FvCr2(CO)4L2 (Fv = fulvalene; L = PMe2Ph, PMe3), metal-metal-bonded compounds which undergo spontaneous thermal homolysis to their biradical isomers. Relevant chemistry of the compounds FvCr2(CO)4L2X2 (X = H, Cl, Br, I)

Article abstract of DOI:10.1021/om960051y

Two new (fulvalene)dichromium carbonyl dihydrides, FvCr₂(CO)₄L₂H₂ (L = PMe₂Ph (2b), PMe₃ (2c)) were prepared by phosphine substitution of FvCr₂(CO)₆H₂ (2a) and studied by variable-temperature ¹H, ¹³C{¹H}, and ³¹P{¹H} NMR. Compound 2b assumes a 77:23 and 2c an 85:15 cis/trans isomeric ratio at -50 °C, while low-temperature NMR spectra of 2b present evidence for the meso and dl forms of the cis,cis isomer. Hydride hydrogen atom abstraction from 2b and 2c by Ph₃C^?, as well as hydrogenation of isoprene and 1,3,5-hexatriene by 2b and 2c, result in the formation of FvCr₂(CO)₄L₂ (L = PMe₂Ph (6a), PMe₃ (6b)). Variable-temperature IR and ¹H, ¹³C{¹H}, and ³¹P{¹H} NMR studies demonstrate that both 6a and 6b maintain facile equilibria between the trans isomers of the chromium-chromium-bonded species and the novel biradical species L(CO)₂Cr(μ-Fv)Cr(CO)₂L at room temperature in solution. Room-temperature reactions of 6a with alkyl halides reflect the radical character of this compound, halogen atom abstractions leading to the new dihalide compounds FvCr₂(CO)₄(PMe₂Ph)₂X₂ (X = I (3b), Br (4), Cl (5)). Compounds 3b, 4, and 5 were obtained alternatively in the hydrogen-halogen atom exchange reactions of 2b with the appropriate alkyl halides. These reactions are shown to proceed via intermediate formation of the corresponding mixed hydrido halide compounds Cis-FvCr₂(CO)₄(PMe₂Ph)₂HX (containing cis-halide halves). FvCr₂(CO)₆I₂ (3a) was prepared analogously from 2a and ethyl iodoacetate and was also identified as the product of iodine atom abstraction from ethyl iodoacetate by FvCr₂(CO)_6. The reaction of FvCr₂(CO)₆I₂ with PMe₂Ph provided access to Cis-(CO)₂(PMe₂Ph)ICr(μ-Fv)[trans-Cr(CO) ₂(PMe₂Ph)₂]I (7); the reaction was shown to proceed via consecutive formation of the intermediates (CO)₃ICr(μ-Fv)[Cr(CO)₃PMe₂Ph]I and Cis-(CO)₂(PMe₂Ph)ICr(μ-Fv)[Cr(CO)₃PMe ₂Ph]I.

Full text of DOI:10.1021/om960051y

3588
Organometallics 1996, 15, 3588-3599
F vCr 2(CO)4L2 (F v ) F u lva len e; L ) P Me2P h , P Me3),
Meta l-Meta l-Bon d ed Com p ou n d s Wh ich Un d er go
Sp on ta n eou s Th er m a l Hom olysis to Th eir Bir a d ica l
Isom er s. Releva n t Ch em istr y of th e Com p ou n d s
F vCr 2(CO)4L2X2 (X ) H, Cl, Br , I)
Istva´n Kova´cs† and Michael C. Baird*
Department of Chemistry, Queen’s University, Kingston, Ontario, Canada K7L 3N6
Received J anuary 29, 1996X
Two new (fulvalene)dichromium carbonyl dihydrides, FvCr2(CO)4L2H2 (L ) PMe2Ph (2b),
PMe3 (2c)) were prepared by phosphine substitution of FvCr2(CO)6H2 (2a ) and studied by
1
variable-temperature H, 13C{1H}, and 31P{1H} NMR. Compound 2b assumes a 77:23 and
2c an 85:15 cis/trans isomeric ratio at -50 °C, while low-temperature NMR spectra of 2b
present evidence for the meso and dl forms of the cis,cis isomer. Hydride hydrogen atom
abstraction from 2b and 2c by Ph3C•, as well as hydrogenation of isoprene and 1,3,5-
hexatriene by 2b and 2c, result in the formation of FvCr2(CO)4L2 (L ) PMe2Ph (6a ), PMe3
1
(6b)). Variable-temperature IR and H, 13C{1H}, and 31P{1H} NMR studies demonstrate
that both 6a and 6b maintain facile equilibria between the trans isomers of the chromium-
chromium-bonded species and the novel biradical species L(CO)2Cr(µ-Fv)Cr(CO)2L at room
temperature in solution. Room-temperature reactions of 6a with alkyl halides reflect the
radical character of this compound, halogen atom abstractions leading to the new dihalide
compounds FvCr2(CO)4(PMe2Ph)2X2 (X ) I (3b), Br (4), Cl (5)). Compounds 3b, 4, and 5
were obtained alternatively in the hydrogen-halogen atom exchange reactions of 2b with
the appropriate alkyl halides. These reactions are shown to proceed via intermediate
formation of the corresponding mixed hydrido halide compounds cis-FvCr2(CO)4(PMe2Ph)2HX
(containing cis-halide halves). FvCr2(CO)6I2 (3a ) was prepared analogously from 2a and
ethyl iodoacetate and was also identified as the product of iodine atom abstraction from
ethyl iodoacetate by FvCr2(CO)6. The reaction of FvCr2(CO)6I2 with PMe2Ph provided access
to cis-(CO)2(PMe2Ph)ICr(µ-Fv)[trans-Cr(CO)2(PMe2Ph)2]I (7); the reaction was shown to
proceed via consecutive formation of the intermediates (CO)3ICr(µ-Fv)[Cr(CO)3PMe2Ph]I and
cis-(CO)2(PMe2Ph)ICr(µ-Fv)[Cr(CO)3PMe2Ph]I.
In tr od u ction
We are currently engaged in an investigation of
phosphine-substituted, homobimetallic (fulvalene)dimo-
lybdenum and -dichromium carbonyl complexes of the
type trans-FvM2(CO)4L2 (M ) Mo, L ) PPh3, PCy3 (Cy
) cyclohexyl), PXy3 (Xy ) 3,5-dimethylphenyl); M ) Cr,
L ) PMe2Ph).1 These compounds are of interest be-
cause the parent dimers FvM2(CO)6 (M ) Cr, Mo, W)
contain extraordinarily long metal-metal bonds but do
not undergo significant thermal metal-metal bond
homolysis to their biradical isomers L(CO)2M(µ-Fv)M-
(CO)2L (L ) CO) (eq 1),2 in striking contrast with [CpCr-
(CO)3]2.4,5 We hoped to induce the formation of biradical
(1)
isomers by using bulky ligands, an approach which has
proven to be very effective for stabilizing the 17-electron,
(5) For more recent information, see the following. CpCr(CO)3: (a)
Goh, L. Y.; Lim, Y. Y. J . Organomet. Chem. 1991, 402, 209. (b) Fortier,
S.; Baird, M. C.; Preston, K. F.; Morton, J . R.; Ziegler, T.; J aeger, T.
J .; Watkins, W. C.; MacNeil, J . H.; Watson, K. A.; Hensel, K.; LePage,
Y.; Charland, J .-P.; Williams, A. J . J . Am. Chem. Soc. 1991, 113, 542.
(c) O’Callaghan, K. A. E.; Brown, S. J .; Page, J . A.; Baird, M. C.;
Richards, T. C.; Geiger, W. E. Organometallics 1991, 10, 3119. (d)
Watkins, W. C.; J aeger, T.; Kidd, C. E.; Fortier, S.; Baird, M. C.; Kiss,
G.; Roper, G. C.; Hoff, C. D. J . Am. Chem. Soc. 1992, 114, 907. (e)
MacConnachie, C. A.; Nelson, J . M.; Baird, M. C. Organometallics 1992,
11, 2521. (f) Yao, Q.; Bakac, A.; Espenson, J . H. Organometallics 1993,
12, 2010. (g) Richards, T. C.; Geiger, W. E.; Baird, M. C. Organome-
tallics 1994, 13, 4494. CpMo(CO)3: (h) Pugh, J . R.; Meyer, T. J . J .
Am. Chem. Soc. 1992, 114, 3784. (i) Scott, S. L.; Espenson, J . H.; Zhu,
Z. J . Am. Chem. Soc. 1993, 115, 1789. CpW(CO)3: Reference 3i and (j)
Scott, S. L.; Espenson, J . H.; Bakac, A. Organometallics 1993, 12, 1044.
(k) Scott, S. L.; Espenson, J . H.; Chen, W.-J . Organometallics 1993,
12, 4077. (l) Zhu, Z.; Espenson, J . H. Organometallics 1994, 13, 1893.
† NATO Science Fellow; Research Group for Petrochemistry of the
Hungarian Academy of Sciences, Veszpre´m, Hungary.
X Abstract published in Advance ACS Abstracts, J uly 1, 1996.
(1) (a) Kova´cs, I.; Baird, M. C. Organometallics 1995, 14, 4074. (b)
Kova´cs, I.; Baird, M. C. Organometallics 1995, 14, 4084. (c) Kova´cs,
I.; Baird, M. C. Organometallics 1995, 14, 5469.
(2) (a) McGovern, P. A.; Vollhardt, K. P. C. Synlett 1990, 493. (b)
Note, however, that FvCr2(CO)6 catalyzes the 1,4-dihydrogenation of
conjugated dienes, as does [CpCr(CO)3]2 via CpCr(CO)3 and CpCr-
(CO)3H,3 which can be taken as indirect evidence for the diradical
isomer of FvCr2(CO)6. Vollhardt, K. P. C. Personal communication.
(3) Miyake, A.; Kondo, H. Angew. Chem., Int. Ed. Engl. 1968, 7, 631.
(4) (a) Baird, M. C. Chem. Rev. 1988, 88, 1217. (b) Trogler, W. C.,
Ed. Organometallic Radical Processes; J . Organomet. Chem. Library
22; Elsevier: Amsterdam, 1990.
S0276-7333(96)00051-9 CCC: $12.00 © 1996 American Chemical Society

