Four-legged piano stool molybdenum(II) compounds without carbonyl ligands. 4. Cyclopentadienylmolybdenum(II) complexes with 16-electron and 18-electron configurations

Article abstract of DOI:10.1021/om960331d

Monocyclopentadienyl complexes of Mo(II) with 16- and 18-electron configurations of the form (Ring)MoCIL_x (x = 2, Ring = Cp, L = PMe₂Ph; x = 2, Ring = Cp *, L = PMe₃, PMe₂Ph, L₂ = dppe; x = 3, Ring =

Full text of DOI:10.1021/om960331d

Organometallics 1996, 15, 4407-4416
4407
F ou r -Legged P ia n o Stool Molybd en u m (II) Com p ou n d s
w ith ou t Ca r bon yl Liga n d s. 4.
Cyclop en ta d ien ylm olybd en u m (II) Com p lexes w ith
16-Electr on a n d 18-Electr on Con figu r a tion s
Fatima Abugideiri, J ames C. Fettinger, D. Webster Keogh, and Rinaldo Poli*
Department of Chemistry and Biochemistry, University of Maryland,
College Park, Maryland 20742
Received May 3, 1996X
Monocyclopentadienyl complexes of Mo(II) with 16- and 18-electron configurations of the
form (Ring)MoClLx (x ) 2, Ring ) Cp, L ) PMe2Ph; x ) 2, Ring ) Cp*, L ) PMe3, PMe2Ph,
L2 ) dppe; x ) 3, Ring ) Cp, L ) PMe2Ph) are described. All of the 16-electron complexes
are paramagnetic with an S ) 1 ground state, as shown by magnetic measurements in the
1
solid state and in solution, and by the contact-shifted H NMR spectra. The structure of
Cp*MoCl(dppe) was determined by X-ray diffraction methods. The 18-electron complex
CpMoCl(PMe2Ph)3 has been synthesized by reduction of {CpMoCl2}n with Na in the presence
1
of 3 equiv of phosphine. It has been fully characterized by H and 31P NMR, chemical
analysis, and X-ray structural determination. Thermolysis of a THF or C6D6 solution of
this 18-electron species generates the 16-electron paramagnetic Mo(II) complex CpMoCl-
(PMe2Ph)2. The Cp*MoClL2 (L ) PMe3 and PMe2Ph) systems react with 2-electron-donor
ligands, i.e. CO and H2, to afford stable 18-electron complexes. The carbonyl derivative
Cp*MoCl(CO)(PMe2Ph)2 has also been characterized by X-ray crystallography.
In tr od u ction
phenomenologically the factors that determine the
choice of spin state.
We are interested in the effect of the spin state on
the energetic stabilization of electronically unsaturated
organometallic complexes. For instance, a 16-electron
complex might have the 2 electrons of highest energy
unpaired and located 1 each in the two highest energy
orbitals of the valence shell, giving rise to a spin triplet,
or they may be paired in the same orbital, leaving a
relatively accessible LUMO:
In simple terms, the choice between the above two
electronic configurations is determined by the relative
magnitude of the separation between the two orbitals
and the pairing energy. Given a certain energetic
separation between the two orbitals in question, the low
effective positive charge on the metal center in typical
low-oxidation-state organometallic compounds results
in diffuse metal orbitals and therefore low pairing
energies, favoring the low-spin configuration. Low-
oxidation-state 16-electron complexes are typically reac-
tive intermediates which easily engage in ligand-
addition or oxidative-addition reactions. In most
instances, the assumption is implicitly made in the
literature that such intermediates have a low-spin
configuration, or the question of their spin state is
simply not addressed. There are, however, important
cases of high-spin intermediates, for instance Fe(CO)4
and CpCo(CO).3-5
A similar situation may be pictured for 15-electron
1
complexes, where the choice is between an S ) /2 and
3
For higher oxidation state complexes, the higher
effective positive charge on the metal center is expected
to result in a greater pairing energy and a higher
relative stability of the high-spin configuration with
respect to the alternative low-spin one. A good example
is provided by the isostructural and isoelectronic series
of four-legged piano stool, d2 (16-electron) compounds
an S ) /2 configuration.
