EPR study of photochemical reactions of fac- and mer-[Cr(CO)3(η1-L2)(η2-L 2)]+ (L2 = bidentate phosphine, arsine, or phosphonite ligand)

Article abstract of DOI:10.1021/om0203734

Rearrangement of fac-[Cr(CO)₃(η²-L₂)(η¹-L ₂)]⁺ to mer-(CO)₃(η²-L₂)(η¹-L₂ )]⁺ (L₂ is a bidentate phosphine or arsine ligand) is a rapid thermally activated process. Loss of CO from mer-[Cr(CO)₃(η²-L₂)(η¹-L ₂)]^- to form trans-[Cr(CO)₂(η²-L₂)₂]^- is shown to be a clean photochemical process, perhaps the first example of a quantitative photochemical transformation for a Cr(I) complex. The bidentate phosphonite ligand, L₂ = (MeO)₂PCH₂CH₂P(OMe)₂, behaves quite differently: fac-[Cr(CO)₃(η²-L₂)(η¹-L ₂)]^- undergoes photochemical loss of CO to form a five-coordinate complex which converts slowly via thermal activation to trans[Cr(CO)₂(η²-L₂)₂]⁺ .

Full text of DOI:10.1021/om0203734

5868
Organometallics 2002, 21, 5868-5873
EP R Stu d y of P h otoch em ica l Rea ction s of fa c- a n d
m er -[Cr (CO)3(η1-L2)(η2-L2)]+ (L2 ) Bid en ta te P h osp h in e,
Ar sin e, or P h osp h on ite Liga n d )
Anne L. Rieger and Philip H. Rieger*
Department of Chemistry, Brown University, Providence, Rhode Island 02912
Received May 9, 2002
Rearrangement of fac-[Cr(CO)3(η2-L2)(η1-L2)]+ to mer-[Cr(CO)3(η2-L2)(η1-L2)]+ (L2 is a
bidentate phosphine or arsine ligand) is a rapid thermally activated process. Loss of CO
from mer-[Cr(CO)3(η2-L2)(η1-L2)]+ to form trans-[Cr(CO)2(η2-L2)2]+ is shown to be a clean
photochemical process, perhaps the first example of a quantitative photochemical transfor-
mation for a Cr(I) complex. The bidentate phosphonite ligand, L2 ) (MeO)2PCH2CH2P(OMe)2,
behaves quite differently: fac-[Cr(CO)3(η2-L2)(η1-L2)]+ undergoes photochemical loss of CO
to form a five-coordinate complex which converts slowly via thermal activation to trans-
[Cr(CO)2(η2-L2)2]+.
In tr od u ction
With the mechanism in mind, our attention was
drawn to the reaction which converts mer-[Cr(CO)3(η2-
dppm)(η1-dppm)]+ to trans-[Cr(CO)2(η2-dppm)2]+ with
loss of CO:
Over the course of two decades, Bond, Colton, and co-
workers1 made many electrochemical and EPR spectro-
scopic contributions to the chemistry of chromium(I)
carbonyl phosphine, arsine, and phosphite complexes.
For example, we know from the work of Bagchi, Bond,
and Colton2 that fac-[Cr(CO)3L3] (L ) phosphine, phos-
phite) is stable in the Cr(0) state but that the Cr(I)
complex rapidly rearranges to the meridional conforma-
tion:
fac-[Cr(CO)3L3]+ f mer-[Cr(CO)3L3]+
(1)
Rate constants for the isomerization of two complexes
were measured, and analysis of the data leads to the
following activation parameters: for L ) PPhMe2, ∆Hq
) 9 ( 2 kJ mol-1 and ∆Sq ) -226 ( 8 J mol-1 K-1; for
L ) P(OMe)3, ∆Hq ) 11 ( 1 kJ mol-1 and ∆Sq ) -220
( 3 J mol-1 K-1. These parameters show that little bond
stretching is involved in the isomerization, but the large
loss of entropy indicates that a very specific conforma-
tion is required: i.e., the transition state is very
crowded, even for monodentate phosphines and phos-
phites.