FvCr2(CO)4L2 Metal-Metal-Bonded Compounds
Organometallics, Vol. 15, No. 16, 1996 3589
Ta ble 1. Ca r bon yl Str etch in g F r equ en ces
metal-centered radicals Cp′M(CO)2L (M ) Cr, Mo; L )
phosphine, phosphite; Cp′ ) substituted cyclopenta-
dienyl).4,5a-d,6 Our general approach to biradical forma-
tion has involved the hydrogen atom abstraction reac-
tion of trityl radicals (Ph3C•)7 with the corresponding
hydrides FvM2(CO)4L2H2 (M ) Cr, Mo), as in eq 2.1
compd
νCO (cm-1
)
(Et4N)2[FvCr2(CO)6] (1)
1890 (vs), 1800 (vs),
1717 (s)a
FvCr2(CO)6H2 (2a )
2008 (vs), 1936 (vs, sh),
1929 (vs)b
FvCr2(CO)4(PMe2Ph)2H2 (2b)
1924.5 (vs), 1856 (vs),
1846 (sh)b
FvCr2(CO)4(PMe3)2H2 (2c)
FvCr2(CO)6I2 (3a )
1921 (vs), 1853 (s)c
2026 (s), 1972 (vs),
1951 (s, br)a
cis,cis-FvCr2(CO)4(PMe2Ph)2I2 (3b)
cis,cis-FvCr2(CO)4(PMe2Ph)2Br2 (4)
trans,trans-FvCr2(CO)4(PMe2Ph)2Br2 (4)
cis,cis-FvCr2(CO)4(PMe2Ph)2Cl2 (5)
trans,trans-FvCr2(CO)4(PMe2Ph)2Cl2 (5)
FvCr2(CO)4(PMe2Ph)2 (6a )
FvCr2(CO)4(PMe3)2 (6b)
cis-PMe2Ph(CO)2ICr(µ-Fv)[trans-
Cr(CO)2(PMe2Ph)2]I (7)
1958 (vs), 1877 (m, br)b
1968 (vs), 1881 (m)d
1954 (m), 1881 (vs)d
1973 (vs), 1881 (m)d
1954 (m), 1881 (vs)d
1926 (s), 1850 (vs)b
1923 (m-s), 1846 (vs)c
1956 (vs), 1878 (vs)
(2)
We were able to obtain indirect verification of the
intermediate formation of metal-centered biradicals,
during both the hydrogen atom transfer process from
FvMo2(CO)4(PPh3)2H2 to Ph3C• and the light-induced
homolytic dissociation of trans-FvMo2(CO)4(PPh3)2, by
formation of FvMo2(CO)4(PPh3)2X2 (X ) Cl, Br, I), the
anticipated products of halogen atom abstraction reac-
tions of the biradicals with alkyl halides.1a,b In a
preliminary communication we also reported the first
direct, variable-temperature IR and 1H, 13C{1H}, and
31P{1H} NMR evidence for the existence of a facile
thermal equilibrium between trans-FvCr2(CO)4(PMe2-
Ph)2 (obtained by hydrogen atom abstraction by Ph3C•
from FvCr2(CO)4(PMe2Ph)2H2) and its biradical isomer,
PMe2Ph(CO)2Cr(µ-Fv)Cr(CO)2PMe2Ph.1c The latter un-
dergoes halogen atom abstraction reactions with alkyl
halides, and there is thus growing evidence that these
phosphine-substituted (fulvalene)molybdenum and chro-
mium carbonyl “dimers” do indeed exhibit dissociative
behavior which resembles that of the corresponding Cp
complexes.5a-d,6
We now report further details of the preparation,
spectroscopic properties and reactions of FvCr2(CO)4-
(PMe2Ph)2 and another derivative, FvCr2(CO)4(PMe3)2.
Since direct phosphine substitution of FvCr2(CO)6 is not
a viable route to prepare such complexes,2a we utilized
the hydride hydrogen atom abstraction reactions of the
corresponding dihydrides with trityl radicals; this ap-
proach required detailed investigations of the com-
pounds FvCr2(CO)4L2H2 (L ) CO, PMe3, PMe2Ph) as
well. In order to verify radical-like reactivities of FvCr2-
(CO)4(PMe2Ph)2, we also investigated its reactions with
various alkyl halides, and thus the necessity to identify
possible products of these reactions required an exami-
nation of the corresponding dihalides FvCr2(CO)4L2X2
(L ) CO, PMe2Ph; X ) Cl, Br, I). In this context we
note that related chromium compounds containing Cp
and substituted cyclopentadienyl ligands have only been
sporadically studied until recently,8 probably because
of low stabilities which make them difficult to handle.
a
b
d
In THF. In toluene. c In benzene. In CH2Cl2.
Relevant data for comparison with (fulvalene)chromium
compounds are even more limited, unlike the situation
with the much more robust molybdenum and tungsten
analogues. Most of the compounds reported in this
article are new.9
Exp er im en ta l Section
Gen er a l Con sid er a tion s. All manipulations were carried
out under purified nitrogen by using standard Schlenk tech-
niques and a Vacuum Atmospheres glovebox. All solvents
were freshly distilled under nitrogen from sodium benzophe-
none ketyl except CH2Cl2, which was distilled from CaH2.
Deuterated solvents were purchased from CDN Isotopes and
CIL and were degassed and stored in the glovebox. Alkyl
halides, PMe3, PMe2Ph, 1,3,5-hexatriene, and isoprene were
purchased from Aldrich and used as received, while trityl
dimer was prepared as previously reported.6c IR spectra were
recorded on a Bruker IFS 25 FT-IR spectrometer using a 0.2
mm NaCl cell; for variable-temperature IR studies, the cell
temperature was controlled by a Fenwall Model 550 thermo-
couple. UV-visible spectra were run on a Hewlett-Packard
8452A diode array spectrophotometer and NMR spectra on
Bruker AC-200 (200.1 MHz, 1H) and AM-400 (400.1 MHz, 1H;
100.6 MHz, 13C{1H}; 162.0 MHz, 31P{1H}) NMR spectrometers.
IR and 1H, 13C{1H}, and 31P{1H} NMR data are listed in Tables
1-4, respectively. EI- and FAB(+)-MS analyses were carried
out on a Fisons VG Quattro triple quadrupole mass spectrom-
eter, the latter using 3-nitrobenzyl alcohol as the matrix.
High-resolution FAB(+) mass spectra were obtained on a
Fisons AutoSpec instrument. Elemental analyses were carried
out by Canadian Microanalytical Services Ltd. (Delta, BC,
Canada).
(Et4N)2[F vCr 2(CO)6] (1). This compound was previously
synthesized by Vollhardt et al.,12 but little information was
provided. In our procedure, a solution of Li2C10H8 (∼15 mmol)
in 135 mL of THF/hexane (8:1)1a was added by syringe to a
mixture of Cr(CO)3(EtCN)3 (9.0 g, 30 mmol) and Et4NBr (6.3
g, 30 mmol). The yellow-brown slurry was refluxed for 1 h,
and at this point an IR spectrum of the reaction mixture
showed complete consumption of the starting chromium
complex and formation of the expected anionic product. Most
of the solvent was evaporated under reduced pressure, and
(6) See also the following. CpCr(CO)2L: (a) Cooley, N. A.; MacCo-
nnachie, P. T. F.; Baird, M. C. Polyhedron 1988, 7, 1965. (b) Watkins,
W. C.; Hensel, K.; Fortier, S.; Macartney, D. H.; Baird, M. C.; McLain,
S. J . Organometallics 1992, 11, 2418. CpMo(CO)2PPh3: (c) Drake, P.
R.; Baird, M. C. J . Organomet. Chem. 1989, 363, 131. CpW(CO)2PPh3:
Reference 5k.
(7) (a) Koeslag, M. D.; Baird, M. C. Organometallics 1994, 13, 11.
(b) Kuksis, I.; Baird, M. C. Organometallics 1994, 13, 1551. (c)
Eisenberg, D. C.; Lawrie, C. J . C.; Moody, A. E.; Norton, J . R. J . Am.
Chem. Soc. 1991, 113, 4888. (d) Eisenberg, D. C.; Norton, J . R. Isr. J .
Chem. 1991, 31, 55 and references therein.
(9) In fact, the only well-characterized (fulvalene)dichromium car-
bonyl complex reported so far is the chromium-chromium-bonded
dimer FvCr2(CO)6.2a,10,11 FvCr2(CO)6I2 has also been mentioned in the
literature but not characterized,11 and we have been informed that
(Et4N)2[FvCr2(CO)6], FvCr2(CO)6H2, and FvCr2(CO)6Me2 were studied
previously by Vollhardt et al.12
(10) Vollhardt, K. P. C.; Weidman, T. W. Organometallics 1984, 3,
82.
(8) Davis, R.; Kane-Maguire, L. A. P. In Comprehensive Organome-
tallic Chemistry; Wilkinson, G., Stone, F. G. A., Abel, E. W., Eds.;
Pergamon Press: Oxford, U.K., 1982; Vol. 3, p 953.
(11) Moulton, R.; Weidman, T. W.; Vollhardt, K. P. C.; Bard, A. J .
Inorg. Chem. 1986, 25, 1846.
(12) McGovern, P. A.; Vollhardt, K. P. C. Private communication.

