbed amount of light (τ) are normalized to
the initial concentration of the starting material (3).34
The X-ray structure analysis of 2 in essence shows
an octahedral coordination geometry (Figure 5, Table
2). The CdC bond of the (E)-cyclooctene ligand is
eclipsed to one of the two equatorial OC-W-CO axes
of the square-pyramidal W(CO)5 skeleton, as indicated
by the respective C(carbonyl)-W-C(11)-C(12) torsional
angles. This is in accord with the results of a theoretical
study of W(CO)5(η2-ethene),46 where the eclipsed orien-
tation of the CdC double bond was predicted to be
slightly favored over the staggered structure. A nearly
eclipsed structure was previously found for W(CO)5(η2-
zco).7,47 Comparison of the W-C(olefin) bond lengths in
the latter compound (2.49 and 2.51 Å)7 with those in 2
[W-C(11) ) 2.451(5) Å, W-C(12) ) 2.425(4) Å; Table
2] again underlines that (E)-cyclooctene is indeed sig-
nificantly stronger bound than (Z)-cyclooctene.
It is interesting to compare the W-C bond distances
to the olefinic ligands and the CdC bond lengths in 2
and 4a (Table 2). The W-C bond distances to the
olefinic carbons in 2 [W-C(11) ) 2.451(5) and W-C(12)
) 2.425(4) Å] are distinctly longer than those in 4a
[W-C(12) ) W-C(12*) ) 2.327(3), W-C(22) ) W-C(22*)
) 2.328(3) Å], while the CdC bond lengths show the
opposite trend [2, C(11)-C(12) ) 1.384(6) Å; 4a , C(12)-
means of quantitative IR spectroscopy. Rather, the
concentration of 3 has to be evaluated as the sum of
the individual concentrations of the two isomers. In
detail, the analysis involves the determination of c3b and
the computer-assisted removal of the entire ν(CO)
spectrum of 3b (cf. Figure 2B) on the basis of the
absorbance at 1894.1 cm-1, followed by the determina-
tion of c3a and the computer-assisted removal of the
entire ν(CO) spectrum of 3a (cf. Figure 2C) on the basis
of the absorbances at 2037.3 and 1933.4 cm-1, leaving
behind the strong absorption of 4 at 1949.9 cm-1 as a
basis for the determination of c4. The latter absorption
nearly coincides with one band each of 3a (1950.0 cm-1
)
and 3b (1948.6 cm-1), so that the procedure is prone to
experimental errors, and a high level of accuracy is
required in order to achieve reliable results. Moreover,
the system is very sensitive to traces of oxygen, which,
particularly under photolytic conditions, gives rise to
oxidative decomposition with loss of material. Therefore,
rigorous degassing is indispensable in order to achieve
a satisfactory material balance.
(46) (a) Pidun, U.; Frenking, G. Organometallics 1995, 14, 5325-
5336. (b) Pidun, U. Diploma Thesis; Universita¨t Marburg, 1995.
(47) In the discussion concerning the structure of W(CO)5(η2-(Z)-
cyclooctene),7 the CdC double bond was mentioned to be staggered
with the carbonyl ligands. However, the calculation of the four
C(olefin)-C(olefin)-W-C(equatorial carbonyl) torsional angles (170.9°,
81.8°, -8.0°, and -97.3°) from the published atomic coordinates reveals
that the CdC bond actually is not far from being eclipsed to one of the
two OC-W-CO axes and perpendicular to the other.
In view of all these possible sources of error it is not
surprising that the quantum yields for the disappear-
ance of 3 and the formation of 4 were somewhat found
to scatter: Φ-3 ) 0.15 and Φ+4 ) 0.12. Because of the
slightly deficitary material balance (92-94%), we take

3286 Organometallics, Vol. 18, No. 17, 1999
Grevels et al.
F igu r e 6. IR spectrum of W(CO)5(η2-eco) (2) in the ν(CO)
region [2079.5 (A), 1992.2 (A), 1966.1 (B), 1954.3 (A), 1947.4
(B) cm-1], recorded in n-hexane.
C(12*) ) 1.424(5) Å and C(22)-C(22*) ) 1.412(6) Å].
This indicates a considerable strengthening of the
metal-olefin bond on passing from 2 to 4a , in accord
with the notion of an enhanced metal d(π) f olefin(π*)
back-donation arising from the single-faced nature of
the olefinic ligands and their trans-orthogonal arrange-
ment in 4a . In complex 2, the CO group trans to the
olefin effectively competes with the olefin for π back-
donation and thus renders the metal-olefin bond
weaker than in 4a .
The W(CO)5 skeleton of 2 in the crystal is somewhat
distorted from the C2 molecular symmetry of the com-
plex. The four equatorial CO ligands are not pairwise
equivalent, but differ by ca. 0.02 Å in each pair [W-C(1)/
C(2) ) 2.047(5)/2.069(5), W-C(3)/C(4) ) 2.050(5)/2.031-
(4) Å]. Similar distortions were found for the parent
W(CO)6 (1) in the crystal,39,41 thus illustrating that
crystal packing effects may be difficult to separate from
subtle, but chemically relevant variations in the M-CO
bonding situation.