Several mysteries remained from the work of Bond
and Colton. We have recently provided an explanation
for the failure to observe isotropic EPR spectra of fac-
[Cr(CO)3L3]+ or cis-[Cr(CO)2L4]+,3 but there are still a
number of unsolved problems related to the mechanisms
of Cr(I) reactions.
Blagg et al.4 reported that reaction 2 could be followed
over the course of 100 min by observation of the EPR
spectra of the complexes. A substantial loss of signal
intensity over the course of the reaction was attributed
to the disproportionation reaction
2 mer-[Cr(CO)3(η2-dppm)(η1-dppm)]+ f
mer-[Cr(CO)3(η2-dppm)(η1-dppm)] +
mer-[Cr(CO)3(η2-dppm)(η1-dppm)]2+
(1) (a) Bowden, J . A.; Colton, R.; Commons, C. J . Aust. J . Chem.
1973, 26, 655. (b) Bond, A. M.; Colton, R.; J ackowski, J . J . Inorg. Chem.
1979, 18, 1977. (c) Bagchi, R. N.; Bond, A. M.; Brain, G.; Colton, R.;
Henderson, T. L. E.; Kevekordes, J . E. Organometallics 1984, 3, 4. (d)
Bond, A. M.; Colton, R.; Kevekordes, J . E. Inorg. Chem. 1986, 25, 749.
(e) Bond, A. M.; Colton, R.; Kevekordes, J . E.; Panagiotidou, P. Inorg.
Chem. 1987, 26, 1430. (f) Bond, A. M.; Colton, R.; Feldberg, S. W.;
Mahon, P. J .; Whyte, T. Organometallics 1991, 10, 3320.
(2) Bagchi, R. N.; Bond, A. M.; Colton, R. J . Electroanal. Chem.
Interfacial Electrochem. 1986, 199, 297.
with the dication decaying rapidly to EPR-inactive
(4) Blagg, A.; Carr, S. W.; Cooper, G. R.; Dobson, I. D.; Gill, J . B.;
Goodall, D. C.; Shaw, B. L.; Taylor, N.; Boddington, T. J . Chem. Soc.,
Dalton Trans. 1985, 1213.
(3) Bond, A. M.; McGarvey, B. R.; Rieger, A. L.; Rieger, P. H. Inorg.
Chem. 2000, 39, 3428.
10.1021/om0203734 CCC: $22.00 © 2002 American Chemical Society
Publication on Web 11/26/2002

fac- and mer-[Cr(CO)3(η2-L2)(η2-L2)]+
Organometallics, Vol. 21, No. 26, 2002 5869
Ta ble 1. In fr a r ed Da ta
ν/cm-1
complex
obsda
lit.b
mer-[Cr(CO)3(η1-dppm) (η2-dppm)]
mer-[Cr(CO)3(η1-dppm) (η2-dppm)]+
trans-[Cr(CO)2(η2-dppm)2]+
1948, 1850, 1830 sh
2031 w, 1971, 1912
1871
1950 w, 1850 1830 shd
2036, 1972, 1912c
1869f
fac-[Cr(CO)3(η1-dppe) (η2-dppe)]
mer-[Cr(CO)3(η1-dppe) (η2-dppe)]
mer-[Cr(CO)3(η1-dppe) (η2-dppe)]+
trans-[Cr(CO)2(η2-dppe)2]+
1927, 1836 br
1928, 1835d
1955 w, 1850, 1830 shd
1965 w, 1910, 1900 shd (?)