The electronic configuration has profound effects on
the reactivity of the molecule; for instance, an S ) 0
16-electron complex has the ability to undergo rapid
ligand additions or oxidative-addition reactions, whereas
the same reactions on the spin triplet counterpart
should be slower because they involve a forbidden (in
principle)1 crossover to the singlet state of the 18-
electron product. The energetics of stable molecules and
reaction intermediates (and therefore the reaction ther-
modynamics, kinetics, and mechanism) can also be
affected by spin changes.2 We therefore wish to study
6
of Zr(II), (η6-C6H5Me)ZrCl2(PMe3)2 (diamagnetic),
Nb(III), Cp*NbCl2(PMe3)27 (paramagnetic), and Mo(IV),
(3) Poliakoff, M.; Weitz, E. Acc. Chem. Res. 1987, 20, 408-414.
(4) Bengali, A. A.; Bergman, R. G.; Moore, C. B. J . Am. Chem. Soc.
1995, 117, 3879-3880.
(5) Siegbahn, P. E. M. J . Am. Chem. Soc. 1996, 118, 1487-1496.
(6) Diamond, G. M.; Green, M. L. H.; Walker, N. M.; Howard, J . A.
K.; Mason, S. A. J . Chem. Soc., Dalton Trans. 1992, 2641-2647.
(7) Siemeling, U.; Gibson, V. C. J . Organomet. Chem 1992, 424,
159-161.
X Abstract published in Advance ACS Abstracts, September 1, 1996.
(1) Detrich, J . L.; Reinaud, O. M.; Rheingold, A. L.; Theopold, K. H.
J . Am. Chem. Soc. 1995, 117, 11745-11748.
(2) Poli, R. Comments Inorg. Chem. 1992, 12, 285-314.
S0276-7333(96)00331-7 CCC: $12.00 © 1996 American Chemical Society

4408 Organometallics, Vol. 15, No. 21, 1996
Abugideiri et al.
[CpMoCl2(PMe3)2]+ (paramagnetic).8 Since pairing en-
ergies are lower for more diffuse orbitals, a higher spin
state is more likely observed for an unsaturated complex
of a 3d metal versus the 4d and 5g congeners. The
isoelectronic intermediates CpM(CO) (M ) Co, Rh)
illustrate the dramatic difference in chemical reactivity
for species with different spin: while the S ) 0 CpRh-
(CO) rapidly binds 2-electron-donor ligands (even the
inert gases Kr and Xe!) and oxidatively add C-H bonds
of saturated hydrocarbons,9,10 the corresponding S ) 1
CpCo(CO) fragment does not engage in either process.4
We have shown that the phosphine ligand exchange on
the 17-electron (S ) 1/2) CpMoX2L2 complexes (X )
halogen; L ) tertiary phosphine) occurs via the 15-
electron intermediate CpMoX2L (rather than through
the associative mechanism which is typical of low-
oxidation-state radicals) and advanced the hypothesis
thermodynamically more favorable combination of (η-
C5R5)MoCl2L2 and [(η-C5R5)MoCl2]2.17
Here we report the results of our investigations on
the class of Mo(II) complexes of general formula (η-
C5R5)MoClLn, for which either an 18-electron (n ) 3)
or a 16-electron (n ) 2) configuration is expected. The
lowering of the metal oxidation state from Mo(IV) to
Mo(III) to Mo(II) is of interest because (i) the preference
for a high-spin electronically unsaturated configuration
should decrease and (ii) lower coordination numbers are
sufficient to achieve electronic saturation; therefore,
electronically unsaturated complexes can be assumed
to be less dependent on steric effects (e.g. compare the
three-legged piano stool CpMoClL2 with the four-legged
piano stool CpMoCl3L, both having a 16-electron count).
Our studies demonstrate that 16-electron Mo(II) com-
plexes (i) unlike the 15-electron Mo(III) CpMoX2L
complexes17 are stable in solution and (ii) like the 16-
electron Mo(IV) CpMoCl3L systems preferably adopt a
high-spin configuration. Part of this work has been
previously communicated.18
3
that a spin state change to a more favorable S ) /2
configuration is at least in part responsible for this
change of mechanism.11 We shall address elsewhere
how pairing energies affect the relative stabilities of 15-
electron spin quartet CpMCl2(PH3) versus 17-electron
spin doublet CpMCl2(PH3)2 for M ) Cr, Mo.12
Exp er im en ta l Section
We have now embarked on a synthetic project de-
signed to probe the stability and spin state of electroni-
cally unsaturated organometallic complexes in different
oxidation states. We have found that for the Mo(IV)
(η-C5R5)MoCl3Ln system, when the less sterically en-
cumbered Cp and L ) PMe3, PMe2Ph ligands are
employed, an equilibrium is established between the 18-
electron CpMoCl3L2 and the 16-electron spin triplet
CpMoCl3L complexes, whereas only the 16-electron
mono(phosphine) adduct could be obtained for the
sterically more encumbered Cp* systems and also for
the Cp system with the bulkier phosphine PMePh2.13
Also, the stable 16-electron [CpMoCl2(PMe3)2]+ complex
reacts promptly and quantitatively with Cl- to afford
CpMoCl3(PMe3)2 (in equilibrium with CpMoCl3(PMe3)),
whereas [CpMoI2(PMe3)2]+ and I- do not show any
tendency to form the corresponding 18-electron ad-
duct.14,15 All these 16-electron four-legged piano stool
Mo(IV) complexes have a spin triplet ground state.