3590 Organometallics, Vol. 15, No. 16, 1996
Kova´cs and Baird
Ta ble 2. 1H NMR Da ta
1H chem shifts (δ, 25 °C)
compd
fulvalene
other
(Et4N)2[FvCr2(CO)6] (1)a
4.13 (“t”, 4H), 4.61 (“t”, 4H)
1.36 (tt, J 1 ) 7 Hz, J 2 ) 2 Hz, 24H, Me), 3.42 (q, J ) 7 Hz, 16H,
CH2N)
-5.46 (s, 2H, CrH)
-6.07 (d, J PH ) 73 Hz, 2H, CrH), 1.26 (d, J PH ) 8 Hz, 12H, PMe),
∼7.06 (m, 6H, m, p-Ph), 7.30 (“t”, 4H, o-Ph)
-6.45 (d, J PH ) 81 Hz, 2H, CrH), 0.98 (d, J PH ) 9 Hz, 18H, PMe)
FvCr2(CO)6H2 (2a )b
4.08 (“t”, 4H), 4.34 (“t”, 4H)
4.04 (“q”, 4H), 4.35 (“q”, 4H)
FvCr2(CO)4(PMe2Ph)2H2 (2b)b
FvCr2(CO)4(PMe3)2H2 (2c)b
FvCr2(CO)6I2 (3a )a
4.13 (“q”, 4H), 4.57 (“q”, 4H)
5.54 (“t”, 4H), 5.90 (“t”, 4H)
4.38 (m, 3H), 4.44, 4.50, 4.61,
4.74, 5.20 (all m, 1H)
3.92, 4.00, 4.04, 4.06, 4.27,
4.46, 4.62, 4.93 (all m, 1H)
3.90 (m, 2H), 4.02, 4.04, 4.27,
4.53, 4.67, 4.86 (all m, 1H)
3.70, 3.81, 3.92, 4.23, 4.42,
4.49, 4.84, 5.29 (all m, 1H)
FvCr2(CO)4(PMe2Ph)2HClc
FvCr2(CO)4(PMe2Ph)2HBrb
FvCr2(CO)4(PMe2Ph)2HIb
FvCr2(CO)4(PMe2Ph)2I2
1.63 (d, J PH ) 8 Hz, 3H, PMe), 1.65 (d, J PH ) 8 Hz, 3H, PMe),
1.68 (d, J PH ) 9 Hz, 3H, PMe), 1.69 (d, J PH ) 9 Hz, 3H, PMe),
∼7.10 (m, 6H, m,p-Ph), 7.33 ("t", 4H, o-Ph)
(cis,cis-3b)b
FvCr2(CO)4(PMe2Ph)2Br2
(cis,cis-4)b
3.70, 3.83, 4.00, 4.21, 4.85,
5.19 (all m, 1H), 4.38 (m, 2H)
1.53 (br m, 18H, PMe), ∼7.08 (m, 6H, m,p-Ph), 7.39 (m, 4H, o-Ph)
FvCr2(CO)4(PMe2Ph)2Cl2
4.37, 4.43, 4.50, 4.54 (all m, 1H), 1.85 (br m, 18H, PMe)
4.73, 5.25 (both m, 2H)
(cis,cis-5)c
FvCr2(CO)4(PMe2Ph)2Cl2
(trans,trans-5)c
3.91, 3.96 (both m, 4H)
1.99 (d, J PH ) 8 Hz, 18H, PMe)
FvCr2(CO)4(PMe2Ph)2 (6a )b
FvCr2(CO)4(PMe3)2 (6b)b
cis-(PMe2Ph)(CO)2ICr(µ-Fv)-
[trans-Cr(CO)2(PMe2Ph)2]I (7)d
3.69 (br m, 4H), 3.92 (br m, 4H)
3.87 (m, 4H), 3.90 (m, 4H)
4.71, 4.74, 4.82, 4.95, 5.26, 5.28, 1.96 (d, J PH ) 9 Hz, 3H, PMe of neutral Cr), 2.02 (m, 12H, PMe
5.33, 5.40 (all m, 1H)
1.68 (br s, 12H, PMe), ∼7.05 (br m, 4H, Ph), 7.41 (br m, 6H, Ph)
1.21 (br d, 18H, PMe)
of Cr+), 2.15 (d, J PH ) 9 Hz, 3H, PMe of neutral Cr), 7.42-7.61
(m, 15H, Ph)
[(CO)3ICr](µ-Fv)[Cr(CO)3
PMe2Ph]Ia
5.45, 5.91, 6.12, 6.44 (all m, 2H) 2.44 (d, J PH ) 11 Hz, 6H, PMe), 7.64 (m, 3H, m, p-Ph), 7.91 (m,
2H, o-Ph)
cis-[PMe2Ph(CO)2ICr]-
4.96, 5.13, 5.54, 5.81, 5.85, 6.04, 2.17 (d, J PH ) 9 Hz, 3H, PMe), 2.26 (d, J PH ) 9 Hz, 3H, PMe),
(µ-Fv)[Cr(CO)3PMe2Ph]Ia
6.26, 6.38 (all m, 1H)
2.42 (d, J PH ) 11 Hz, 3H, PMe), 2.43 (d, J PH ) 11 Hz, 3H,
PMe), 7.42 (m, 3H, m,p-Ph), 7.63 (m, 3H, m,p-Ph), 7.78 (m, 2H,
o-Ph), 7.90 (m, 2H, o-Ph)
a
b
d
Acetone-d6. Toluene-d8. c CDCl3. DMSO-d6.
Ta ble 3. 13C{1H} NMR Da ta
13C chem shifts (δ, 25 °C)
compound
fulvalene
other
(Et4N)2[FvCr2(CO)6] (1)a
FvCr2(CO)6H2 (2a )b
80.0, 81.7, 100.9 (C-1)
83.7, 85.9, 99.8 (C-1)
83.7, 84.8, 99.9 (C-1)
7.7 (Me), 52.8 (CH2N), 247.4 (CO)
234.7 (br, CO)
128.5 (d, J PC ) 8.5 Hz, Ph), 129.2 (s, Ph), 129.6 (d, J PC ) 9 Hz, Ph),
143.0 (d, J PC ) 36 Hz, Ph)
FvCr2(CO)4(PMe2Ph)2H2 (2b)c
FvCr2(CO)4(PMe3)2H2 (2c)c
FvCr2(CO)6I2 (3a )d
83.0, 84.2, 100.0 (C-1)
91.2, 92.1, 103.2 (C-1)
83.8, 84.0, 89.2, 89.6,
92.1, 92.9, 95.3, 95.4,
101.8 (C-1), 102.0 (C-1)
235.9 (s, CO cis to I), 247.5 (s, CO trans to I)
FvCr2(CO)4(PMe2Ph)2I2 (3b)d
19.3 (d, J PC ) 30 Hz, PMe), 20.38 (d, J PC ) 28 Hz, PMe), 20.43 (d,
J PC ) 28 Hz, PMe), 127.8 (d, J PC ) 10 Hz, Ph), 129.6 (s, Ph),
131.6 (d, J PC ) 8 Hz, Ph), 137.4 (d, J PC ) 44 Hz, Ph), 252.2 (d,
J PC ) 6.5 Hz, CO cis to I), 260.0 (d, J PC ) 57 Hz, CO trans to I)
129.3, 130.0, 130.6 (Ph)
FvCr2(CO)4(PMe2Ph)2 (6a )a
FvCr2(CO)4(PMe3)2 (6b)c
82.6, 88.4
81.8, 85.1, 87.3 (C-1)
83.0, 88.9, 89.0, 89.2, 89.7,
90.0, 92.2, 96.9, 98.4 (C-1),
102.8 (C-1)
253.6 (d, J PC ) 45 Hz, CO)
cis-(PMe2Ph)(CO)2ICr(µ-Fv)-
17.6 (m, PMe of Cr+), 19.4 (d, J PC ) 30 Hz, PMe of neutral Cr),
20.6 (d, J PC ) 30 Hz, PMe of neutral Cr), 127.8 (d, J PC ) 10 Hz,
Ph of neutral Cr), 128.7 (m, Ph of Cr+), 129.1 (br s, Ph of Cr+),
[trans-Cr(CO)2(PMe2Ph)2]I (7)d
129.8 (s, Ph of neutral Cr), 130.6 (s, Ph of Cr+), 131.6 (d, J PC
)
8 Hz, Ph of neutral Cr), 136.8 (d, J PC ) 46 Hz, Ph of Cr+), 137.0
(d, J PC ) 43 Hz, Ph of neutral Cr), 245.8 (t, J PC ) 50 Hz, CO of
Cr+), 251.4 (d, J PC ) 7 Hz, CO cis to I), 259.3 (d, J PC ) 55 Hz, CO
trans to I)
a
b
d
Acetone-d6. C6D6. c Toluene-d8. DMSO-d6.
the remaining oil was shaken with 100 mL of hexane to give
a yellow gum. After the liquid phase was decanted, the sticky
material was dried under reduced pressure. It was then
washed with 50 mL portions of ether until colorless washings
were obtained, and the resulting yellow solid was thoroughly
dried again to yield ∼18 g of air-sensitive, impure (Et4N)2-
[FvCr2(CO)6]. This product contained insoluble byproduct
(LiBr) which could be separated, if necessary, by extraction
with THF followed by precipitation with hexane; traces of THF
and ether could not be completely removed. However, both
impure and purified products were satisfactory starting ma-
terials for further syntheses.
F vCr 2(CO)6H2 (2a ). Compound 2a was typically generated
in situ from 2.0 g of impure (Et4N)2[FvCr2(CO)6] (containing
∼1.62 mmol of the dianion) and 0.6 mL of glacial acetic acid
in 75 mL of toluene at room temperature. The dark yellow
toluene solution was filtered into another flask, diluted with
25 mL of hexane, and used immediately.
Rea ction of 2a w ith Isop r en e. A 0.6 mL acetone-d6
solution of 1 (10 mg, 1.5 × 10-3 mmol) was treated with a drop
of glacial acetic acid to generate hydride 2a , and a 1H NMR
spectrum was recorded. Into this solution was injected 10 µL
(0.1 mmol) of isoprene at room temperature, resulting in rapid
darkening of the solution and complete transformation of 2a

FvCr2(CO)4L2 Metal-Metal-Bonded Compounds
Organometallics, Vol. 15, No. 16, 1996 3591
δ (ppm)
Ta ble 4. 31P {1H} NMR Da ta
compd
FvCr2(CO)4(PMe2Ph)2H2 (2b)
FvCr2(CO)4(PMe2Ph)2H2 (2b)
FvCr2(CO)4(PMe3)2H2 (2c)
FvCr2(CO)4(PMe3)2H2 (2c)
60.2a
61.2 (br), 61.3 (br)a,b
48.2c
48.80, 48.84 (both cis,cis), 48.9 (cis,trans), 49.05
(cis,cis), 50.64 (cis,trans), 50.69 (trans,trans)b,c
cis-FvCr2(CO)4(PMe2Ph)2HI
cis,cis-FvCr2(CO)4(PMe2Ph)2I2 (3b)
cis-FvCr2(CO)4(PMe2Ph)2HBr
cis,cis-FvCr2(CO)4(PMe2Ph)2Br2 (4)
cis-FvCr2(CO)4(PMe2Ph)2HCl
cis,cis-FvCr2(CO)4(PMe2Ph)2Cl2 (5)
trans,trans-FvCr2(CO)4(PMe2Ph)2Cl2 (5)
trans-FvCr2(CO)4(PMe2Ph)2 (6a )
trans-FvCr2(CO)4(PMe3)2 (6b)
31.4, 59.1 (br)c
31.15, 31.20;c 33.17, 33.20a
35.5, 59.0d
35.2;d 35.16, 35.25;c 37.4a
39.4, 59.1d
39.6d
62.8d
66.0b,c
57.4b,c
cis-PMe2Ph(CO)2ICr(µ-Fv)[trans-Cr(CO)2(PMe2Ph)2]I (7)
cis-PMe2Ph(CO)2ICr(µ-Fv)[Cr(CO)3PMe2Ph]I
cis-FvCr2(CO)5(PMe2Ph)I2
32.9 (1P), 51.1 (2P)
32.7 (1P), 42.0 (1P)
33.0
(CO)3ICr(µ-Fv)[Cr(CO)3PMe2Ph]I
41.7
a
b
d
In acetone-d6. At -50 °C. c In toluene-d8. In CDCl3.
into FvCr2(CO)6, identified by a second 1H NMR spectrum,
which exhibited resonances identical with those previously
reported.10 IR (THF): νCO 2013 (s), 1951 (vs), 1933 (s), 1911
(sh), 1890 (sh) cm-1. (Lit.10 IR (KBr): 2000 (s), 1920 (vs), 1900
(vs) cm-1.) 1H NMR (acetone-d6): δ 4.53 (“t”, 4H, Fv), 5.24 (“t”,
4H, Fv). (Lit.10 1H NMR: δ 4.54, 5.25.) 13C{1H} NMR (acetone-
d6): δ 84.5 (s, Fv), 88.3 (s, Fv), 92.1 (s, C-1 Fv). No CO carbon
resonance could be identified.
characterized spectroscopically. High-resolution FAB(+)-
MS: m/ z 497.017 167, calcd for [M - H]+ 497.016 428. EI-
MS: m/ z (%) (no [M]+ appeared) [M - 2H]+ ) 496 (2.8), [M -
2H - 2CO]+ ) 440 (7.2), [M - 2H - 4CO]+ ) 384 (4.4), [M -
2H - PMe3 - 3CO]+ ) 336 (3.9), [M - 2H - 2PMe3 - CO]+
)
316 (3.4), [M - 2H - PMe3 - 4CO]+ ) 308 (13.4), [M - 2H -
2PMe3 - 2CO]+ ) 288 (3.6), [M - 2H - 2PMe3 - 3CO]+
260 (21.0), [M - 2H - 2PMe3 - 4CO]+ ) 232 (3.8), [FvCr]+
)
)
180 (15.5), [Fv]+ ) 128 (100), [OdPMe3]+ ) 92 (21.0), [PMe3]+
F vCr 2(CO)4(P Me2P h )2H2 (2b). To a stirred solution of 2a ,
prepared as described above, was added 0.80 mL of PMe2Ph
(5.6 mmol), resulting in gas evolution and slow formation of a
yellow precipitate (1 day). When no more 2a was present in
the reaction mixture (IR), the supernatant solution was
separated, concentrated under reduced pressure to ∼5 mL, and
diluted with 20 mL of hexane to give a second crop of solid
material. The combined precipitates were washed with 3 ×
10 mL of hexane and dried under reduced pressure to yield
0.74 g (1.19 mmol, 73% based on Cr(CO)3(EtCN)3) of a yellow
powder, which was recrystallized from a 2:1 THF-hexane
solvent mixture. Complex 2b was indefinitely stable in the
solid state but slowly decomposed in solution to an unidentified
green precipitate. It is soluble in many aromatic and polar
solvents. Anal. Calcd for C30H32Cr2O4P2: C, 57.88; H, 5.18.
Found: C, 57.30, H, 5.33. High-resolution FAB(+)-MS: m/ z
621.050 089, calcd for [M - H]+ 621.050 781. EI-MS: m/ z (%)
(no [M]+ appeared) [M - 2H - CO]+ ) 592 (0.2), [M - 2H -
2CO]+ ) 564 (0.8), [M - 2H - 3CO]+ ) 538 (2.2), [M - 2H -
4CO]+ ) 508 (0.5), [M - 2H - PMe2Ph]+ ) 482 (0.4), [M - 2H
- PMe2Ph - CO]+ ) 454 (0.2), [M - 2H - PMe2Ph - 2CO]+
) 426 (0.2), [M - 2H - PMe2Ph - 3CO]+ ) 398 (0.2), [M -
2H - PMe2Ph - 4CO]+ ) 370 (3.3), [FvCrH]+ ) 181 (11), [Fv]+
) 76 (61.3). 1H NMR (toluene-d8, -50 °C): δ -6.82 (d, J PH
)
93 Hz, CrH, cis), -6.77 (d, J PH ) 92 Hz, CrH, cis), -5.53 (d,
J PH ) 28 Hz, CrH, trans), 0.85 (br d, PMe), 4.09-4.54 (br m,
Fv). 13C{1H} NMR (toluene-d8, 25 °C): δ 83.0 (s, Fv), 84.2 (s,
Fv), 100.0 (s, C-1 Fv). No CO or PMe resonances were
observed.
F vCr 2(CO)6I2 (3a ). This compound was prepared previ-
ously by iodine cleavage of FvCr2(CO)6, but no experimental
data were given.11 We have developed alternative methods
of preparation, as follows. Into a vigorously stirred solution
of 2a , prepared as above, were injected 2 mL (16.9 mmol) of
ethyl iodoacetate, and stirring was continued for 3 h in the
dark. The reaction mixture was then decanted to leave a fine
solid, which was washed with 3 × 10 mL of hexane and dried
under reduced pressure. Yield: 0.67 g (1.02 mmol, 63% based
on Cr(CO)3(EtCN)3). Compound 3a is a light-sensitive, rust
brown solid which is poorly soluble in nonpolar organic
solvents. It was stored at -20 °C in the dark. Anal. Calcd
for C16H8Cr2I2O6: C, 29.38; H, 1.23. Found: C, 29.95; H, 1.48.
Formation of ethyl acetate was verified by NMR spectros-
copy when the reaction of 2a with excess ethyl iodoacetate was
carried out in toluene-d8. 1H NMR of ethyl acetate: δ 0.95 (t,
) 128 (33). 1H NMR (toluene-d8, -50 °C): δ -6.43 (d, J PH
)
J HH ) 7 Hz, 3H, MeCH2), 1.65 (s, 3H, MeCO), 3.88 (q, J HH )
89 Hz, CrH, cis), -6.39 (d, J PH ) 89 Hz, CrH, cis), -5.43 (d,
J PH ) 26 Hz, CrH, trans), 1.06 (br s, PMe), 1.20 (br s, PMe),
3.94-4.23 (br m, Fv), 7.01 (br s, Ph). 13C{1H} NMR (toluene-
d8, 25 °C): δ 83.7 (s, Fv), 84.8 (s, Fv), 99.9 (s, C-1 Fv), 128.5
(d, J PC ) 8.5 Hz, m-Ph), 129.2 (s, p-Ph), 129.6 (d, J PC ) 9 Hz,
o-Ph), 143.0 (d, J PC ) 36 Hz, ipso-Ph) (no CO or PMe
resonances were observed). 13C{1H} NMR (acetone-d6, -60
°C): δ 21.8 (d, J PC ) 31 Hz, PMe), 22.5 (br d, J PC ) 33 Hz,
PMe), 82.6, 83.4, 84.3, 85.5, 85.7, 85.9 (s, all Fv), 100.0, 100.5
(s, both C-1 Fv), 128.9 (d, J PC ) 8 Hz, m-Ph), 129.7 (s, p-Ph),
130.1 (d, J PC ) 8 Hz, o-Ph), 142.5 (d, J PC ) 42 Hz, ipso-Ph),
243.7 (d, J PC ) 6.5 Hz, CO cis to H), 251.4 (d, J PC ) 42 Hz, CO
trans to H).
F vCr 2(CO)4(P Me3)2H2 (2c). Compound 2c was obtained
as an olive green solid by adding 0.7 mL of PMe3 (6.8 mmol)
to a solution of 2a . Yield: 0.50 g (1.00 mmol, 62% based on
Cr(CO)3(EtCN)3). It was found to be more soluble in common
organic solvents and less stable than 2b, and thus recrystal-
lization resulted in substantial loss and an analytically pure
sample could not be isolated. The compound was therefore
7 Hz, 2H, CH2). While the fulvalene resonances of 3a were
observed at δ 4.14 (“t”, 4H) and 4.41 (“t”, 4H), this compound
quickly precipitated from solution. The anticipated intermedi-
ate FvCr2(CO)6HI was not detected.
Compound 3a was also prepared by treating 0.02 g (0.05
mmol) of FvCr2(CO)6 in 10 mL of THF with 20 µL (0.16 mmol)
of ethyl iodoacetate at room temperature. The mixture was
stirred under fluorescent lights, and IR spectroscopic monitor-
ing showed slow transformation of the starting dimer to 3a
as the sole carbonyl-containing product. The reaction was
complete in ∼1 day. No attempt was made to isolate 3a from
this mixture.
F vCr 2(CO)4(P Me2P h )2HX (X ) Cl, Br , I). Reactions of
2b with ∼10-fold excesses of CCl4, CHBr3, or ICH2CO2Et were
carried out at room temperature with NMR-scale samples
containing ∼5 mg (8.0 × 10-3 mmol) of 2b dissolved in 0.6 mL
of toluene-d8 or CDCl3 and 10 µL of alkyl halide. Transient
formation of the hydrido halide intermediates FvCr2(CO)4(PMe2-
Ph)2HX (X ) I, Br, Cl, respectively) was detected in situ by
1H NMR spectroscopy in the characteristic fulvalene proton