1850g
1965 w, 1909 br
1849
1950 w, 1850, 1830 sh
2029, 1951, 1909
1850
1927, 1832
2029w, 1925, 1911
1851
1919, 1824
2029, 1961, 1896
1828
1919, 1826, 1817 sh
2023, 1952, 1900
1842
2008 w, 1853, 1824 sh
2031 w, 1907, 1882 sh
1858
mer-[Cr(CO)3(η1-arphos) (η2-arphos)]
mer-[Cr(CO)3(η1-arphos) (η2-arphos)]+
trans-[Cr(CO)2(η2-arphos)2]+
1960 w, 1855, 1835 she
1970 w, 1915, 1895e
fac-[Cr(CO)3(η1-dpae) (η2-dpae)]
mer-[Cr(CO)3(η1-dpae) (η2-dpae)]+
trans-[Cr(CO)2(η2-dpae)2]+
1930, 1830 bre
fac-[Cr(CO)3(η1-dmpm) (η2-dmpm)]
mer-[Cr(CO)3(η1-dmpm) (η2-dmpm)]+
trans-[Cr(CO)2(η2-dmpm)2]+
mer-[Cr(CO)3(η1-dmpe) (η2-dmpe)]
mer-[Cr(CO)3(η1-dmpe) (η2-dmpe)]+
trans-[Cr(CO)2(η2-dmpe)2]+
mer-[Cr(CO)3(η1-dppbz) (η2-dppbz)]
mer-[Cr(CO)3(η1-dppbz) (η2-dppbz)]+
trans-[Cr(CO)2(η2-dppbz)2]+
fac-[Cr(CO)3(η1-pompom) (η2-pompom)]
fac-[Cr(CO)3(η1-pompom) (η2-pompom)]+
[Cr(CO)2(η1-pompom) (η2-pompom)]+
trans-[Cr(CO)2 (η2-pompom)2]+
mer,mer-[{Cr(CO)3(η2-dppe)}2(µ-dppe)]
mer,mer-[{Cr(CO)3(η2-dppe)}2(µ-dppe)]+
1948, 1868
1967, 1871
2011 w, 1869
1892
1950 w, 1850, 1835
2029 w, 1957, 1909
1950, 1850, 1834 sh
a
b
d
In CH2Cl2/C2H4Cl2 solution unless otherwise noted. In CH2Cl2 solution unless otherwise noted. c Reference 4. Reference 11.
e Reference 5. f Reference 17. Reference 18. Reference 12. Obtained by ferrocenium oxidation in a dry ice/acetone slush bath.
g
h
i
Diethyl ether was distilled from sodium benzophenone ketyl
under nitrogen. Acetone was dried over potassium carbonate
and distilled from potassium permanganate. Other solvents
were used as supplied.
products. This explanation seems unlikely since, from
electrochemical studies,5 we know that ∆G° ) 1.09 eV
(105 kJ mol-1) for the disproportionation step. Subse-
quent experiments by Bond, Colton, and co-workers5
showed that solutions of mer-[Cr(CO)3(η2-dppm)(η1-
dppm)]+ were stable indefinitely when kept in the dark
in a sealed tube, but the reaction was found to occur
when the tube was opened or purged with argon. These
authors6 later showed that the oxidation of fac-[Mn-
(CO)3(dppm)Cl] involves a photochemical fac f mer
rearrangement of the Mn(I) species prior to oxidation:
[Cr(CO)6], [(C7H8)Cr(CO)3], Ph2PCH2CH2PPh2 (dppe), Ph2-
PCH2PPh3 (dppm), Me2PCH2PMe2 (dmpm), Me2PCH2CH2PMe2
(dmpe), 1,2-(Ph2P)2C6H4 (dppbz), and Ph2AsCH2CH2AsPh2
(dpae) were obtained from Aldrich or Strem, and Ph2PCH2-
CH2AsPh2 (arphos) was obtained from Pressure Chemical Co.
(MeO)2PCH2CH2P(OMe)2 (pompom) was prepared by the
method of King and Rhee8,9 from Cl2PCH2CH2PCl2 (Strem).
Ferrocenium tetrafluoroborate was prepared as described by
Connelly and Geiger.10 (p-Nitrophenyl)diazonium tetrafluo-
roborate (Aldrich) was recrystallized from acetone/diethyl
ether.
fac-[Mn(CO)3(dppm)Cl] hν8
mer-[Mn(CO)3(dppm)Cl] f
The Cr(0) complexes were prepared by literature meth-
ods5,6,11 or small variations thereon. All operations were
performed under argon using either Schlenk techniques or a
Vacuum Atmospheres drybox. The identities of the Cr(0)
complexes were confirmed by mass and infrared spectra.