Gen er a l Da ta . All operations were carried out under an
atmosphere of argon. Solvents were dehydrated by conven-
tional methods and distilled directly from the dehydrating
agent prior to use (THF and Et2O from Na/benzophenone,
heptane and toluene from Na, and CH2Cl2 from P2O5). NMR
spectra were recorded on Bruker WP200 and AF200 spectrom-
eters; the peak positions are reported with positive shifts
downfield of TMS as calculated from the residual solvent peaks
(1H) or downfield of external 85% H3PO4 (31P). For each 31P
NMR spectrum, a sealed capillary containing H3PO4 was
immersed in the same NMR solvent used for the measurement
and this was used as the external reference. EPR spectra were
recorded on a Bruker ER200 spectrometer equipped with an
X-band microwave generator. Cyclic voltammograms were
recorded with an EG&G 362 potentiostat connected to a
Macintosh computer through MacLab hardware/software; the
electrochemical cell was a locally modified Schlenk tube with
a Pt counter electrode sealed through uranium glass/Pyrex
glass seals. The cell was fitted with a Ag/AgCl reference
electrode and a Pt working electrode. All potentials are
reported vs the Cp2Fe/Cp2Fe+ couple, which was introduced
into the cell at the end of each measurement. The solid-state
magnetic susceptibility measurements were performed with
a J ohnson Matthey magnetic susceptibility balance. The
elemental analyses were by M-H-W Laboratories, Phoenix, AZ
or Galbraith Laboratories, Inc., Knoxville, TN. Cp*MoCl4,17,19
Cp*MoCl2(PMe3)2,20 Cp*MoCl(PMe3)2(N2),20 {CpMoCl2}n,21 and
Cp*MoCl(PMe3)322 were prepared by literature methods. PMe3
(Aldrich), PMe2Ph (Aldrich), dppe (Strem), H2 (Air Products),
and CO (Air Products) were used without further purification.
P r ep a r a tion of Cp MoCl(P Me2P h )3 (1) a n d F or m a tion
of Cp MoCl(P Me2P h )2 (2). {CpMoCl2}n (306 mg, 1.32 mmol)
and THF (25 mL) were introduced into a Schlenk tube
equipped with a magnetic stirring bar, and PMe2Ph (563 µL,
547 mg, 3.96 mmol; PMe2Ph/Mo ratio 3.0) was added to the
For the Mo(III) (η-C5R5)MoCl2Ln system, we have not
yet been able to produce a stable 15-electron monophos-
phine adduct, whereas a number of 17-electron bis-
adducts have been obtained, isolated, and character-
ized.16 We have shown that this is not due to insufficient
stabilization of the alleged (η-C5R5)MoCl2L complex by
the expected S ) 3/2 electronic configuration with respect
to the bis(phosphine) adduct but rather to a ligand
disproportionation reaction to ultimately afford the
(8) Krueger, S. T.; Poli, R.; Rheingold, A. L.; Staley, D. L. Inorg.
Chem. 1989, 28, 4599-4607.
(9) Schultz, R. H.; Bengali, A. A.; Tauber, M. J .; Weiller, B. H.;
Wasserman, E. P.; Kyle, K. R.; Moore, C. B.; Bergman, R. G. J . Am.
Chem. Soc. 1994, 116, 7369-7377.
(10) Bengali, A. A.; Schultz, R. H.; Moore, C. B.; Bergman, R. G. J .
Am. Chem. Soc. 1994, 116, 9585-9589.
(17) Abugideiri, F.; Keogh, D. W.; Kraatz, H.-B.; Poli, R.; Pearson,
W. J . Organomet. Chem. 1995, 488, 29-38.
(11) Cole, A. A.; Fettinger, J . C.; Keogh, D. W.; Poli, R. Inorg. Chim.