3592 Organometallics, Vol. 15, No. 16, 1996
Kova´cs and Baird
region (the CrH, PMe, and Ph resonances extensively over-
lapped with those of 2b and the product dihalides). The
identity of these intermediates was supported by the transient
31P{1H} NMR spectra, which featured two singlet resonances
of equal intensity in the upfield halide and downfield hydride
region,1a,b both being different from the resonances of 2b and
the corresponding dihalide. The mixed hydrido halides were
found to readily react further with the alkyl halides to give
the expected dihalo complexes FvCr2(CO)4(PMe2Ph)2X2 (X ) I
(3b), Br (4), Cl (5)) and completely disappeared after ∼1 h.
When less reactive alkyl halides such as t-BuI and BzBr were
used, the intermediates remained longer, but their formation
from 2b was also slow and the reactions were difficult to
monitor because of decomposition and precipitation of the
unstable and poorly soluble dihalides. In view of the transient
nature of the hydrido halides, they could not be isolated and
further characterized.
F vCr 2(CO)4(P Me2P h )2X2 (X ) I (3b), Br (4), Cl (5)). The
dibromide compound 4 and dichloride 5 proved to be very
unstable at room temperature and under ambient light and
were therefore characterized only in situ by 1H and 31P{1H}
NMR and IR spectroscopy. In contrast, 3b was isolated in
analytically pure form as the cis,cis isomer and characterized
spectroscopically. Thus, data for cis,cis-3b were used for
comparisons to support the identities of the less stable analogs
4 and 5.
Compound 3b was isolated as a purple-brown microcrys-
talline solid from the reaction of 2b (0.30 g, 0.47 mmol) with
1.5 mL (12.7 mmol) of ethyl iodoacetate in 30 mL of toluene
at room temperature and in the dark for 4 h. After the liquid
phase had been removed by syringe, the solid product was
washed with 3 × 10 mL of hexane and dried under reduced
pressure, yielding 0.33 g (79%) of cis,cis-3b. It is soluble only
in THF and is light-sensitive even in the solid state, although
it can be stored in the dark at -20 °C without considerable
degradation. Anal. Calcd for C30H30Cr2I2O4P2: C, 41.21; H,
3.46. Found: C, 40.93; H, 3.48. Compound 3b was also
formed in an NMR-scale experiment by adding a solution of
I2 in toluene-d8 dropwise to ∼5 mg (8.0 × 10-3 mmol) of 2b
dissolved in 0.6 mL of toluene-d8. 1H NMR spectra of the
reaction mixture showed instant formation of a mixture of
FvCr2(CO)4(PMe2Ph)2HI and 3b with less than 2 molar equiv
of iodine, while 3b was the sole product when the reaction was
completed.
tunately, the compound decomposed extensively during the
experiment, particularly above 50 °C, and no quantitative
information could be acquired, although the conversion was
clearly shown to be reversible. Decomposition could be due
to the thermal sensitivity of 6a or its radical isomer but might
be facilitated by a reaction between the radicals and the cell
window (NaCl) as well. It has been mentioned previously that
CpCr(CO)3 radicals react with CaF2 windows.5d
For multinuclear NMR studies, typically 0.02 mmol of each
of the starting hydrides was dissolved in 0.6 mL of toluene-d8
or acetone-d6 and a previously prepared solution of the trityl
dimer in the appropriate solvent was added dropwise by
microsyringe. Completion of the reactions was monitored by
1H NMR spectroscopy, which showed formation of 6a and 6b
as the sole fulvalene compounds formed, along with 2 molar
equiv of Ph3CH (δ 5.37 (CH)); no unreacted trityl dimer
remained. NMR data for 6a : 13C{1H} NMR (acetone-d6, 25
°C) δ 82.6 (s, Fv), 88.4 (s, Fv), 129.3, 130.0, 130.6 (all s, Ph)
(no quaternary and PMe carbon resonances appeared); 13C-
{1H} NMR (acetone-d6, -50 °C) δ 18.7 (d, J PC ) 32 Hz, PMe),
82.5 (s, Fv), δ 87.3 (s, C-1 Fv), 88.1 (s, Fv), 129.0, 129.7, 129.8
(s, all Ph), 143.8 (d, J PC ) 33.5 Hz, ipso-Ph), 254.8 (d, J PC
)
35 Hz, CO). 13C{1H} NMR for 6b (toluene-d8, 25 °C): δ 81.8
(s, Fv), 85.1 (s, Fv), 87.3 (s, C-1 Fv), 253.6 (d, J PC ) 45 Hz,
CO) (no PMe resonance appeared).
In an approach complementary to that reported above, 0.02
mmol of 2b or 2c was mixed with 10 µL (0.09 mmol) of 1,3,5-
hexatriene or 30 µL (0.3 mmol) of isoprene in the dark. The
reaction mixtures changed color to yellow-brown, but while
1,3,5-hexatriene reacted instantaneously, the reaction with
isoprene took ∼0.5 h to be completed, in accord with the
reported difference in reactivity of these conjugated systems
toward CpM(CO)3H (M ) Cr, Mo, W).13 The products of these
reactions, i.e. 6a and 6b, were identical with those obtained
via the above hydride hydrogen atom abstraction by trityl
radicals, as shown by IR and 1H, 31P{1H}, and 13C{1H} NMR
spectroscopy.
Rea ction s of 6a w ith Alk yl Ha lid es. Freshly prepared
toluene (3 mL) solutions of 6a (13 mg, 0.02 mmol), prepared
as described above, were treated in the dark with 10-fold molar
excesses (∼10 µL) of the following alkyl halides: CCl4, CHBr3,
BrCH2CN, ICH2CO2Et, allyl iodide, benzyl bromide, BrCH2-
CO2Me, ethyl iodide, isopropyl iodide, tert-butyl iodide. The
reaction mixtures changed color to red in a few seconds with
CCl4, CHBr3, BrCH2CN, ICH2CO2Et, and allyl iodide, and in
these cases complete consumption of 6a was established by
IR spectroscopy. The reaction products were solely the cor-
responding dihalides 3b, 4, and 5 for allyl iodide, CHBr3, and
CCl4, respectively, all being identified on the basis of compari-
sons with IR data of the independently characterized dihalides.
In the reaction of 6a with BrCH2CN, dibromide 4 was the
major product, although the presence of another significant
product was also apparent from the IR spectrum (νCO 1943
and 1872 cm-1). The 31P{1H} NMR spectrum (toluene-d8) of
a similar reaction mixture exhibited, in addition to strong
resonances of cis,cis-4 at δ 35.16 and 35.25, several weaker
resonances at δ ∼48.1-48.6 and at δ 34.45, 34.95, and 35.15.
For IR spectroscopic characterization, 10 mg (1.6 × 10-2
mmol) of 2b was dissolved in 2 mL of CH2Cl2, a ∼20-fold excess
of CCl4, CHBr3, or ICH2CO2Et was added at room temperature
and in the dark, and the spectra were recorded after ∼0.5 h;
only absorbances of the dihalides were observed.
The hydrogen-halogen exchange reaction of 2c (∼5 mg, 0.01
mmol) with 10 µL (0.11 mmol) of CHBr3 was briefly investi-
gated, FvCr2(CO)4(PMe3)2Br2 being obtained. IR (CH2Cl2):
1967 (vs, br), 1877 (vs) cm-1
.
1H NMR of the cis,cis isomer
(toluene-d8): δ 1.17 (br d, J PH ) 9 Hz, 18H, PMe), 3.79, 3.85,
4.16, 4.29, 4.33, 4.36, 4.89, 5.19 (m, 1H, all Fv).
F vCr 2(CO)4L2 (L ) P Me2P h (6a ), P Me3 (6b)). These
compounds were investigated only in solution, since all at-
tempts to isolate them resulted in extensive decomposition to
unidentified species, as indicated by color changes to greenish
brown, formation of insoluble precipitates, and broadening of
the 1H NMR resonances. For IR spectroscopic studies, typi-
cally 0.02 mmol of the corresponding dihydride (2b, 2c) was
dissolved in 3 mL of benzene or toluene and the solution
treated with an equivalent amount of the trityl dimer (0.1 M
solution in the same solvents) in the dark. The reaction
mixtures changed color from yellow to yellow-brown in a few
seconds, and IR analyses performed after ∼15 min indicated
complete consumption of the starting materials and formation
solely of the products.
A
1H NMR spectrum of this solution exhibited strong reso-
nances of cis,cis-4, the major product, in addition to many other
weak, broad, overlapping resonances. Similar results were
obtained with ICH2CO2Et; in this case the IR spectrum
exhibited strong carbonyl bands of 3b and absorptions of a
minor product at 1938 and 1863 cm-1
.
The 31P{1H} NMR
spectrum (toluene-d8) exhibited, in addition to resonances of
3b, weaker resonances at δ ∼50.8-51.0 and at δ 30.8, 31.3.
Benzyl bromide, BrCH2CO2Me, and tert-butyl iodide reacted
with 6a in ∼4 h, but the reaction mixtures decomposed during
this time and only the formation of cis,cis-3b, from t-BuI, could
1
be verified by IR and H and 31P{1H} NMR. No reaction was
observed with ethyl and isopropyl iodide over ∼4 h.
Variable-temperature IR spectroscopic studies carried out
on 6a in the temperature range 25-80 °C showed a decrease
of the absorbances of 6a and formation of a new species (νCO
1949, 1800 (br) cm-1) as the temperature increased. Unfor-
(13) Miyake, A.; Kondo, H. Angew. Chem., Int. Ed. Engl. 1968, 7,
880.