Cr(I) complexes were prepared by ferrocenium or diazonium
oxidation of Cr(0) complexes. Carbonyl stretching frequencies
for the Cr(0) and Cr(I) complexes are given in Table 1. mer,-
mer-[{Cr(CO)3(η2-dppe)}2(µ-dppe)] was prepared by a modifica-
tion of the reported synthesis.12 A 4:1 mixture of dppe and
[(C7H8)Cr(CO)3] was refluxed for 14 h in hexane. The super-
natant was removed by needle under Ar. The solid was
transferred to a Soxhlet extractor under Ar and extracted with
hexane for 48 h. The extract contained mostly dppe. The
extractor was then charged with degassed CH2Cl2 and the
complex extracted into the solvent. The identity of the complex
was confirmed by its infrared (Table 1) and mass spectra.
mer-[Mn(CO)3(dppm)Cl]+ + e- (3)
This result suggests that some reactions of Cr(I) com-
plexes could be photochemical. We will show below that
reaction 2, and related reactions of many Cr(I) carbonyl
phosphine and arsine complexes, are indeed photochemi-
cal in nature and proceed virtually quantitatively under
controlled conditions. Photochemical substitution reac-
tions have been reported for 18-electron complexes such
as Cr(CO)6,7 but this may be the first report of a
quantitative photochemical reaction of a 17-electron
chromium complex.
Exp er im en ta l Section
Ma ter ia ls. Chlorinated solvents were distilled from calcium
hydride under nitrogen. Hydrocarbons were dried over sodium.
(8) King, R. B.; Rhee, W. M. Inorg. Chem. 1978, 17, 2961.
(9) Cummings, D. A.; McMaster, J .; Rieger, A. L.; Rieger, P. H.
Organometallics 1997, 16, 4362.
(5) Bagchi, R. N.; Bond, A. M.; Colton, R.; Creece, I.; McGregor, K.;
Whyte, T. Organometallics 1991, 10, 2611.
(6) Compton, R. G.; Barghout, R.; Eklund, J . C.; Fisher, A. C.; Bond,
A. M.; Colton, R. J . Phys. Chem. 1993, 97, 1661.
(7) (a) Cotton, F. A.; Wilkinson, G. Advanced Inorganic Chemistry,
5th ed.; Wiley-Interscience: New York, 1988; p 1047. (b) Cruse, H. A.;
Leadbeater, N. E. Inorg. Chem. 1999, 38, 4149.
(10) Connelly, N. G.; Geiger, W. E. Chem. Rev. 1996, 96, 904.
(11) Bond, A. M.; Colton, R.; McGregor, K. Inorg. Chem. 1986, 25,
2378 and references therein.
(12) Bond, A. M.; Colton, R.; Cooper, J . B.; McGregor, K.; Walter, J .
N.; Way, D. M. Organometallics 1995, 14, 49.