The IR spectrum of 2 exhibits four prominent bands
in the CO stretching vibrational region (Figure 6)
instead of the typical three-band pattern commonly
observed for M(CO)5L complexes with (local) C4v sym-
metry [3 IR active ν(CO) modes: 2 A1, E].48 Obviously,
the degeneracy of the E mode is lifted, due to the single-
face π-acceptor character of the olefin ligand. At C2
symmetry, all five ν(CO) modes [3 A, 2 B] are IR active.
However, one has to keep in mind that one of the three
A modes corresponds to the IR inactive B2 mode of the
undistorted fragment and, therefore, is intrinsically
weak in intensity. The two strongest bands in the
spectrum apparently correspond to the two components
of the very intense E mode at C4v symmetry and thus
are assigned as the B modes of 2. The knowledge of the
CO force constants would be very helpful with respect
to a better understanding of the directional influence
of the olefin ligand on the individual CO groups of the
W(CO)5 moiety. However, the evaluation of such data
requires complementary frequencies of 13CO-labeled
derivatives,49 since a rigorous treatment of the W(CO)5
unit with C2 symmetry within the framework of the
F igu r e 7. IR Spectra in the ν(CO) region from a low-
temperature matrix experiment with W(CO)5(η2-eco) (2) in
solid argon at 10 K: (A) after deposition [2084.9, 1972.2,
1961.0, 1951.8 cm-1]; (B) after 30 min irradiation (λ ) 334
nm); (C) difference spectrum [spectrum B - (0.32 ×
spectrum A)], indicating the formation of the fragments
W(CO)5 [O; 1961.5 and 1928 cm-1] and W(CO)4(η2-eco) [9;
2052.5, 1951.5, and 1894 cm-1].
energy-factored CO force field approximation,50-52 in-
volves a total number of nine parameters (three prin-
cipal and six interaction force constants).
The 13C NMR spectrum of 2 shows, in addition to the
resonances associated with the (E)-cyclooctene ligand,
two lines in the carbonyl region in an approximately
4:1 ratio. The appearance of only one signal for the four
equatorial CO groups, even at temperatures down to 193
K, indicates that the molecule is highly fluxional with
respect to rotation about the metal-olefin bond axis.
The coordination shift to higher field of the olefin carbon
atoms,42 ∆δ ) 50.0, is in accord with the value expected
for (E)-cyclooctene in trans position to a CO group (cf.
the corresponding data of 3).
The photochemistry of W(CO)5(η2-eco) (2) involves two
primary processes, loss of CO and detachment of the
olefin ligand, as monitored by IR spectroscopy under
low-temperature matrix isolation conditions (Scheme 6).
Sch em e 6
The CO stretching vibrational pattern of 2, deposited
in argon at 10 K, is displayed in Figure 7A. Upon
irradiation at λ ) 334 nm (Figure 7B), the high-
frequency A band (2084.9 cm-1) and the two B bands
(1972.2 and 1951.8 cm-1) of 2 gradually decrease, while
three new bands grow in at 2052.5, 1928, and 1894,
(48) Braterman, P. S. Metal Carbonyl Spectra; Academic Press:
London, 1975.
(49) A comparative investigation into the energy-factored CO force
field of W(CO)6 (1), W(CO)5(η2-eco) (2), and trans-W(CO)4(η2-eco)2 (4),
including the syntheses of 13CO-labeled derivatives, is in progress and
will be communicated elsewhere.
(50) Cotton, F. A.; Kraihanzel, C. S. J . Am. Chem. Soc. 1962, 84,
4432-4438.
(51) Haas, H.; Sheline, R. K. J . Chem. Phys. 1967, 47, 2996-3021.
(52) Bor, G. Inorg. Chim. Acta 1967, 1, 81-92.

Photosubstitution of CO by (E)-Cyclooctene
Organometallics, Vol. 18, No. 17, 1999 3287
along with a weak feature at 2138 cm-1, which is readily
assigned to free CO. The absorption at the position of
the low-frequency A vibration (1961 cm-1) of 2 main-
tains nearly its initial intensity, which indicates that
an additional product absorption band emerges at
almost the same frequency. This becomes more obvious
when the residual absorptions of the starting material
2 are removed by computer-assisted subtraction (Figure
7C). Comparison with literature data53 identifies the
bands at 1928 and 1961.5 cm-1 as the low-frequency
A1 and E modes of the W(CO)5 fragment, respectively.
The other three product bands at 2052.5, 1951.5, and
1894 cm-1 are logically attributed to the CO loss
product, the W(CO)4(η2-eco) fragment (5). Subsequent
long-wavelength irradiation (λ g 400 nm), i.e., into a
broad band emerging in the electronic spectrum at
around 420 nm, resulted in partial reformation of 2 at
the expense of both of the two fragments, thus indicat-
ing that both of them absorb in this spectral region and
undergo photoinduced re-coordination of the respective
photodissociated ligand from the matrix cage environ-
ment.
and, therefore, is accounted for in Scheme 7. Any reverse
process involving replacement of (E)-cyclooctene by CO
can be left out of consideration under the conditions
employed, i.e., in the presence of a large excess of the
olefin under inert atmosphere.