FvCr2(CO)4L2 Metal-Metal-Bonded Compounds
Organometallics, Vol. 15, No. 16, 1996 3593
ci s-(P Me 2P h )(C O )2I C r (µ-F v )[t r a n s-C r (C O )2(P Me 2-
P h )2]I (7). To a stirred solution of 0.20 g (0.31 mmol) of FvCr2-
(CO)6I2 in 20 mL of THF was quickly added 0.44 mL (3.1 mmol)
of PMe2Ph, resulting in rapid gas evolution. The reaction
mixture was stirred for ∼1 h at room temperature in the dark,
resulting in formation of a crystalline precipitate. Precipita-
tion was completed by adding 20 mL of hexane to the reaction
mixture, the supernatant liquid was decanted and the remain-
ing solid was washed with 5 × 10 mL of hexane and dried
under reduced pressure. Yield: 0.23 g (0.23 mmol, 73%).
Compound 7 is a purple-brown, crystalline powder which is
insoluble in acetone or acetonitrile but of limited solubility in
THF and DMSO, giving cherry red solutions. Anal. Calcd for
hydrogen atom abstraction from FvCr2(CO)4L2H2 (eq 1)
and/or reduction of FvCr2(CO)4L2I2, as demonstrated
previously for molybdenum-containing analogues.1a,b
Indeed, substitution of the cyclopentadienyl analogues
CpCr(CO)3X (X ) H, I) with phosphines as bulky as
PPh3 has been reported6a,14 and similar behavior of the
fulvalene complexes was anticipated.
The anionic complex (Et4N)2[FvCr2(CO)6] (1) was
synthesized from Cr(CO)3(EtCN)3, Li2C10H8, and Et4-
NBr in a “one pot” reaction (eq 3). This procedure is
C
38H41Cr2I2O4P3: C, 45.08; H, 4.08. Found: C, 44.66; H, 4.28.
FAB(+)-MS: m/ z (%) [M]+ ) 885 (3), [M - 2CO]+ ) 829 (9),
[M - I]+ ) 758 (4), [M - PMe2Ph - 2CO]+ ) 691 (43), [M -
PMe2Ph - 4CO]+ ) 635 (44), [M - I - PMe2Ph - 2CO]+
)
564 (7), [M - 2PMe2Ph - 4CO]+ ) 497 (100), [M - 3PMe2Ph
- 4CO]+ ) 359 (81).
(3)
In ter m ed ia tes in th e Rea ction of F vCr 2(CO)6I2 w ith
P Me2P h . A 0.03 g (0.05 mmol) amount of FvCr2(CO)6I2 was
dissolved in 5 mL of THF, and 8 µL (0.05 mmol) of PMe2Ph
was added dropwise at room temperature. An IR spectrum
of the reaction mixture taken at this point exhibited strong,
similar to that developed by us to obtain (Et4N)2[FvMo2-
(CO)6]1a and is based on the method of Smart and Curtis
for the in situ synthesis of Li2[FvMo2(CO)6].15 Com-
pound 1 was obtained in large quantities and in good
yield; although air-sensitive, it can be stored for long
periods under nitrogen. It was fully characterized in
broad carbonyl bands at 2033, 1969 (sh), 1956, and 1882 cm-1
.
After addition of an extra 20 µL (0.10 mmol) of PMe2Ph, only
those at 1956 and 1878 cm-1 remained (the latter was slightly
shifted from 1882 cm-1). Solvent was evaporated under
reduced pressure, and the remaining solid was redissolved in
acetone-d6 for NMR investigations; 7 was identified as the only
product. 1H NMR (acetone-d6): δ 2.18 (d, J PH ) 8 Hz, 3H,
PMe), 2.20 (d, J PH ) 9 Hz, 12H, PMe), 2.21 (d, J PH ) 8 Hz,
3H, PMe), 4.73, 4.81, 4.85, 4.93, 5.07, 5.12 (m, 1H, all Fv), 5.43
(m, 2H, Fv), 7.43 (m, 3H, m,p-Ph), 7.56 (m, 6H, m,p-Ph), 7.71
(m, 2H, o-Ph), 7.80 (m, 4H, o-Ph).
1
solution by IR and H and 13C{1H} NMR spectroscopy,
and all data are consistent with the formulation. Thus,
the IR spectrum exhibits carbonyl absorbances at 1890,
1800, and 1717 cm-1, similar to those of the Cp analogue
(1877, 1765, 1735 cm-1 (KBr)),16 while the 1H NMR
spectrum exhibits, in addition to Et4N+ resonances, two
fulvalene multiplets at δ 4.13 and 4.61. The 13C{1H}
NMR spectrum exhibits three fulvalene resonances (δ
80.0, 81.7, 100.9) and a single resonance in the CO
In a separate experiment, 10 mg of FvCr2(CO)6I2 was mixed
with 0.6 mL of acetone-d6 in an NMR tube, resulting in only
partial dissolution of the solid. Into this mixture was injected
region at δ 247.4, identical with that of Et4N[CpCr-
2a,10,11
(CO)3].16 Finally, the known FvCr2(CO)6
was
formed as the oxidation product when solutions of 1
were exposed briefly to air, confirming the nature of 1.
As shown in eq 4, protonation of 1 with glacial acetic
acid readily afforded the unstable dihydride FvCr2-
(CO)6H2 (2a ), which was generated in situ for instant
1
µL of PMe2Ph, and formation of [(CO)3ICr](µ-Fv)[Cr-
(CO)3PMe2Ph]I was established by 1H and 31P{1H} NMR
spectroscopy. This compound readily transformed into a more
stable one in ∼30 min, and no attempt was made to isolate it.
It could be regenerated from the unreacted starting material
by addition of 1 µL of PMe2Ph every ∼30 min. During the
consumption of [(CO)3ICr](µ-Fv)[Cr(CO)3PMe2Ph]I, a steady
increase in the concentration of cis-[PMe2Ph(CO)2ICr](µ-Fv)-
[Cr(CO)3PMe2Ph]I was observed. This was characterized
again by 1H and 31P{1H} spectroscopy. When this sample was
kept at room temperature in the dark for ∼2 days (containing
unreacted FvCr2(CO)6I2), formation of a ca. 1:1 mixture of cis-
FvCr2(CO)5(PMe2Ph)I2 and cis,cis-FvCr2(CO)4(PMe2Ph)2I2
could be established. 1H NMR for cis-FvCr2(CO)5(PMe2Ph)I2
1
use and was characterized by IR and H and 13C{1H}
NMR spectroscopy. The IR spectrum exhibited CO
2- + 2H+ f
(4)
[FvCr2(CO)6]
FvCr2(CO)6H2
anion of 1
2a
absorptions at 2014.5, 1943 and 1940 cm-1, at consider-
ably higher frequencies than for 1 and very similar to
those of CpCr(CO)3H (2018, 1946, 1936 cm-1),6a while
(acetone-d6): δ 2.13 (d, J PH ) 9 Hz, 3H, PMe), 2.24 (d, J PH
)
1
9 Hz, 3H, PMe), 4.92, 5.18, 5.27, 5.44, 5.48, 5.67 (m, 1H, all
Fv), 5.78 (m, 2H, Fv). 1H NMR for cis,cis-FvCr2(CO)4(PMe2-
Ph)2I2 (acetone-d6): δ 2.11 (d, J PH ) 9 Hz, 6H, PMe), 2.21 (d,
J PH ) 9 Hz, 6H, PMe), 4.81, 4.91, 4.95, 5.03, 5.17, 5.23, 5.52,
5.67 (m, 1H, all Fv), 7.42 (m, 6H, m, p-Ph), 7.74 (m, 4H, o-Ph).
Attempts, however, to prepare [cis-PMe2Ph(CO)2ICr](µ-Fv)[Cr-
(CO)3PMe2Ph]I in pure form failed in THF, as mixtures with
cis,cis-FvCr2(CO)4(PMe2Ph)2I2 and FvCr2(CO)6I2 were obtained.
the H NMR spectrum exhibited two fulvalene (δ 4.08,
4.34) multiplets and a CrH (δ -5.46) resonance. The
13C{1H} NMR spectrum exhibited three fulvalene reso-
nances (δ 83.7, 85.9, 99.8) and a CO resonance (δ 234.7);
the CO chemical shift is very similar to that of CpCr-
(CO)3H (δ 233.5, 242.0 at -80 °C).17 Furthermore, when
2a was treated with an excess of isoprene at room
temperature, rapid formation of FvCr2(CO)6, character-
istic of the hydridic nature of 2a , was formed. Similar
reactions of CpCr(CO)3H with conjugated dienes, lead-
ing solely to [CpCr(CO)3]2, have been reported.3,6a,13
Phosphine substitution reactions of 2a were at-
tempted with PPh3, PPh2Me, PMe2Ph, and PMe3 with,
Resu lts a n d Discu ssion
Syn th esis, Ch a r a cter iza tion , a n d Rea ction s of
(Et4N)2[F vCr 2(CO)6] (1), F vCr 2(CO)6H2 (2a ), a n d
F vCr 2(CO)6I2 (3a ). These compounds were synthe-
sized in order to secure easily available starting materi-
als for preparation of the phosphine-substituted com-
plexes FvCr2(CO)4L2X2 (X ) H, I). It was anticipated
that the latter would in turn serve as precursors for the
syntheses of the compounds FvCr2(CO)4L2 via hydride
(14) Hackett, P.; O’Neill, P. S.; Manning, A. R. J . Chem. Soc., Dalton
Trans. 1974, 1625.
(15) Smart, J . C.; Curtis, C. J . Inorg. Chem. 1977, 16, 1788.
(16) Behrens, U.; Edelmann, F. J . Organomet. Chem. 1984, 263, 179.
(17) Alt, H. G.; Engelhardt, H. E.; Kla¨ui, W.; Mu¨ller, A. J . Orga-
nomet. Chem. 1987, 331, 317.