5870 Organometallics, Vol. 21, No. 26, 2002
Rieger and Rieger
Ta ble 2. EP R P a r a m eter sa
Oxidation of fac-[Cr(CO)3(η1-L2)(η2-L2)] (L2 ) dpae,
dppe, dmpm) results in the expected rapid rearrange-
ment to the mer isomer:
complex
isotropica
g
A
mer-[Cr(CO)3(η2-dppm)-
(η1-dppm)]+
2.016
2.034
2.034
1.982
2.031
24 (3 P)
24
22.8 (1 P)
19.6 (2 P)
2.004
21.1
trans-[Cr(CO)2(η2-
26.4 (4 P)e
25.0
dppm)2]+ b
25.82 (4 P) 2.014
1.973
2.025
25.5
mer-[Cr(CO)3(η2-dppe)-
(η1-dppe)]+
[2.055]c [22.9] (3 P)c
22.7 (3 P)
2.037
1.983
2.027
22.0
23.2
trans-[Cr(CO)2(η2-dppe)2]+ b 2.015
28.5 (4 P)e
26.8
26.91 (4 P) 2.025
1.980
2.025
26.5d
mer-[Cr(CO)3(η2-arphos)-
(η1-arphos)]+
[2.068]c [24] (2 P),
[21] (1 As)c
25, 25
24 (2 P)
23 (1 As)
2.025
2.024
1.983
f
23, 23
f
mer-[Cr(CO)3(η1-dpae)-
(η2-dpae)]+
24.3 (3 As)d
trans-[Cr(CO)2(η2-dpae)2]+ 2.008
2.034
31 (4 As)
28.5 (4 As) 2.008
1.980
2.016
31
28
mer-[Cr(CO)3(η2-dmpe)-
(η1-dmpe)]+
[2.01]c [22] (3 P)c
24.3 (3 P)
2.04
1.994
f
26
24.9
f
mer-[Cr(CO)3(η2-dmpm)-
(η1-dmpm)]+
2.018
23.7 (3 P)
trans-[Cr(CO)2(η2-dmpm)2]+ 2.004
f
f
26.76 (4 P) 2.011
29 (4 P)
f
28.8 (4 P)
27.4
f
trans-[Cr(CO)2(η2-dmpe)2]+ 2.016
2.030
27.82 (4 P) 2.023
1.993
In all three cases, the mer isomer was formed too rapidly
to obtain well-resolved EPR or IR spectra of the fac
cation.
The oxidized solutions are stable, provided they are
kept air-free and in the dark. The integrated EPR signal
intensity remained constant within experimental error
for at least 2 h. On standing under room light for several
hours, spectral features due to trans-[Cr(CO)2(L2)2]+
(L2 ) dppm, dppe, dmpm, dmpe, arphos, dpae, dppbz)
can be detected. As shown in Figure 1 for the reaction
27.4
trans-[Cr(CO)2(η2-dppbz)2]+ 2.015
2.058e
26.75 (4 P) 2.019e
1.967e
25.1 (4 P)e
27.1e
24.9e
30.7 (3 P)
28.8
fac-[Cr(CO)3(η2-pompom)-
(η1-pompom)]+
1.990
2.050
2.050
28.8
f
[Cr(CO)2 (η2-pompom)-
(η1-pompom)]+
2.017
2.046
2.021
1.987
13.8 (1 P)
24.6 (1 P)
31.6 (1 P)
2.023
trans-[Cr(CO)2(η2-
2.049
31.1 (4 P)e
pompom)2]+ b
31.49 (4 P) [2.029]c [31.5]c
1.991
31.9
mer,mer-[{Cr(CO)3(η2-
dppe)}2(µ-dppe)]+
2.025
22.7 (3 P)
[2.049]c [22.4] (3 P)c
2.037
1.989
23.2
22.5
a
Hyperfine couplings in units of 10-4 cm-1; uncertainties are
(1 in last digit. Data from ref 9. c Value in brackets computed
b
d
from measured components and the isotropic parameters. Uncer-
tainty (2 in last decimal place. e Uncertainty (3 in last decimal
place. f Hyperfine structure poorly resolved.
In str u m en ta tion . EPR spectra were obtained using Bruk-
er ER-220D or EMX X-band spectrometers equipped with a
Bruker variable-temperature accessory, a Systron-Donner
microwave frequency counter, and a Bruker gauss meter.
Infrared spectra were recorded with a Mattson FT-IR spec-
trometer. Mass spectra were obtained with a Kratos MS80RFA
spectrometer in positive-ion mode with fast atom bombard-
ment using m-nitrobenzyl alcohol as a matrix.
Resu lts
Bid en ta te P h osp h in es a n d Ar sin es. One-electron
oxidation of mer-[Cr(CO)3(η1-L2)(η2-L2)] (L2 ) dppm,
dmpe, arphos, dppbz) by [Fe(η-C5H5)2]+ or [O2NC6H4N2]+
in CH2Cl2/C2H4Cl2 results in the corresponding Cr(I)
complexes. In each case, the paramagnetic product was
identified by its EPR and infrared spectra. CO stretch-
ing frequencies and EPR parameters are given in Tables
1 and 2, respectively.
the transformation is complete after several days under
room light or after several minutes of illumination with
300 nm lamps in a Rayonet reactor. In all cases, the
integrated intensities of the isotropic EPR spectra were

fac- and mer-[Cr(CO)3(η2-L2)(η2-L2)]+
Organometallics, Vol. 21, No. 26, 2002 5871
cation has sufficiently even line spacings that AP and
AAs differ by no more than 0.1 × 10-4 cm-1
.