As shown in the Appendix, the concentrations of the
starting material and the products in such a sequence
of photoreactions can be obtained as implicit functions
of the total amount of light absorbed by the system, the
quantum yields of the individual photoprocesses, and
the UV-vis molar absorbance coefficients of the com-
pounds involved. The functional relationships derived
in the Appendix (eqs A-12, A-8, A-10, and A-11) refer
to a four-component system (A, B, C, and D) involving
six photoprocesses with the quantum yields ΦAB, ΦAC
,
ΦAD, ΦBC, ΦBD, and ΦCD, but are readily adapted to the
special case given in Scheme 7 by setting ΦAB ) Φ12,
ΦAC ) ΦAD ) 0, ΦBC ) Φ23, ΦBD ) Φ24, and ΦCD ) Φ34.
Furthermore, for convenience the concentrations (ci) of
the four compounds and the total amount of light
absorbed by the system are normalized to the initial
concentration (c0) of the starting material by setting ci/
1
The observed ν(CO) pattern of 5 is consistent with a
“seesaw”-type arrangement of the carbonyl ligands in
the W(CO)4 unit48 rather than a square-planar W(CO)4
skeleton. In the latter case, only one strong ν(CO)
absorption (or a closely spaced pair of bands) would be
expected, as long as the W(CO)4 moiety is not too much
distorted from planarity.
c0 ) c˜i and (1/c0) ∫ Qabs dt ) τ. With these substitutions
t
1
1
0
one obtains the functional relationships of eqs 1-4.
â
γ
p1[1 - c˜1] + p2[1 - c˜1 ] + p3[1 - c˜1 ] -
ꢀ4 ln c˜1 ) ꢀ1Φ12τ (1)
c˜2
c˜1
) q[c˜1(â - 1) - 1]
(2)
c˜3
c˜1
(γ - 1)
) r1 + r2c˜1(â - 1) + r3c˜1
(3)
(4)
c˜4 ) 1 - c˜1 - c˜2 - c˜3
ꢀ2(Φ23 + Φ24)
ꢀ3Φ34
P h otok in etics a n d Qu a n tu m Yield s. The overall
process of the formation of trans-W(CO)4(η2-eco)2 (4)
from W(CO)6 (1) and excess (E)-cyclooctene essentially
involves three consecutive photochemical steps (Scheme
7).
where â )
γ )
ꢀ1Φ12
ꢀ1Φ12
(ꢀ2 - ꢀ4)
p1 ) (ꢀ1 - ꢀ4) -
+
(1 - â)
(ꢀ3 - ꢀ4)
Φ23
â
Sch em e 7
(1 - γ) (1 - â) (Φ23 + Φ24)
(ꢀ2 - ꢀ4)
(ꢀ3 - ꢀ4) Φ23
p2 )
+
â(1 - â) (1 - â)(γ - â) (Φ23 + Φ24)
(ꢀ3 - ꢀ4) Φ23
â
p3 )
q )
γ(1 - γ) (â - γ) (Φ23 + Φ24)
1
(1 - â)
Φ23
1
â
r1 )
r2 )
r3 )
In contrast to the gas-phase photolysis of W(CO)6 (1)
in the presence of ethene with formation of W(CO)4(η2-
ethene)2,19 there is no indication of any direct conversion
of 1 into either of the two disubstituted products 3 and
4 in our solution experiments. However, the direct
photoproduction of 4 from 2 cannot a priori be excluded
(1 - γ) (1 - â) (Φ23 + Φ24)
Φ23
â
(1 - â)(γ - â) (Φ23 + Φ24)
Φ23
1
â
(1 - γ) (â - γ) (Φ23 + Φ24)
(53) (a) Graham, M. A.; Poliakoff, M.; Turner, J . J . J . Chem. Soc.
(A) 1971, 2939-2948. (b) Perutz, R. N.; Turner, J . J . Inorg. Chem. 1975,
14, 262-270. (c) Perutz, R. N.; Turner, J . J . J . Am. Chem. Soc. 1975,
97, 4791-4800.
These equations can be numerically solved for the
concentrations. Using a given set of quantum yields and

3288 Organometallics, Vol. 18, No. 17, 1999
Grevels et al.
F igu r e 8. Photochemical conversion of W(CO)5(η2-eco) (2,
0) into cis-W(CO)4(η2-eco)2 (3, O) and trans-W(CO)4(η2-eco)2
(4, 4) upon irradiation (λ ) 365 nm) in the presence of
excess (E)-cyclooctene in n-hexane solution. The symbols
indicate the experimental data points, while the curves
represent c˜i ) f(τ) computed with Φ23 ) 0.34, Φ24 ) 0.16,
Φ34 ) 0.15, and the ꢀ values of the involved complexes. The
concentrations (c˜i) and the absorbed amount of light (τ) are
normalized to the initial concentration of the starting
material (2).34
F igu r e 9. Photochemical conversion of W(CO)6 (1, ]) into
W(CO)5(η2-eco) (2, 0), cis-W(CO)4(η2-eco)2 (3, O), and trans-
W(CO)4(η2-eco)2 (4, 4) upon irradiation (λ ) 365 nm) in the
presence of excess (E)-cyclooctene in n-hexane solution. The
symbols indicate the experimental data points, while the
curves represent c˜i ) f(τ) computed with Φ12 ) 0.73, Φ23
)
0.34, Φ24 ) 0.16, Φ34 ) 0.15, and the ꢀ values of the
involved complexes. The concentrations (c˜i) and the ab-
sorbed amount of light (τ) are normalized to the initial
concentration of the starting material (1).34
t
0
the starting material 2 (ci/c0 ) c˜i, and (1/c0)∫ Qabs dt )
molar absorbance coefficients, the concentrations can
thus be computed as functions of the absorbed amount
of light. With these functional relationships the quan-
tum yields can be evaluated in an iterative way on the
basis of a sufficiently large number of experimental data
as follows.