3594 Organometallics, Vol. 15, No. 16, 1996
Kova´cs and Baird
respectively, decreasing cone angles and, consequently,
decreasing steric requirements.18 Unfortunately, de-
composition to FvCr2(CO)6 rather than substitution took
place with PPh3 and PPh2Me in refluxing hexane.
However, the smaller phosphines PMe2Ph and PMe3
readily reacted with 2a at room temperature, affording
the symmetrically substituted compounds FvCr2(CO)4-
L2H2 (L ) PMe2Ph (2b), PMe3 (2c)) (eq 5), which were
FvCr2(CO)6H2
+ 2L f
2a
FvCr2(CO)4L2H2
2b (L ) PMe2Ph), 2c (L ) PMe3)
+ 2CO (5)
isolated and characterized (see below). These results
show that the chromium compound 2a exhibits reactiv-
ity similar to that of the tungsten analogue FvW2-
19
(CO)6H2 but lower than that of the molybdenum
analogue FvMo2(CO)6H2.1a
Hydrogen-halogen exchange reactions of 2a with
CCl4, CHBr3, and ICH2CO2Et were subsequently at-
1
F igu r e 1. Stacked variable-temperature H NMR spectra
of compound 2b in the hydride region in toluene-d8.
1a
identical conditions;1a the byproducts of this reaction
were not investigated further.
Ch ar acter ization of FvCr 2(CO)4L2H2 (L ) P Me2P h
(2b), P Me3 (2c)). These compounds were isolated in
good yields by direct substitution of 2a with the corre-
sponding phosphine. They are fairly stable as solids and
tempted but, unlike the case for FvMo2(CO)6H2 and
FvW2(CO)6H2,19 no reaction occurred with CCl4 and
CHBr3 at room temperature; only slow (1 day) degrada-
tion of 2a to FvCr2(CO)6 was observed. However,
formation of FvCr2(CO)6I2 (3a ) from ICH2CO2Et readily
occurred (eq 6), and the dihalide was isolated and fully
1
were characterized by high-resolution MS, IR, and H,
FvCr2(CO)6H2
+ 2ICH2CO2Et f
13C{1H}, and 31P{1H} NMR techniques, as well as by
elemental analysis for 2b. While the room-temperature
1H NMR spectra of 2b and 2c were much as expected,
with doublets in both the PMe and hydride regions and
a pair of virtual quartets in the fulvalene region, no CO
or PMe carbon resonances could be observed in the 13C-
{1H} NMR spectra, although resonances of the three
different fulvalene carbons in both 2b and 2c and all
the Ph carbons in 2b were present. Similar observa-
tions have been made with FvMo2(CO)4(PPh3)2H2, which
was shown by variable-temperature 1H and 31P{1H}
NMR spectroscopy to be undergoing rapid exchange
between cis and trans isomers.1a A brief variable-
temperature NMR study of the complex CpCr(CO)2-
(PMe3)H has also been reported.17
2a
FvCr2(CO)6I2
+ 2MeCO2Et (6)
3a
1
characterized by IR and H and 13C{1H} NMR spectros-
copy and by elemental analyses. The IR spectrum
exhibited CO absorptions at 2026, 1972, and 1951 cm-1
as anticipated at considerably higher frequencies than
for 2a , while the 1H NMR spectrum exhibited two
fulvalene (δ 5.54, 5.90) multiplets. The 13C{1H} NMR
spectrum exhibited three fulvalene resonances (δ 91.2,
92.1, 103.2) and two CO resonances at δ 235.9 (cis to I)
and 247.5 (trans to I) in a 2:1 ratio, assigned on the basis
1a
of precedents established previously for FvMo2(CO)6X2
and CpMo(CO)3X20 (X ) Cl, Br, I). The presence of the
byproduct, ethyl acetate, was confirmed by 1H NMR
spectroscopy.
We therefore carried out variable-temperature (+25
to -60 °C) 1H, 13C{1H}, and 31P{1H} NMR investigations
of 2b and 2c in order to study possible isomerization
processes and in so doing found that the FvCr(CO)2LH
moieties exist in cis and trans isomeric forms. The
results of the variable-temperature 1H NMR experiment
with 2b are illustrated in the stacked spectra in Figure
1, which demonstrate the dynamic behavior of this
compound. As can be seen, the somewhat asymmetric
hydride doublet resonances broaden significantly below
room temperature and eventually decoalesce into at
least three doublets.
Compound 3a was also produced in the reaction of
FvCr2(CO)6 with ICH2CO2Et. Formation of 3a in this
case probably results from iodine atom abstraction from
ICH2CO2Et by chromium-centered radicals generated
either by heat or by fluorescent lighting and provides
additional indirect evidence for the existence of the
biradical isomer of FvCr2(CO)6.2b
A reaction of 3a with PPh3 was attempted as a
possible route to substituted (fulvalene)chromium com-
plexes containing bulky phosphines. However, when a
toluene solution of 3a containing an excess of PPh3 was
heated to 110 °C for 10 min, 3a completely decomposed
to FvCr2(CO)6 (IR, NMR); no substitution was observed.
Note that similar but only partial decomposition of
FvMo2(CO)6X2 (X ) Cl, Br, I) was also observed under
Although the 1H NMR spectra of 2b and 2c were
similar in the Cr-H region at -50 °C, we refer here to
the better resolved spectrum of 2c (Figure 2) to discuss
the most plausible assignments (see ref 1a or 19 for
schemes showing possible isomers). On the basis of
characteristic values of the P-H coupling constants for
isomers of CpCr(CO)2(PMe3)H17 and FvMo2(CO)4L2H2
(L ) PPh3, PMe3),1a,19 the two equal upfield (J PH ) 92
Hz) doublets probably belong to cis,cis isomers (meso
and dl) while the weak downfield doublet (J PH ) 28 Hz)
may represent the trans half of the cis,trans isomer. As
(18) Dias, P. B.; Minas de Piedade, M. E.; Martinho Simo˜es, J . A.
Coord. Chem. Rev. 1994, 135/ 136, 737.
(19) Tilset, M.; Vollhardt, K. P. C.; Boese, R. Organometallics 1994,
13, 3146.
(20) Todd, L. J .; Wilkinson, J . R.; Hickey, J . P.; Beach, D. L.; Barnett,
K. W. J . Organomet. Chem. 1978, 154, 151.

FvCr2(CO)4L2 Metal-Metal-Bonded Compounds
Organometallics, Vol. 15, No. 16, 1996 3595
spectrum of CpMo(CO)2(PMe2Ph)I,21c or to the ligands
of different meso- and dl-cis,cis isomers.
Hyd r id e Hyd r ogen Atom Abstr a ction fr om 2b
a n d 2c by Tr ityl Ra d ica ls. Ch a r a cter iza tion of
F vCr 2(CO)4L2 (L ) P Me2P h (6a ), P Me3 (6b)). Com-
pounds 2b and 2c were both treated with 2 equiv of the
trityl radical at room temperature, resulting in char-
acteristic color changes from yellow to yellow-brown and
complete consumption of the starting materials along
with formation of triphenylmethane (1H NMR) and a
single organometallic product, e.g. 6a and 6b, respec-
tively (eq 7). The products were thoroughly character-
F igu r e 2. Cr-H proton resonances of compound 2c at -50
°C in toluene-d8.
+ 2Ph C• f
FvCr2(CO)4L2H2
3
2b, 2c
the shoulders evidently indicate, resonances attribut-
able by default to a trans,trans and cis half of the
cis,trans isomer remain hidden underneath the much
stronger peaks. Similar assignments can be made for
2b. Integrations of the resonances were consistent with
a 77:23 cis:trans ratio for 2b and an 85:15 ratio for 2c;
these differ considerably (in favor of the cis form) from
those of FvM2(CO)4L2H2 (M ) Mo, W; L ) PMe3, PMe2-
Ph)19 and CpM(CO)2LH (M ) Cr, Mo, W; L ) PMe3,
PMe2Ph),17,21 which typically appear in cis:trans ratios
close to 1. It is also interesting to note that there are
separate hydride resonances for the meso and dl forms
of the cis,cis isomers of 2b and 2c.
FvCr2(CO)4L2
+ 2Ph3CH (7)
6a (L ) PMe2Ph), 6b (L ) PMe3)
ized in solution by spectroscopic means but were too
unstable to isolate pure. However, the analogous reac-
tions of FvMo2(CO)4L2H2 (L ) CO, PMe3, PPh3, PXy3)
have been shown to afford the molybdenum-molybde-
num-bonded compounds trans-FvMo2(CO)4L2 (contain-
ing both phosphines trans to the Mo-Mo bond) as the
sole organometallic products,1b and similar results for
the reactions of 2b and 2c would be anticipated. Indeed,
the IR spectra of the reaction mixtures exhibited pat-
terns of carbonyl absorbances very similar to those of
trans-FvMo2(CO)4L2, suggesting trans structures for 6a
and 6b and, consequently, a metal-metal bond; medium
and very strong bands were observed at 1925 and 1851
cm-1, respectively, for 6a and at 1923 and 1849 cm-1
for 6b. While the frequencies of these absorbances are
almost identical with those of 2b and 2c, respectively,
the carbonyl bands of the two pairs of compounds 2b
and 2c versus 6a and 6b exhibit significantly different
relative intensities.
Consistent with these interpretations, the single 31P-
{1H} resonances observed for 2b and 2c at room
temperature decoalesced at lower temperatures to two
distinct, equal cis,cis resonances, at δ 61.2 and 61.3 (-50
°C) for 2b and at δ 48.5 and 48.7 (-30 °C) for 2c,
although the weaker resonances of the minor isomers
could not be distinguished. Interestingly, the upfield
resonance of 2c further split into two closely spaced
singlets (δ 48.80 and 48.84) upon cooling to -50 °C,
probably attributable to the meso- and dl-cis,cis isomers.
For all previously reported examples of phosphine-
substituted hydrido fulvalene complexes FvM2(CO)4L2H2
(M ) Mo, L ) PMe3, PPh3; M ) W, L ) PMe3, PMe2-
Ph), low-temperature studies revealed the presence of
The UV-vis spectra of the products were also con-
sistent with structures containing metal-metal bonds,
since both exhibited an intense σ f σ* transition, at
1
460 and 455 nm, respectively. The H NMR spectra of
1
only one doublet in the H and one singlet in the 31P-
6a and 6b provided complementary structural informa-
tion, exhibiting for PMe, Fv, and Ph resonances with
appropriate integrals. In support of the identification
of the products as trans-FvCr2(CO)4L2 (L ) PMe2Ph
(6a ), PMe3 (6b)), they were also prepared via alternative
routes, i.e. by reacting 2b and 2c with 1,3,5-hexatriene
and isoprene. We have demonstrated previously that
2a readily hydrogenates isoprene to form the well-
known2a,10,11 FvCr2(CO)6, and analogous reactions of the
hydrides CpM(CO)3H (M ) Cr, Mo, W) with these
conjugated olefins are also known to give the corre-
sponding metal-metal-bonded dimers [CpM(CO)3]2 via
metal-centered radicals.3,6a,13
{1H} NMR spectra, assignable to both the meso- and
dl-cis,cis isomers.1a,19
The 13C{1H} NMR spectrum of 2b recorded at -60
°C was also consistent with the low-temperature 1H and
31P{1H} NMR observations. The resonances of the
major cis,cis (meso and dl) isomers were dominant and
exhibited CO and PMe resonances which were absent
in the room-temperature spectra, confirming the broad-
ening effects of dynamic behavior. Thus, resonances
attributable to all types of carbons present in 2b were
clearly visible, including two CO doublets at δ 243.7 (J PC
) 6.5 Hz) and 251.4 (J PC ) 42 Hz), which probably
indicate ligands cis and trans to the hydride ligand,
respectively, on the basis of the P-C coupling con-
stants.20 There were also observed two doublets at δ
21.8 and 22.5, which may be assigned to the nonequiva-
lent PMe carbon atoms. It is not evident at this point,
however, whether the two PMe doublets are to be
assigned to diastereotopic carbons of the same ligand
bonded to a chiral cis isomer, as reported for the proton
1
On the other hand, the room-temperature H NMR
spectra exhibited anomalous behavior not completely
consistent with these formulations of compounds 6a and
1
6b. Thus, while the H NMR spectrum of 6a exhibits
two fulvalene (δ 3.69, 3.92) and a PMe (δ 1.68) reso-
nance, all are broadened and the PMe proton resonance
1
does not exhibit coupling to phosphorus. The H NMR
spectrum of 6b was rather similar, the PMe3 resonance
being a poorly resolved doublet. The room-temperature
13C{1H} NMR spectrum of 6a exhibited only two tertiary
fulvalene resonances (δ 82.6, 88.4) but no resonances
for the PMe group or any of the quaternary carbon (CO,
(21) (a) Faller, J . W.; Anderson, A. S. J . Am. Chem. Soc. 1970, 92,
5852. (b) Kalck, P.; Poilblanc, R. J . Organomet. Chem. 1969, 19, 115.
(c) Faller, J . W.; Anderson, A. S.; J akubowski, A. J . Organomet. Chem.
1971, 27, C47.