Frozen-solution EPR spectra range considerably in
resolution. All spectra of trans-[Cr(CO)2(η2-L2)2]+, except
those of the arphos, dmpm, and dppbz complexes,
showed well-resolved 31P or 75As hyperfine coupling on
two of the three g components (the L2 ) dppe, dpae
derivatives showed one or more resolved features for the
third component). Resolution of the frozen-solution
spectra of the mer dmpm and dppe cations and the trans
dmpm cation was not sufficient to quote anisotropic
parameters with certainty, although the overall shapes
of the spectra suggest g components similar to those of
the mer dppm and trans dppm cations, respectively.
Frozen-solution spectra of the mer dppm and dppe
cations and the trans dppe cation have resolved features
for two of the g components and a variably resolved
shoulder corresponding to the lowest-field feature of the
F igu r e 1. EPR spectra showing conversion of mer-Cr-
[(CO)3(η2-dppe)(η1-dppe)]+ to trans-[Cr(CO)2(η2-dppe)2]+.
Integrated intensities of the spectra are (top to bottom) 51,
55, and 52.
g
max series. Analysis of the EPR spectra thus is straight-
forward. Although spectra of mer,mer-[{Cr(CO)3(η2-
dppe)}2(µ-dppe)]+ are slightly better resolved than those
of mer-[Cr(CO)3(η2-dppe)(η1-dppe)]+, the EPR param-
eters are identical within experimental uncertainty. The
frozen-solution spectrum of the mer dpae cation is
completely unresolved, and no parameter estimates
could be obtained.
identical within experimental error before and after the
transformation.
Oxidation of mer,mer-[{Cr(CO)3(η2-dppe)}2(µ-dppe)] by
[Fe(η-C5H5)2]+ resulted in an isotropic EPR spectrum
similar to that reported by Bond et al.12 and virtually
indistinguishable from those of mer-[Cr(CO)3(η2-dppe)-
(η1-dppe)]+. However, under no circumstances did we
observe the very broad line attributed to mer,mer-[{Cr-
(CO)3(η2-dppe)}2(µ-dppe)]2+; indeed, the spectra re-
mained unchanged when an excess of oxidant was
employed. Furthermore, on irradiation, the spectrum of
trans-[{Cr(CO)2(η2-dppe)2]+ was observed with a factor-
of-2 decrease in integrated intensity. This suggests that
the spectra of mer-[Cr(CO)3(η2-dppe)(η1-dppe)]+ and
mer,mer-[{Cr(CO)3(η2-dppe)}2(µ-dppe)]2+ are essentially
identical: i.e., that there is negligible spin exchange
through the PCH2CH2P bridge.
The isotropic EPR spectra of mer-[Cr(CO)3(η1-L2)(η2-
L2)]+ (L2 ) dppm, dppe, dmpm, dmpe, dppbz) are
approximate 1:3:3:1 quartets with relatively broad lines,
although the spectrum of the dppm complex showed
sufficient departure from 1:3:3:1 intensity ratios that a
least-squares analysis of the digitized spectrum gave
couplings for one unique and two equivalent 31P nuclei.
Lines in the other spectra were too broad to attempt
such an analysis. The isotropic spectrum of the mer dpae
cation consists of 10 lines (three approximately equiva-
lent 75As nuclei, I ) 3/2). The spectrum of the mer arphos
cation has 6 lines (2 equivalent 31P nuclei and 1 75As
nucleus) with AP ) AAs ; small variations in the line
spacings suggest that the 31P coupling is slightly larger
than the 75As coupling.