In two series of experiments W(CO)6 (1) and W(CO)5-
(η2-eco) (2), respectively, were irradiated in the presence
of a 10-fold excess of (E)-cyclooctene for varying periods
of time. In each individual run the total amount of light
absorbed by the system was measured with an elec-
tronically integrating actinometer, while the concentra-
tions of the starting materials and products were
determined by quantitative IR spectroscopy. UV-vis
spectroscopy is clearly not suitable for this purpose
because of the broadness and the substantial overlap
of the absorption bands throughout the entire spectral
region (Figure 3).
In principle, the quantum yields of all four processes
involved in the overall reaction (Scheme 7) can be
determined from the experiments starting with W(CO)6
(1). However, for the sake of accuracy the quantum
yields Φ23 and Φ24 were separately determined from the
irradiation of W(CO)5(η2-eco) (2) in the presence of a 10-
fold excess of (E)-cyclooctene, taking Φ34 ) 0.15 as a
fixed value known already from the cis f trans photoi-
somerization of W(CO)4(η2-eco)2 (3 f 4, vide supra).
The gradual conversion of 2 into 3 and 4 upon
irradiation with λ ) 365 nm is displayed in Figure 8. It
includes the results of 12 individual runs with a
conversion of the starting material up to 70% and a
satisfactory material balance throughout (g96%). The
concentration of the intermediate product 3 reaches a
maximum of ca. 33% of the entire reaction mixture. The
quantum yields Φ23 and Φ24 were evaluated on the basis
of eqs 5-7, which are adapted from the Appendix (eqs
A13-A15) by setting ΦAB ) Φ23, ΦAC ) Φ24, ΦBC ) Φ34
()0.15) and normalizing the concentrations and the
absorbed amount of light to the initial concentration of
2
2
τ).
â
u1[1 - c˜2] + u2[1 - c˜2 ] - ꢀ4 ln c˜2 ) ꢀ2(Φ23 + Φ24)τ
(5)
c˜3
c˜2
) q[c˜2(â - 1) - 1]
(6)
(7)
c˜4 ) 1 - c˜2 - c˜3
ꢀ3Φ34
where â )
ꢀ2(Φ23 + Φ24)
(ꢀ3 - ꢀ4)
Φ23
u1 ) (ꢀ2 - ꢀ4) -
(1 - â) (Φ23 + Φ24)
(ꢀ3 - ꢀ4)
u2 )
Φ23
â(1 - â) (Φ23 + Φ24)
Φ23
(1 - â) (Φ23 + Φ24)
1
q )
Starting with a reasonable guess for the quantum
yields Φ23 and Φ24, the concentrations (c˜2, c˜3, c˜4) were
computed as functions of the amount of absorbed light
(τ). In an iterative procedure54 the quantum yield input
data were varied until the best fit of the computed
concentrations to the measured concentrations was
achieved (Figure 8): Φ23 ) 0.34 and Φ24 ) 0.16.
These quantum yields were used as fixed input values
in the analogous treatment of the experimental data
obtained from the irradiation of W(CO)6 (1) under the
same conditions (λ ) 365 nm, 10-fold excess of (E)-
cyclooctene), yielding Φ12 ) 0.73. The combined results
of 10 individual runs are displayed in Figure 9. Again,
(54) The Fortran computer program is available on request from
one of the authors (F.M.).

Photosubstitution of CO by (E)-Cyclooctene
Organometallics, Vol. 18, No. 17, 1999 3289
F igu r e 10. Qualitative d-level diagrams for W(CO)6, W(CO)5(η2-olefin), cis-W(CO)4(η2-olefin)2, and trans-W(CO)4(η2-olefin)2.
Sch em e 8
the material balance was satisfactory throughout (g90%)
up to the highest conversion of the starting material (ca.
80%). The solid curves in Figure 9 represent the
concentrations computed on the basis of the functional
relationships, eqs 1-4, using the complete set of four
quantum yields. The intermediate products 2 and 3 did
not accumulate to concentrations higher than ca. 20%.
Clearly, the larger ꢀ values at the excitation wavelength
(365 nm) of 2 (1010 L‚mol-1‚cm-1) and 3 (1540 L‚
mol-1‚cm-1), compared with those of 1 (305 L‚mol-1‚cm-1
)
introduction of a second olefin in cis position to the first
one, pushing the b2 above the a1 level, as sketched out
in a qualitative molecular orbital diagram of W(CO)4-
(η2-ethene)2.19 In either case, a low-energy ligand field
transition is plausible, in accord with the appearance
of the shoulder around 390-400 nm in the absorption
spectra of 2 and 3 (Figure 3),60 which is responsible for
the (pale) yellow color of these two complexes. The
situation is different when the second olefin is oriented
trans-orthogonal to the first one. In this case, two
d-levels, each interacting with one of the two olefin(π*)
orbitals, are equally stabilized, probably down to almost
the same energy as that in W(CO)6 (1). On the basis of
this consideration, the colorless appearance of the trans-
W(CO)4(η2-olefin)2 complexes is readily understood.