Acta 1995, 240, 355-366.
(18) Abugideiri, F.; Keogh, D. W.; Poli, R. J . Chem. Soc., Chem.
Commun. 1994, 2317-2318.
(12) Cacelli, I.; Poli, R.; Rizzo, A. To be submitted for publication.
(13) Abugideiri, F.; Gordon, J . C.; Poli, R.; Owens-Waltermire, B.
E.; Rheingold, A. L. Organometallics 1993, 12, 1575-1582.
(14) Poli, R.; Owens, B. E.; Linck, R. G. J . Am. Chem. Soc. 1992,
114, 1302-1307.
(19) Murray, R. C.; Blum, L.; Liu, A. H.; Schrock, R. R. Organome-
tallics 1985, 4, 953-954.
(20) Baker, R. T.; Calabrese, J . C.; Harlow, R. L.; Williams, I. D.
Organometallics 1993, 12, 830-841.
(21) Linck, R. G.; Owens, B. E.; Poli, R.; Rheingold, A. L. Gazz. Chim.
Ital. 1991, 121, 163-168.
(15) Poli, R.; Rheingold, A. L.; Owens-Waltermire, B. E. Inorg. Chim.
Acta 1993, 203, 223-227.
(16) Poli, R. J . Coord. Chem. B 1993, 29, 121-173.
(22) Abugideiri, F.; Kelland, M. A.; Poli, R.; Rheingold, A. L.
Organometallics 1992, 11, 1303-1311.

Four-Legged Piano Stool Mo(II) Compounds
Organometallics, Vol. 15, No. 21, 1996 4409
resulting suspension, which was then stirred until the Cp-
MoCl2 precipitate reacted with PMe2Ph to form CpMoCl2(PMe2-
Ph)2 (ca. 1 h), according to published procedures.23 The
solution was transferred using a filter cannula into a Schlenk
tube containing amalgamated Na (30 mg, 1.32 mmol in 11 g
of Hg). The mixture was stirred for 4 days, the color changing
from purple-brown to orange-brown, and then filtered through
Celite. The solvent was removed under reduced pressure, and
the residue was extracted with toluene (200 mL), filtered, and
placed in a -20 °C freezer for 10 days. The resulting red
crystals were filtered and dried in vacuo. Yield: 218 mg, 27%.
Anal. Calcd for C29H38ClMoP3: C, 57.01; H, 6.27. Found: C,
56.80; H 6.25. 1H NMR (C6D6, δ): 7.6-6.7 (m, 15H, PPh), 4.59
(d, 5H, J H-P ) 2 Hz, Cp), 1.82 (vt, 6H, J H-P ) 7 Hz, P-CH3),
1.43 (vt, 6H, J H-P ) 6 Hz, P-CH3), 0.73 (d, 6H, J H-P ) 3 Hz,
PMe2Ph). 31P NMR (C6D6, δ): 34.7 (t, 1P, J P-P ) 53 Hz), 21.9
(d, 2P, J P-P ) 53 Hz). Crystals for the X-ray analysis were
obtained by extraction of the crude material into Et2O and
cooling overnight at -20 °C.
Cp*MoCl4 (0.105 g, 0.282 mmol) was added to a THF solution
(15 mL) of amalgamated Na (0.021 g, 0.913 mmol, in 2 g of
Hg) and PMe2Ph (0.080 mL, 0.564 mmol). After it was stirred
overnight, the resulting orange-brown solution was evaporated
to dryness and the residue was extracted with heptane (20
mL) and filtered through Celite until the washings were
colorless. The compound could not be crystallized out of
solutions of saturated hydrocarbons, and these solutions were
therefore used directly for the derivatization reactions with
CO, H2, and PMe3 (vide infra). 1H NMR (C6D6, δ): 71 (br s,
w1/2 ) 230 Hz, 15 H, C5Me5), 16.3 and 15.3 (1:1 br s, w1/2
)
115 Hz, 12 H, PMe2Ph). 8.9 (s, w1/2 ) 22 Hz, 2 H, p-Ph), 7.8
(s, w1/2 ) 26 Hz, 4 H, m-Ph), 6.1 (s, w1/2 ) 150 Hz, 4 H, o-Ph).