3596 Organometallics, Vol. 15, No. 16, 1996
Kova´cs and Baird
ipso-Ph, C-1 Fv) sites. Similarly, while the room-
temperature 13C{1H} NMR spectrum of 6b exhibited a
CO doublet and three fulvalene resonances, no PMe
resonance was observed. In addition, the 31P{1H} NMR
spectra of neither 6a nor 6b exhibited resonances at
room temperature, and it thus seemed likely that
compounds 6a and 6b exists in rapid equilibrium with
one or more other isomers, possibly a paramagnetic
diradical species which would be responsible for exten-
sive line broadening in the observed, time-averaged
spectra.
(8)
In order to support this hypothesis, variable-temper-
ature IR and NMR investigations were carried out with
6a and 6b. When solutions of both were cooled below
room temperature, resonances in the 31P{1H} NMR
spectra appeared and, at -40 °C, the spectra exhibited
narrow singlet resonances at δ 66.0 and 57.4, respec-
tively (see ref 1c for figure). Similarly, the 13C{1H}
NMR spectrum of 6a was recorded at -50 °C and all of
the previously missing quaternary and PMe resonances
were observed. In particular, a CO doublet at δ 254.8
(J PC ) 35 Hz) suggested identical CO ligands being cis
to PMe2Ph, supporting a trans structure.1b The CO
carbon resonance of 6b, evident at room temperature
at δ 253.6 (d, J PC ) 45 Hz), was also consistent with a
trans structure.
temperature, and the paramagnetic, biradical isomers
are the predominant species at ∼80 °C.
No attempt was made to carry out solution magnetic
susceptibility or EPR experiments. The extent of dis-
sociation is too small for usefully accurate magnetic
susceptibilities to be measured by the Evans method,22
and experience has shown that facile monomer-dimer
equilibration in systems such as this invariably leads
to sufficient EPR line broadening such that resonances
cannot be observed.
The fulvalene 1H and 13C{1H} NMR resonances of the
chromium-chromium-bonded form of 6a and 6b merit
additional comment, since several analogous molybde-
num complexes are available for comparison.1b,23 It has
been proposed that the magnetic anisotropy induced by
the metal-metal bonds in the dimers trans-FvMo2-
(CO)4L2 (L ) CO, PMe3) not only generates a larger
chemical shift difference for the fulvalene proton sites
(∼0.7-0.8 ppm) than is found for many analogous non-
metal-metal-bonded complexes (∼0.4 ppm) but also
results in inversion of the relative positions of the
While the 1H NMR spectrum of 6a did not exhibit
significant changes below room temperature, both the
fulvalene and the methyl resonances gradually shifted
downfield and broadened on raising the temperature
from 20 to 80 °C; at 80 °C, all of these resonances had
broadened sufficiently that they could not be identified.
Reversibility of these changes was demonstrated when
the sample was cooled back to room temperature,
although significant decomposition had taken place.
Analogous chemical shift changes and line broadening
1
resonances. If so, then the fulvalene H chemical shifts
could be diagnostic of the presence of metal-metal
bonds in fulvalene complexes in general.23 We have
previously shown, however, that while trans-FvMo2-
(CO)4(PCy3)2 obeys this “rule”, the compounds trans-
FvMo2(CO)4L2 (L ) PPh3, PXy3) do not,1b and we now
provide further contradictions. The chemical shift dif-
ference between the fulvalene proton resonances is only
0.23 ppm in 6a and is apparently negligible in 6b,
showing that the proposed effects of the metal bond23
are not applicable to these compounds.
1
have also been observed in the variable-temperature H
NMR spectra of compounds of the type [CpCr(CO)2L]2
and have been attributed to extensive dissociation to
17-electron monomers.5a,b,d
In a complementary variable-temperature IR spec-
troscopic study, carried out on 6a in the temperature
range 25-80 °C, the two absorbances of 6a at 1925 and
1851 cm-1 gradually decreased and two new absor-
bances evolved at 1949 (s) and 1800 (s, br) cm-1 as the
temperature increased. Only the new bands were
We also previously found that the 13C{1H} NMR
spectra of the metal-metal-bonded compounds trans-
FvMo2(CO)4L2 (L ) PPh3, PXy3) exhibit bridgehead (C-
1) fulvalene carbon resonances upfield from one of the
tertiary fulvalene carbon resonances; this was unusual
since, in all other fulvalene complexes, the bridgehead
carbons are normally deshielded relative to both the CR
and Câ carbons.23 Now 6a provides the third example
where the C-1 carbon resonance is also irregularly
shielded, although 6b seems to be normal.
Rea ction s of 6a w ith Alk yl Ha lid es. A reaction
typical of 17-electron, metal-centered radicals is the
abstraction of halogen atoms from organic halides,4 and
the extensive chemistry exhibited by [CpCr(CO)3]2 with
alkyl halides5e,24 is actually a reflection of the reactivity
1
present at 80 °C, consistent with the H NMR observa-
tion, but the reversibility of the process could not be
confirmed because of very extensive decomposition in
the IR cell. However, we note that the carbonyl stretch-
ing frequencies of the high-temperature species are very
similar to those of the persistent metal-centered radical
CpCr(CO)2PMe2Ph (1925 (s), 1807 (m, br) cm-1),6a
supporting the identification of this species as the
biradical (Me2PhP)(CO)2Cr(µ-Fv)Cr(CO)2(PMe2Ph).
In summary, the most plausible rationale for the
temperature dependence of the NMR and IR spectra is
that the chromium-chromium-bonded forms of 6a and
6b maintain a facile equilibrium with the corresponding
radical isomers, as shown in eq 8. The “monomer-
dimer” equilibrium shifts toward the diamagnetic,
metal-metal-bonded isomer and exchange between the
two species slows down considerably at low tempera-
tures; thus, only the “dimers” are observable by IR and
NMR spectroscopy below about -40 °C. However, the
equilibrium shifts significantly to the right above room
(22) Evans, D. F. J . Chem. Soc. 1959, 2003.
(23) Meyerhoff, D. J .; Nunlist, R.; Tilset, M.; Vollhardt, K. P. C.
Magn. Reson. Chem. 1986, 24, 709.
(24) (a) Goulin, C. A.; Huber, T. A.; Nelson, J . M.; Macartney, D.
H.; Baird, M. C. J . Chem. Soc., Chem. Commun. 1991, 798. (b) Huber,
T. A.; Macartney, D. H.; Baird, M. C. Organometallics 1993, 12, 4715.
(c) Huber, T. A.; Macartney, D. H.; Baird, M. C. Organometallics 1995,
14, 592.
(25) McLain, S. J . J . Am. Chem. Soc. 1988, 110, 643.
(26) Kova´cs, I.; Baird, M. C. Can. J . Chem., in press.

FvCr2(CO)4L2 Metal-Metal-Bonded Compounds
Organometallics, Vol. 15, No. 16, 1996 3597
of the monomer CpCr(CO)3, which occurs in the equi-
librium mixture as about 10% of the dimer.5d,25 In
general, CpCr(CO)3 abstracts halogen atoms from a
number of activated alkyl halides and generates the
halide complexes CpCr(CO)3X (X ) Br, I); a second
CpCr(CO)3 species then traps the alkyl moieties as the
alkyl compounds CpCr(CO)3R (eq 9).4,5e,24 In contrast,
CN)Br and FvCr2(CO)4(PMe2Ph)2(CH2CO2Et)I. It is
certain, however, that the dialkyl compound FvCr2-
(CO)4(PMe2Ph)2(CH2CN)2 was not formed, as it exhibits
quite different IR and 31P{1H} NMR spectroscopic
properties.26
The relative reactivities observed in these experi-
ments, although qualitative, are in good agreement with
earlier reports on the relative reactivities of the metal-
centered radicals CpM(CO)3 (M ) Cr, Mo, W) with alkyl
halides4,5e,i,j,24 and are as anticipated for halogen ab-
straction reactions for the types of biradical species
shown in eq 8. Formation of the halide complexes
FvCr2(CO)4(PMe2Ph)2X2 (X ) Cl, Br, I) as the sole or
major carbonyl-containing products implies that cou-
pling processes to give Cr-alkyl bonds are not competi-
tive with halogen atom abstractions from excess alkyl
halides5i,j and is thus consistent with our finding that
the equilibrium of eq 8 lies far to the side of the metal-
metal-bonded isomer at room temperature. In passing,
we note that the types of alkyl halides (XCH2CN and
XCH2CO2Et) which appear to give chromium alkyl
compounds in the fulvalene systems are also those
which give more stable Cp chromium alkyl compounds,
presumably because of the effects of electronegative
substituents.5e,24c
However, the apparent disinclination of the presumed
intermediate metal-centered monoradical species PMe2-
Ph(CO)2Cr(µ-Fv)Cr(CO)2PMe2PhX to couple with or-
ganic radicals does seem strange, given the necessarily
close proximity of the two metal centers in this com-
pound. Possibly relevant to this anomaly, it has been
shown that CpCr(CO)3 and CpCr(CO)3I take part in an
iodine atom exchange process which involves an inter-
mediate of the type {CpCr(CO)3- - -I- - -Cr(CO)3Cp},
formed via interaction of a CpCr(CO)3I iodine lone pair
with the singly occupied orbital of CpCr(CO)3.5e If a
chelate effect were to apply in the fulvalene system, it
is possible that the halogen atom of PMe2Ph(CO)2Cr-
(µ-Fv)Cr(CO)2(PMe2Ph)X bridges the chromium atoms
as in A, thereby providing steric hindrance to the
[CpCr(CO)3]2 + RX f CpCr(CO)3X + CpCr(CO)3R
(9)
analogous alkyl complexes are not formed in reactions
of organic halides with the short-lived, photochemically
generated radicals CpM(CO)3 (M ) Mo, W) because the
low steady-state concentrations of the organic and
metal-centered radicals prevent the coupling processes
necessary for formation of the products CpM(CO)3R.5i,j
To test the chemistry of 6a and 6b for radical-like
properties, their reactions with alkyl halides have been
studied. We find that both 6a and 6b react with a
variety of activated alkyl halides at room temperature,
and the reactions of 6a have been investigated in detail.
IR spectroscopic investigations revealed that 6a is
consumed instantaneously in the dark in the presence
of an excess of allyl iodide, CHBr3, or CCl4 and that the
dihalides 3b, 4, and 5 (see below for discussion of these
compounds) were formed as the sole carbonyl-containing
products with allyl iodide, CHBr3, and CCl4, respectively
(eq 10). The reaction of 6a with t-BuI was somewhat
FvCr2(CO)4(PMe2Ph)2
+ RX f
6b
FvCr2(CO)4(PMe2Ph)2X2
(10)
3b (X ) I), 4 (X ) Br), 5 (X ) Cl)
slower (∼1 h) and resulted in the formation of 3b, while
those with PhCH(Br)Me, PhCH2Br, and BrCH2CO2Me
took place at much slower rates (>3 h) such that the
expected atom abstraction products decomposed; thus,
only degradation of 6a could be observed. Less active
alkyl halides (MeI, EtI, and i-PrI) did not exhibit any
reactivity at room temperature.
In contrast to the above, the activated alkyl halides
BrCH2CN and ICH2CO2Et reacted quickly with 6a to
form the corresponding dihalides 4 (νCO 1968, 1881
cm-1) and 3b (νCO 1958, 1877 cm-1), respectively, in
yields of ∼70%. Byproducts were also formed with
carbonyl stretching bands at 1943, 1872 cm-1 and 1938,
1863 cm-1, respectively. These are at somewhat lower
frequencies than the dihalides, as would be anticipated
for the anticipated halo-alkyl products FvCr2(CO)4(PMe2-
Ph)2(CH2CN)Br and FvCr2(CO)4(PMe2Ph)2(CH2CO2Et)I.
While chromatographic separations of the highly labile
materials failed and the 1H NMR spectra were too
complicated to be interpreted, the 31P{1H} NMR spectra
exhibited, in addition to resonances of 4 and 3b, two
groups of relatively weak resonances, one with chemical
shifts nearly coincident with those of 4 and 3b, and a
second at ∼δ 50. By analogy with the hydrido-halides
FvCr2(CO)4(PMe2Ph)2HX and compounds of the type
FvCr2(CO)4(PMe2Ph)2RR′,26 the 31P chemical shift of a
PMe2Ph coordinated to the alkyl “half” of a halo-alkyl
Fv compound is expected to be ∼δ 58, and thus the 31P-
{1H} NMR data may be consistent with the partial
formation of the compounds FvCr2(CO)4(PMe2Ph)2(CH2-
“second” chromium or possibly even affecting its elec-
tronic properties such that radical-like reactivity is
decreased. In fact, a molybdenum analogue of A was
previously invoked as an intermediate during the
hydrogen-halogen atom exchange reactions of alkyl
halides with (fulvalene)molybdenum dihydrides FvMo2-
(CO)4L2H2 (L ) CO, PPh3), where the only significant
forward reaction in the presence of an excess of alkyl
halide was the abstraction of a second halogen atom to
furnish FvMo2(CO)4L2X2.1a
Hyd r ogen -Ha logen Atom Exch a n ge Rea ction s
of 2b w ith Activa ted Alk yl Ha lid es. In d ep en d en t
Ch a r a cter iza tion of F vCr 2(CO)4(P Me2P h )2X2 (X )
I (3b), Br (4), Cl (5)). To confirm the identities of 3b,
4, and 5 as the products of reactions of 6a with alkyl
halides, and possibly to learn more about these new
compounds, we developed an alternative route to their
syntheses. As demonstrated previously for the PPh3-
substituted molybdenum system, dihalides FvMo2(CO)4-
(27) Alt, H. G.; Schwarzle, J . A. J . Organomet. Chem. 1978, 162,
45.