Bid en ta te P h osp h on ite. Oxidation of fac-[Cr(CO)3-
(η1-pompom)(η2-pompom)] yields the corresponding fac
cation, identified by its frozen-solution EPR spectra and
the absence of an isotropic spectrum. The frozen-
solution EPR spectrum is clearly axial with three
hyperfine features resolved on both the g| and g
components (the fourth features in each series overlap
but are clearly present); the spectrum is better resolved
than, but similar to, those of fac-[Cr(CO)3(triphos)]+ and
fac-[Cr(CO)3(PPhMe2)3]+.9 g| is close to the free-electron
g value as expected for a C3v complex where the metal
2
contribution to the a1 SOMO is necessarily dz .
Isomerization is very slow in the dark, even at room
temperature. However, on irradiation for a few minutes
in the Rayonet reactor, an EPR spectrum is observed
which was at first thought to be that of the mer cation.
However, further irradiation does not lead to the trans
cation, and we conclude that irradiation leads to loss of
CO,7 presumably to form a square-pyramidal intermedi-
ate.13 On standing at room temperature either in the
dark or under room light, the spectrum slowly changes
to that of trans-[Cr(CO)2(η2-pompom)2]+.9 No isotropic
EPR spectrum of the fac cation is detected, so that no
integration could be performed for the first step, but the
integrated intensity remained constant over the second
step. The proposed reaction steps are shown in Scheme
1.
EPR spectra of fac-[Cr(CO)3(η1-pompom)(η2-pom-
pom)]+, the photolysis product, and trans-[Cr(CO)2(η2-
pompom)2]+ are shown in Figures 2 and 3, and isotropic
and anisotropic parameters are given in Table 2. The
isotropic spectrum obtained on photolysis of fac-[Cr-
(CO)3(η2-pompom)(η1-pompom}]+ consists of eight re-
markably well-resolved lines, corresponding to hyperfine
coupling to three nonequivalent 31P nuclei, AP ) 13.8
Isotropic spectra were not detected for any of the fac
cations, consistent with previous experience: we were
unable to obtain isotropic spectra for fac-[Cr(CO)3-
(triphos)]+ or fac-[Cr(CO)3(PPhMe2)3]+,9 and Bond and
co-workers saw no spectrum for solutions which almost
certainly contained fac-[Cr(CO)3(PPhMe2)3]+ or fac-[Cr-
(CO)3{P(OMe)3}3]+.2,5 The reasons for this apparent
anomaly are discussed in detail elsewhere.3
Isotropic EPR spectra of trans-[Cr(CO)2(η2-L2)2]+ all
showed well-resolved 31P and/or 75As hyperfine coupling,
and all (except the trans dppbz cation) exhibited 53Cr
satellites. The nine-line spectrum of the trans dpae
(13) There is ample precedent for isoelectronic square-pyramidal
Fe(III) complexes; see: Cotton, F. A.; Wilkinson, G. Advanced Inorganic
Chemistry, 5th ed., Wiley-Interscience: New York, 1988; p 720.

5872 Organometallics, Vol. 21, No. 26, 2002
Rieger and Rieger
Sch em e 1
broadening; better resolution was observed in spectra
of mer-[Cr(CO)3L3]+ species derived from fac precur-
sors.2
Identification of the intermediate as mer-[Cr(CO)3-
(η1-pompom)(η2-pompom)]+ is impossible on several
grounds: (i) it is surprising that it is generated photo-
chemically; (ii) if the intermediate were the expected
mer cation, it would be surprising to have CO loss to
form the trans cation be thermally activated; (iii) other
mer cations show three equivalent (or two small and
one slightly larger) 31P couplings, while the intermediate
shows three quite different couplings.
This system was studied using cyclic voltammetry. A
CH2Cl2 solution of fac-[Cr(CO)3(η1-pompom)(η2-pom-
pom)] exhibits a reversible oxidation at -0.04 V vs
ferrocene. On irradiation (15 min with stirring in
Rayonet reactor), a new reversible couple is observed
at -0.34 V, which is not the reduction of trans-[Cr(CO)2-
(η2-pompom)2]+.