The quantum yield Φ12 ) 0.73 for the conversion of
W(CO)6 (1) into the monosubstituted olefin complex 2
is in good agreement with the previously reported value
for the analogous photosubstitution of CO by pyridine
as the incoming ligand (Φ ) 0.78 ( 0.05, at λ ) 366
nm).56b Hence, it seems that these data essentially
represent the efficiency of the photolytic generation of
the W(CO)5(solv) fragment, which has been discussed
in terms of a model where most of the ejected CO ligands
can escape from the coordination site without leaving
the solvent cage.61 In principle, re-coordination of liber-
ated CO, still present in the bulk solution, may compete
with the formation of the olefin-substituted product 2
(Scheme 8). However, as can be estimated from the
previously determined rate constants,28,62 the reforma-
tion of 1 should be negligible since, due to the presence
of (E)-cyclooctene in sufficiently large excess, kCO[CO]
, keco[eco] at any stage of the conversion.
and 4 (170 L‚mol-1‚cm-1), favor the further conversion
into the final product 4, despite the decreasing quantum
yields for the respective steps in the reaction sequence.
Mech a n istic Asp ects. The photosubstitution of CO
in group 6 metal carbonyls has been known for a long
time as a highly efficient process.55,56 It was thought to
involve loss of CO from the 3T1g ligand field excited state
of M(CO)657 as the actual photochemical step, although
the notion of internal conversion from singlet to the
triplet manifold has been invalidated on the basis of
femtosecond transient absorption measurements.58 More-
over, recent theoretical studies have shown that the
photodissociation does not necessarily require ligand
field excitation, but may also occur from a charge-
transfer excited state.59
The olefin-substituted complexes have received much
less attention in this respect so that a sound theoretical
basis for a discussion of the electronic absorption spectra
(Figure 3) and the excited-state reactivities is not
available. Qualitative d-level diagrams (Figure 10) may
nevertheless be helpful in order to rationalize the
distinct differences in the long-wavelength region of the
spectra. Taking into account the single-face π-acceptor
character of the olefin, one can expect that the replace-
ment of one CO group by such a ligand leads to a
substantial destabilization of one of the three d(π)
orbitals, labeled b2 under C2v symmetry. Further de-
stabilization of this orbital should occur upon the
(55) (a) Strohmeier, W.; Gerlach, K. Chem. Ber. 1961, 94, 368-406.
(b) Strohmeier, W.; von Hobe, D. Chem. Ber. 1961, 94, 761-765. (c)
Strohmeier, W.; von Hobe, D. Chem. Ber. 1961, 94, 2031-2037. (d)
Strohmeier, W.; von Hobe, D. Z. Phys. Chem N. F. 1962, 34, 393-400.
(e) Kling, O.; Nikolaiski, E.; Schla¨fer, H. L. Ber. Bunsen-Ges. Phys.
Chem. 1963, 67, 883-892. (f) Strohmeier, W. Ber. Bunsen-Ges. Phys.
Chem. 1963, 67, 892-893. (g) Vogler, A. Z. Naturforsch. 1970, 25b,
1069-1070.
As known from the low-temperature matrix experi-
ments, W(CO)5(η2-eco) (2) undergoes photolytic detach-
ment of the olefin ligand and loss of CO as well, yielding
(56) (a) Nasielski, J .; Colas, A. J . Organomet. Chem. 1975, 101, 215-
219. (b) Nasielski, J .; Colas, A. Inorg. Chem. 1978, 17, 237-240.
(57) Beach, N. A.; Gray, H. B. J . Am. Chem. Soc. 1968, 90, 5713-
5721.
(58) J oly, A. G.; Nelson, K. A. Chem. Phys. 1991, 152, 69-82.
(59) (a) Pollak, C.; Rosa, A.; Baerends, E. J . J . Am. Chem. Soc. 1997,
119, 7324-7329. (b) Pierloot, K.; Tsokos, E.; Vanquickenborne, L. G.
J . Phys. Chem. 1996, 100, 16545-16550.
(60) By analogy with the recent reassignment of the respective
transition in Cr(CO)6,59 these features may just as well be attributed
to metal d(π) f ligand(π*) charge-transfer transitions, which, however,
does not disturb the essence of the argumentation.
(61) Burdett, J . K.; Grzybowski, J . M.; Perutz, R. N.; Poliakoff, M.;
Turner, J . J .; Turner, R. F. Inorg. Chem. 1978, 17, 147-154.
(62) Schaffner, K.; Grevels, F.-W. J . Mol. Struct. 1988, 173, 51-65.

3290 Organometallics, Vol. 18, No. 17, 1999
Grevels et al.
to the olefin),13 one readily understands why cis-W(CO)4-
(η2-eco)2 (3) is more easily formed than trans-W(CO)4-
(η2-eco)2 (4), Φ23 ) 0.34 versus Φ24 ) 0.16.