3598 Organometallics, Vol. 15, No. 16, 1996
Kova´cs and Baird
(PPh3)2X2 (X ) Cl, Br, I) are readily prepared by the
hydrogen-halogen atom exchange reactions of FvMo2-
(CO)4(PPh3)2H2 with appropriate alkyl halides.1a Analo-
gous reactions of (cyclopentadienyl)- or (fulvalene)-
chromium hydrides have not previously been inves-
tigated, but the successful preparation of 3a from 2a
and ICH2CO2Et (eq 6) suggested that the dihalides
FvCr2(CO)4(PMe2Ph)2X2 (X ) Cl, Br, I) might be ob-
tained similarly. It was also anticipated that phos-
phine-substituted chromium carbonyl halides could be
more stable than the parent carbonyl halide compounds.
Thus, while the extremely unstable CpCr(CO)3Cl may
be only briefly detected by IR spectroscopy,5e CpCr(CO)-
(PMe3)2Cl has been isolated and fully characterized.27
Indeed, 2b reacted readily with ICH2CO2Et, CHBr3,
and CCl4 at room temperature and in the dark, giving
the dihalides FvCr2(CO)4(PMe2Ph)2X2 (X ) I (3b), Br
(4), Cl (5), respectively) (eq 11). The very labile com-
F igu r e 3. IR spectroscopic monitoring of the spontaneous
thermal decomposition of compound 5 in CH2Cl2.
identical with the products of halogen atom abstraction
observed in the reaction of 6b with alkyl halides. The
IR spectra of cis,cis-3b, cis,cis-4, and cis,cis-5 exhibited
carbonyl bands at 1958 and 1877, 1968 and 1881, and
1973 and 1881 cm-1, in the order anticipated.
FvCr2(CO)4(PMe2Ph)2H2
+ RX f
2b
FvCr2(CO)4(PMe2Ph)2HX RX8
FvCr2(CO)4(PMe2Ph)2X2
3b (X ) I), 4 (X ) Br), 5 (X ) Cl)
(11)
1
Formation of 3b, 4, and 5 was established by both H
and 31P{1H} NMR to proceed stepwise via the interme-
diate formation of the corresponding hydrido halides cis-
FvCr2(CO)4(PMe2Ph)2HX (eq 11), similar again to the
molybdenum-containing systems.1a It is also interesting
to note that only a single intermediate containing a cis-
halide half was detected in each reaction, suggesting
that cis-trans isomerization of 5, for example, did not
occur before complete halogenation of 2b. In contrast,
FvMo2(CO)4(PPh3)2HI, the intermediate in the reaction
of FvMo2(CO)4(PPh3)2H2 with ICH2CO2Et, existed as
both cis- and trans-iodide halves and was ultimately
formed in a mixture of cis,cis, cis,trans, and trans,trans
diodides.1a Another interesting observation was that
the radical reaction of 2b with alkyl iodides and the
oxidative cleavage by I2 both resulted in the sole
formation of cis,cis-3b, although the analogous reactions
of FvMo2(CO)4(PPh3)2H2 afforded different isomers of
FvMo2(CO)4(PPh3)2I2.1a Direct substitution of 3a with
PMe2Ph was also attempted to provide yet another path
to 3b, but this reaction proved to be rather complicated
due to formation of various cationic complexes (see
below).
pounds 4 and 5 were characterized in situ by IR and
1H and 31P{1H} NMR spectroscopy, as only the iodide
3b could be isolated analytically pure. In contrast to
FvMo2(CO)4(PPh3)2X2 (X ) Cl, Br, I),1a 3b was found to
exist as an approximately 1:1 mixture of meso- and dl-
cis,cis isomers; thus, eight single-hydrogen fulvalene
proton, ten fulvalene carbon, and two (very closely
spaced) phosphorus resonances were exhibited in the
1H, 13C{1H}, and 31P{1H} NMR spectra, as anticipated.1a
For the same reason, four PMe proton doublets were
exhibited in the 1H NMR spectrum, since the chiral
chromium centers further split the two doublets of the
meso and dl forms, rendering the methyl protons of the
PMe2Ph ligand diastereotopic.21a Although only three
PMe carbon doublets were exhibited in the 13C{1H}
NMR spectrum, that at δ 19.3 appears to be two
1
completely overlapped doublets, consistent with the H
NMR behavior.
In contrast, compound 4 occurred in solution as the
cis,cis isomer and compound 5 as a mixture of cis,cis
and trans,trans isomers, the latter being the major
product. There was no evidence for formation of the
mixed cis,trans isomers or, in the case of 4, of the
trans,trans isomer. These results seem strange, and
while it is possible that weak resonances were obscured,
in fact the spectral integrations were as expected. Thus,
any minor isomers formed must have been present in
very low concentrations.
Interestingly, the preferred isomers of 3b, 4, and 5
are the opposite of those of the analogous halo com-
pounds CpMo(CO)2(PMe2Ph)X, where the chloride and
bromide exist predominantly as the cis isomer and the
iodide exists as a mixture of cis and trans isomers.21c
When solutions of the isomeric mixtures of 4 and 5 in
CH2Cl2 were kept in the dark at room temperature, IR
spectroscopic monitoring demonstrated (Figure 3) that
thermal decomposition of the cis isomers took place
considerably faster than that of the trans isomers,
indicating higher thermal stability of the latter. Com-
pounds cis,cis-3b, cis,cis-4, and isomers of 5 were
Syn th esis of cis-P Me2P h (CO)2ICr (µ-F v)[tr a n s-
Cr (CO)2(P Me2P h )2]I (7). When the preparation of 3b
by direct substitution of FvCr2(CO)6I2 with excess PMe2-
Ph in THF was attempted, a type of reaction which is
well documented for CpCr(CO)3I,14 an IR spectrum of
the reaction mixture revealed the rapid formation of a
compound displaying two equally strong carbonyl ab-
sorbances at 1956 and 1878 cm-1. These frequencies
are very similar to those of 3b (1958, 1877 cm-1) but
are of different relative intensities and initially sug-
gested the formation of trans isomers of FvCr2-
1
(CO)4(PMe2Ph)2I2. However, a combined H, 13C{1H},
and 31P{1H} NMR investigation unambiguously re-
vealed that the substitution product was in fact the
mixed cationic iodide [cis-PMe2Ph(CO)2ICr](µ-Fv)[trans-
Cr(CO)2(PMe2Ph)2]I (7) (Scheme 1).
Compound 7 was subsequently isolated and charac-
terized by elemental analysis as well. The PMe2Ph
ligands of the cationic chromium center are coordinated
in mutually trans positions, but the pairs of fulvalene

FvCr2(CO)4L2 Metal-Metal-Bonded Compounds
Organometallics, Vol. 15, No. 16, 1996 3599
Sch em e 1
A complementary IR spectroscopic study in THF
solution showed that four broad absorbances at 2033,
1969 (sh), 1956, and 1882 cm-1 appeared after the
addition of only ∼1 equiv of PMe2Ph. These were
replaced by the two relatively sharp bands of 5 at 1956
and 1878 cm-1 when an additional 2 equiv of PMe2Ph
was introduced. It seems reasonable that the cationic
moieties [Cr(CO)3PMe2Ph]I and [trans-Cr(CO)2(PMe2-
Ph)2]I exhibit carbonyl bands which overlap extensively
with those of FvCr2(CO)6I2 and cis,cis-FvCr2(CO)4(PMe2-
Ph)2I2, respectively, and thus that IR spectroscopy
cannot be used to unambiguously identify the interme-
diates observed by NMR spectroscopy.
An example of a cationic fulvalene iodo complex,
[FvMo2(CO)5(dmpm)I]I, was obtained recently by react-
ing the zwitterionic compound FvMo2(CO)5(dmpm) with
I2,19 but the corresponding PMe2Ph-substituted deriva-
tive, [(CO)3ICr](µ-Fv)[Cr(CO)2(PMe2Ph)2]I, was not ob-
served here. Furthermore, although this is the first
report of formation of cationic chromium complexes from
the addition of phosphines to halo complexes of the type
Cp′Cr(CO)3X (Cp′ ) Cp-like ligand, X ) halogen),
similar reactions of molybdenum and tungsten Cp
complexes are well-documented.8 However, monoden-
tate phosphines instead substitute one or two CO
ligands in CpM(CO)3X (M ) Mo, W), and cationic
derivatives of the type [CpM(CO)2L2]X (L ) CO and/or
phosphine) were prepared via alternative ways.8
proton and carbon atoms are diastereotopic because of
the chiral neutral cis-Cr(CO)2(PMe2Ph)I moiety.1a,21a
Although 7 contains neutral and cationic moieties, both
chromium centers are formally in the 2+ oxidation state
and only a relatively small difference was observed
between the bridgehead carbon resonances, at δ 98.4
and 102.8. The 31P{1H} NMR spectrum exhibited two
phosphorus resonances at δ 32.9 and 51.1, in a 1:2 ratio
of intensities and consistent with the proposed struc-
ture.
The formation of 7 was then monitored by 31P{1H}
and 1H NMR spectroscopy in acetone-d6 solution to
ascertain the order in which the two metal centers react
with the 3 equiv of PMe2Ph, and possibly to gain
evidence for intermediates. The phosphine was added
in small aliquots to a suspension of (excess) FvCr2-
(CO)6I2, and spectra were recorded without delay. It
was found that a relatively short-lived species was first
formed; this exhibited a single phosphorus resonance
at δ 41.7 and four fulvalene proton resonances downfield
of those of 7, at δ 5.45, 5.91, 6.12, and 6.44. This
compound readily transformed into a more stable spe-
cies exhibiting two phosphorus resonances of equal
intensity at δ 32.7 and 42.0 and eight fulvalene proton
resonances at δ 4.96, 5.13, 5.54, 5.81, 5.85, 6.04, 6.26
and 6.38. The phosphorus resonance at δ 32.7, in the
region of those of 3b (δ 33.20 and 33.17), and the
presence of eight diastereotopic proton resonances,
suggested the formation of a cis-Cr(CO)2(PMe2Ph)I
moiety. Addition of an excess of PMe2Ph to this mixture
resulted in the formation of compound 7. On the basis
of this evidence, the second intermediate was tentatively
identified as [cis-PMe2Ph(CO)2I]Cr(µ-Fv)[Cr(CO)3PMe2-
Ph]I and the first, by default, as [(CO)3ICr](µ-Fv)[Cr-
(CO)3PMe2Ph]I (Scheme 1).
Ack n ow led gm en t. We thank NATO for a Science
Fellowship to I.K. (administered by the Natural Sciences
and Engineering Research Council), the NSERC for a
Research Grant to M.C.B., and the Hungarian Science
Fund for additional support to I.K. provided by Grant
No. OTKA F7419.
OM960051Y