F igu r e 2. EPR spectra of CH2Cl2/C2H4Cl2 solutions of (a)
fac-[Cr(CO)3(η2-pompom)(η1-pompom)]+ at 120 K and the
photolysis product at (b) 120 K and (c) 300 K.
With the structure of the five-coordinate intermediate
as shown above, an extended Hu¨ckel MO calculation14
suggests a dxz-based SOMO and dyz-based HOMO. We
then expect two larger, but unequal, 31P couplings (P
atoms on the x and z axes) and one smaller coupling (P
atom on the y axis). This stereochemistry has the three
phosphonite groups in the facial conformation, as in the
starting material, and makes the slow transformation
to trans-[Cr(CO)2(η2-pompom)2]+ plausible, since the CO
ligands would have to move to the z axis in order to
allow ring closure. Alternatively, if one CO moves to the
z axis, the product would be cis-[Cr(CO)2(η2-pompom)2]+,
which is EPR silent in liquid solution.3 Isomerization
to the trans isomer is thermodynamically favored but
would probably be slow.
F igu r e 3. EPR spectra of trans-[Cr(CO)2(η2-pompom)2]+
in CH2Cl2/C2H4Cl2 at (a) 120 K and (b) 290 K.
Discu ssion
Tricarbonyl Cr(I) complexes of bidentate phosphines
and arsines all behave very similarly. The facial com-
× 10-4, 24.6 × 10-4, and 31.6 × 10-4 cm-1. The sharper
lines may be due to the absence of the neutral precursor
and the consequent absence of electron-exchange line
(14) Extended Hu¨ckel MO calculations were performed using the
CAChe program and Alvarez parameters.

fac- and mer-[Cr(CO)3(η2-L2)(η2-L2)]+
Organometallics, Vol. 21, No. 26, 2002 5873
plexes isomerize to meridional ones rapidly via a
thermally activated process. The meridional complexes
lose CO in a photochemical process to produce trans-
[Cr(CO)2(L2)2]+. Since cis-[Cr(CO)2(L2)2]+ is expected to
be EPR silent in liquid solution,3 and there is no loss in
integrated EPR intensity, we can be quite certain that
only the trans-dicarbonyl complexes are ultimately
produced after photochemical loss of CO from the mer-
tricarbonyls. However, previous work by Bond and co-
workers15 has shown that the cis-dicarbonyl complexes
rapidly isomerize to trans, so that we cannot rule out
the cis isomer as an intermediate in the photochemical
process. We believe these reactions to be the first
examples of photochemical reactions of 17-electron
organometallics leading quantitatively to substitution
products. It should be noted that the trans-dicarbonyls
are probably not photoactive if they are analogues of
trans-[Cr(CO)2(dppe)2]+, which has been shown to be
unaffected by light.16
The case of the bidentate phosphonite ligand is more
complex. As discussed above, the experimental results
are consistent with the formation of a five-coordinate
(CO)2 complex on photolysis of fac-[Cr(CO)3(η2-pom-
pom)(η1-pompom)]+ followed by slow ring closure to
produce the trans-(CO)2 cation. We have no convincing
evidence for this hypothesis, but the alternatives
formation of the mer-(CO)3 cation on photolysissis not
plausible.
Ack n ow led gm en t. This research was supported by
the Department of Chemistry, Brown University.
OM0203734
(16) Compton, R. G.; Barghout, R.; Eklund, J . C.; Fisher, A. C.;
Davies, S. G.; Metzler, M. R.; Bond, A. M.; Colton, R.; Walter, J . N. J .
Chem. Soc., Dalton Trans. 1993, 3641.
(17) Bond, A. M.; Colton, R.; J ackowski, J . Inorg. Chem. 1975, 14,
2526.
(15) Bond, A. M.; Grabaric, B. S.; J ackowski, J . J . Inorg. Chem. 1978,
17, 2153.
(18) Bond, A. M.; Colton, R.; Cooper, J . B.j; Traeger, J . C.; Walter,
J . N.; Way, D. M. Organometallics 1994, 13, 3434.