Sch em e 9
Concrete information on the mechanism of the cis- f
trans-W(CO)4(η2-eco)2 photoisomerization (3 f 4) is not
yet available. Nevertheless, photolytic detachment of an
olefin ligand from 3 with formation of the W(CO)4(η2-
eco) fragment 5, followed by competitive capturing of
cis-5 or trans-5 by (E)-cyclooctene according to Scheme
9, seems to be a reasonable proposal which also accounts
for the moderate quantum yield (Φ34 ) 0.15). The much
lower quantum yield for the reverse reaction (Φ43
e
0.01) may be due to the particularly high stability of
the trans-orthogonal M(η2-olefin)2 arrangement, which
could render the (reversible) dissociation of CO from 4
a more favorable process than loss of the olefin.
Con clu sion
The mechanism, elucidated in great detail for the
particular case of the photochemical synthesis of trans-
W(CO)4(η2-eco)2 (4), should be generally applicable to
the conversion of group 6 metal hexacarbonyls into
disubstituted trans-M(CO)4(η2-olefin)2 derivatives. It
involves a cis-M(CO)4(η2-olefin)2 complex as a key
intermediate en route from M(CO)5(η2-olefin) to the final
product, while the direct trans-disubstitution is of minor
importance. Hence, accumulation of cis-M(CO)4(η2-ole-
fin)2 to a concentration sufficiently high for competitive
light absorption in the reaction mixture is essential in
order to render the cis f trans photoisomerization
feasible. Taking into account that, in general, complexes
of the type cis-M(CO)4(η2-olefin)2 are thermally labile,
one can understand that the synthesis of trans-Cr(CO)4-
(η2-ethene)2,2,3 for example, requires low-temperature
conditions. In such cases, ambient temperature causes
the photosubstitution of Cr(CO)6 to stop at the stage of
the monosubstituted Cr(CO)5(η2-olefin) complex,4,6 which
in this way can be produced on preparative scale
without significant contamination, if any,6 by the di-
substituted complex. Thus, the intimate knowledge of
the mechanism enables one to select the conditions for
directing the reaction to the desired product.
the fragments W(CO)5 and W(CO)4(η2-eco), respectively.
In solution containing an excess of (E)-cyclooctene, the
former species is rapidly captured with regeneration of
2. Hence it is not surprising that the quantum yield for
the disappearance of 2 (Φ23 + Φ24 ) 0.50) is substan-
tially lower than Φ12 (0.73).
What remains to be addressed is the nature of the
W(CO)4(η2-eco) fragment (5) and its possible role as a
common intermediate en route to the disubstituted
products 3 and 4. In a theoretical study,13 various
geometries of the related Mo(CO)4(η2-ethene) have been
examined. A distorted trigonal-bipyramidal structure
(90° angle between the equatorial CO groups, olefin
upright oriented in an equatorial position) was calcu-
lated to be the most stable one, but at the same time
was thought to be less reactive toward an entering
ligand compared with a square-pyramidal geometry.
Hence we suggest this trigonal-bipyramidal structure
for the observed fragment 5, which moves energetically
uphill on the reaction coordinate to assume a square-
pyramidal geometry prior to taking up the second olefin
(Scheme 9). The present knowledge does not allow the
prediction of whether W(CO)5(η2-eco) (2) loses CO from
a cis or trans position to the olefin. Taking into account
that, according to the aforementioned theoretical study,13
the square pyramid having the olefin situated in a basal
position (cis-5, i.e., the vacant site cis to the olefin)
should be lower in energy than the one with the olefin
in the apical position (trans-5, i.e., the vacant site trans
Ack n ow led gm en t. It is our pleasure to dedicate this
paper to Professor Dietrich Do¨pp on the occasion of his
60th birthday. S.O¨ . is grateful to the Alexander von
Humboldt-Stiftung for a research fellowship. The au-
thors thank P. Bayer, C. J a¨ger, G. Klihm, D. Kreft, R.
Schrader, and the staff of the service laboratories at the
MPI fu¨r Strahlenchemie and the MPI fu¨r Kohlenfor-
schung for their skilled experimental assistance.
Ap p en d ix
Kin etic Equ a tion s for Mu ltistep P h otoch em ica l
Rea ction s. Scheme A-1 represents the photochemical
conversion of a starting material A into the products
B, C, and D in a multistep process involving both
consecutive and parallel pathways.

Photosubstitution of CO by (E)-Cyclooctene
Organometallics, Vol. 18, No. 17, 1999 3291
Integration of eq A-7, for the general case of â * 1, yields
(â - 1)
Sch em e A-1
cB
cA
cA
) q
- 1
(A-8)
0
(
)
[
]
cA
ΦAB
where
q )
ΦA(1 - â)
Dividing eq A-3 by eq A-1 and inserting eq A-8, one
obtains
The rate expressions for the four components of the
system (A, B, C, D) are given in eqs A-1 to A-4.
dcC
dcA
cC
ΦBC cB ΦAC
) γ - â
-
(A-9)
cA
ΦB cA
ΦA
ꢀCΦCD
ꢀAΦA
dc
where
γ )
-
A ) (ΦAB + ΦAC + ΦAD)øAQabs
) [ΦABøA - (ΦBC + ΦBD)øB]Qabs
) [ΦACøA + ΦBCøB - ΦCDøC]Qabs
) [ΦADøA + ΦBDøB + ΦCDøC]Qabs
(A-1)
(A-2)
(A-3)
(A-4)
dt
dcB
dt
Integration of eq A-9, for the general case of â * 1 and
γ * 1, yields
dcC
dt
(â - 1)
(γ - 1)
cC
cA
cA
cA
) r1 + r2
+ r3
(A-10)
0
cA
0
cA
(
)
(
)
dcD
dt
ΦBC ΦAB ΦAC
1
â
where r1 )
-
[
]
ΦB ΦA
ΦA
(1 - γ) (1 - â)
The factors øA, øB, øC, and øD are the so-called “molar
fractions of light”,22c,63 which represent the portion of
light absorbed by the individual compounds A, B, C,
and D:
ΦBC ΦAB
ΦB ΦA
â
r2 )
(1 - â)(γ - â)
ΦAC
ΦA
ΦAB ΦBC
ΦA ΦB
1
â
r3 )
-
[
]
(1 - γ)
(γ - â)
ꢀAcA
ꢀAcA
ꢀ c
øA )
)
etc. (A-5)
ꢀAcA + ꢀBcB + ꢀCcC + ꢀDcD
The concentration of D is determined by the stoichiom-
∑
i
i
etry condition
i
0
The light absorption rate, Qabs [einstein‚L-1‚s-1], is
cD ) cA - cA - cB - cC
(A-11)
given by
Insertion of eqs A-8, A-10, and A-11 into eq A-1 yields
a differential equation for the time dependence of cA
which can be solved by integration. In general Qabs is a
function of the (time-dependent) concentrations (eq A-6),
the explicit consideration of which would render the
integration rather complicated. However, the integra-
tion can easily be carried out for two practically impor-
tant cases. Either the irradiation is performed under
total absorption conditions (which renders the power
term in eq A-6 negligible), or an integrating actinometer
is used which provides a numerical value for the total
I0
(1 - 10-l∑ꢀ c
)
(A-6)
i
i
Qabs
)
l
where I0 is the incident photon flux (which may vary
with time) and l is the path length.
The integration of eqs A-1 to A-4 ultimately yields the
concentrations as functions of the amount of absorbed
light, thus providing the functional relationships needed
to determine the quantum yields. To simplify matters,
the initial concentrations of B, C, and D are taken as
t
amount of light absorbed by the system, ∫0Qabs dt. This
0
0
0
latter technique also eliminates any problem arising
from nonconstant incident photon fluxes. For these two
cases, cA is obtained as an implicit function of the
amount of absorbed light, eq A-12.
zero (cB ) cC ) cD ) 0). Dividing eq A-2 by eq A-1,
one obtains
dcB
dcA
cB ΦAB
) â
-
(A-7)
â
γ
cA
ΦA
cA
cA
cA
p1 1 -
+ p2 1 -
+ p3 1 -
-
0
cA
0
cA
0
cA
(
)
(
)
[
]
[
]
[
]
ꢀBΦB
ꢀAΦA
where
â )
cA
ꢀAΦA
t
ꢀD ln
)
Q
dt (A-12)
∫
abs
0
0
cA
0
(
)
cA
ΦA ) ΦAB + ΦAC + ΦAD
ΦB ) ΦBC + ΦBD
(63) Mark. G.; Mark, F. Z. Naturforsch. 1974, 29a, 610-613.

3292 Organometallics, Vol. 18, No. 17, 1999
Grevels et al.
where
also be integrated in closed form to yield cA as an
implicit function of the amount of absorbed light.
Tr ea tm en t of Sp ecia l Ca ses. It should be empha-
sized that the eqs A-8, A-10, and A-12 become singular
for the special cases of â ) 1 and/or γ ) 1 and/or â ) γ
and therefore require modifications. For each of the
various combinations of these special cases the integra-
tion of the differential equations will give different
analytical formulas. For example, for â ) 1 the solution
(ꢀB - ꢀD) ΦAB (ꢀC - ꢀD)
p1 ) (ꢀA - ꢀD) -
+
ΦA
(1 - â)
(1 - γ)
ΦBC ΦAB ΦAC
â
-
[
]
ΦB ΦA
ΦA
(1 - â)
(ꢀB - ꢀD) ΦAB
(ꢀC - ꢀD) ΦAB ΦBC
p2 )
p3 )
+
-
of eq A-7 takes the form cB/cA ) (ΦAB/ΦA) ln(cA0/cA),
which is different from eq A-8. A more convenient
approach, however, is to start with the equations
derived for the general case (eqs A-8, A-10, and A-12)
and to transform the indeterminate forms into deter-
minate forms by applying L’Hoˆpital’s rule once or, if
necessary, twice.
ΦA
ΦA ΦB
â(1 - â)
(1 - â)(γ - â)
(ꢀC - ꢀD) ΦAC
ΦAB ΦBC
ΦA ΦB
â
[
]
ΦA
γ(1 - γ)
(γ - â)
Recall that the concentration of A in eq A-12 is not given
as an explicit, but rather as an implicit function of the
amount of absorbed light. Therefore the value of cA has
to be evaluated numerically. The concentrations of B,
C, and D can be obtained subsequently from eqs A-8,
A-10, and A-11.
Moreover, general formulas, valid for all values of â
and γ, have been developed. All singularities can be
removed by inserting the Taylor expansion
The photokinetic equations for a system involving
only three compounds A, B, and C (Scheme A-2) can be
obtained in an analogous way.
∞
1
yx ) ex ln y
)
(x ln y)ν
(A-16)
∑
ν)0ν!
Sch em e A-2
into these equations and collecting those terms of the
infinite sum which do not cancel into remainder func-
tions R(n) and R˜ (n). These functions are defined by
(ln y)ν
∞
R
(n)(y,ê) )
êν-1
(A-17)
∑
ν!
ν)n
â
cA
cA
cA
u1 1 -
+ u2 1 -
- ꢀC ln
ꢀAΦA
)
0
0
0
cA
(
)
(
)
[
]
[
]
cA
cA
and
t
Q
dt (A-13)
∫
abs
0
cA
0
R˜ (n)(y,η,ê) ) (R(n + 1)(y,η + ê) -
(ln y)ν+1
ν (ν + 1)!
(â - 1)
∞
cB
cA
1
) q
- 1
(A-14)
(A-15)
R
(n + 1)(y,η)) )
g
(A-18)
0
∑
cA
(
[
)
]
cA
ê
ν)n
0
cC ) cA - cA - cB
ΦA ) ΦAB + ΦAC
where
where
(η + ê)ν - ην
(ꢀB - ꢀC) ΦAB
gν )
and n g 1
u1 ) (ꢀA - ꢀC) -
ê
ΦA
(1 - â)
Φ
1
u2 )
q )
AB(ꢀB - ꢀC)
After some algebraic manipulations the following
expression is obtained from eq A-12:
ΦA
â(1 - â)
ΦAB
1 - â ΦA
1
d1(c˜A - 1) + d2c˜AR(1)(c˜A,â - 1) + d3c˜AR˜ (1)(c˜A,â -
1,γ - â) + ꢀD ln c˜A + d4(γ - â)c˜AR˜ (1)(c˜A,â - 1,γ -
The photokinetic formalism outlined above can easily
be extended to systems with more than four compo-
nents. The concentrations of all products B, C, D, E,
etc. can be expressed as functions of the concentration
of the starting material A since the differential equa-
tions relating cA to the concentration of any consecutive
product can be integrated in closed form, beginning with
B and proceeding stepwise to the ultimate product.
Hence, in the final step the rate expression for A can
ꢀAΦA
â) ) -
tQ dt (A-19)
∫
abs
0
cA
0
where
0
c˜A ) cA/cA , â ) (ꢀBΦB)/(ꢀAΦA), γ ) (ꢀCΦCD)/(ꢀAΦA),

Photosubstitution of CO by (E)-Cyclooctene
Organometallics, Vol. 18, No. 17, 1999 3293
and
cC ΦAB ΦBC
)
âR˜ (1)(c˜A,â - 1,γ - â) -
cA
ΦA ΦB
ꢀB - ꢀD ΦAB
ΦAC
R(1)(c˜A,γ - 1) (A-21)
d1 ) (ꢀA - ꢀD) +
+
â
ΦA
ΦA
ꢀC - ꢀD ΦAC ΦAB ΦBC
By noting that R(1)(y,0) ) ln y and R˜ (1)(y,0,0) ) (ln y)2/2,
explicit formulas can be derived for the special cases
from eqs A-19, A-20, and A-21. Numerical values for the
remainder functions A-17 and A-18 can be conveniently
+
(
)
γ
ΦA
ΦA ΦB
ꢀB - ꢀD ΦAB ꢀC - ꢀD ΦAC ΦAB ΦBC
d2 ) -
-
+
(
)
â
ΦA
γ
ΦA
ΦA ΦB
calculated in a recursive way as follows. Set Tn
)
ê
n-1[(ln y)n/n!] and Sn ) 0. Then, for ν g n, calculate
ΦAB ΦBC â
Sν+1 ) Sν + Tν and update Tν+1 ) Tνê[ln y/(ν + 1)] as
long until Tν+1 becomes sufficiently small compared to
Sν+1, such that R(n) ≈ Sν+1. Values of R˜ (n) can be
calculated in an analogous way, whereby the expansion
coefficients gν also can be obtained recursively according
to gν+1 ) (η + ê)gν + ην, starting from g0 ) 0.
d3 ) (ꢀC - ꢀD)
ΦA ΦB
γ
(ꢀC - ꢀD) ΦAC
d4 ) -
γ
ΦA
Similarly, eq A-8 can be cast into the form
Su p p or tin g In for m a tion Ava ila ble: Tables of bond
lengths and angles, atomic coordinates, and thermal param-
eters for W(CO)6 (1), W(CO)5(η2-(E)-cyclooctene) (2), trans-
W(CO)4(η2-(E)-cyclooctene)2 (meso compound 4a with S4 sym-
metry), and cis-W(CO)4(η2-(Z)-cyclooctene)2. This material is
available free of charge via the Internet at http://pubs.acs.org.
cB
cA
ΦAB
ΦA
) -
R(1)(c˜A,â - 1)
(A-20)
and eq A-10 into the form
OM990300T