Syn th esis of Cp *MoCl(d p p e) (5). Cp*MoCl4 (0.234 g,
0.627 mmol) was added to Na (0.049 g, 2.13 mmol), naphtha-
lene (0.015 g, 0.117 mmol), dppe (0.249 g, 0.625 mmol), and
THF (15 mL). After it was stirred for 2 days, the orange
solution was evaporated to dryness. The residue was extracted
with hot heptane (4 × 10 mL) and the extract filtered through
Celite. The solution was placed at -80 °C overnight, which
precipitated an orange microcrystalline solid. The solid was
filtered, dried, and isolated. Yield: 0.260 g, 62%. Anal. Calcd
for C36H39ClMoP2: C, 65.02; H, 5.91. Found: C, 65.19; H, 6.01.
1H NMR (C6D6, δ): 75.5 (br s, w1/2 ) 50 Hz, 4 H, o-Ph), 62.6
(br s, w1/2 ) 150 Hz, 15 H, C5Me5). 15.2 and 11.9 (s, w1/2 ) 22
Hz, 8 H, m-Ph), 14.6 (br s, w1/2 ) 90 Hz, 4 H, o-Ph), 6.4 and
4.3 (s, 4 H, p-Ph), -1.2 (br s, w1/2 ) 118 Hz, 4 H, Ph2PCH2CH2-
PPh2). µeff ) 2.65 µB. A single crystal for X-ray analysis of
Cp*MoCl(dppe) was obtained by dissolving the crude product
in hot heptane, filtering, and cooling the solution to room
temperature slowly in an oil bath.
In a repeat experiment which was run under identical
conditions starting from 578 mg (2.49 mmol) of {CpMoCl2}2,
115 mg (4.98 mmol) of Na amalgamated with 20 g of Hg, and
1.10 mL (7.48 mmol) of PMe2Ph in 90 mL of THF, the solution
obtained after filtration from Celite was evaporated to dryness
and the residue was extracted with n-heptane (70 mL). After
filtration, the solution was once more evaporated to dryness
and the residue was crystallized from 40 mL of Et2O at -80
°C, giving 540 mg of a brown solid. 1H NMR inspection of this
solid revealed that it mainly consists of a paramagnetic species,
with CpMoH(PMe2Ph)3 impurity (ca. 5% by NMR integration).
1H NMR (C6D6, δ): (a) resonances assigned to CpMoCl(PMe2-
Ph)2, 21.5 (br s, w1/2 ) 79 Hz, 4H, o-Ph), 16.6 (br s, w1/2 ) 80
Hz, 12H, PMe), 10.9 (br s, w1/2 ) 25 Hz, 2H, p-Ph), 0.85 (br s,
w1/2 ) 42 Hz, 4H, m-Ph); (b) resonances attributed to
CpMoH(PMe2Ph)3, ca. 7 (m, Ph), 4.47 (s, 5H, Cp), 1.37 (d, 18H,
Me, J P-H ) 2.1 Hz), -7.7 (q, 1H, Mo-H, J H-P ) 51 Hz).
Th er m a l Tr ea tm en t of Com p ou n d 1. A sample of 1 (20
mg, 0.033 mmol) was dissolved in C6D6 (ca. 1 mL). The
resulting solution was warmed to 45 °C in a sealed NMR tube
for 6 h. 1H and 31P{1H} NMR analyses showed the presence
of a mixture of starting material, free PMe2Ph (Me resonance
Th er m a l Decom p osition of 3. A sample of 3 prepared
as described above was dissolved in C6D6 and the solution
sealed in an NMR tube and heated to 85 °C for 1/2 h, then an
1
additional 2 h at 100 °C, and finally to 135 °C with H and 31P
NMR monitoring after each heating. After the complete
1
heating period the H NMR indicated the complete decomposi-
tion of 3. The final 1H and 31P NMR spectra showed the typical
resonances24 of Cp*MoH(PMe3)3.
Rea ction of Cp *MoCl2L2 w ith Na . (a ) L ) P Me3.
Equimolar quantities of Cp*MoCl2(PMe3)2 and amalgamated
Na (ca. 0.5%) were allowed to react in THF (ca. 0.02 M) with
1H NMR monitoring by periodically withdrawing an aliquot
of the solution, evaporating it to dryness, and redissolving it
in the NMR solvent (C6D6). After 1-2 days, the 1H NMR
spectrum indicated the complete disappearance of the reso-
nance at δ -2.3 of Cp*MoCl2(PMe3)2 and conversion to the 16-
electron Cp*MoCl(PMe3)2 complex (3).
1
at δ 1.12 in the H spectrum and resonance at δ -45.5 in the
31P{1H} spectrum), and CpMoCl(PMe2Ph)2 (paramagnetically
shifted resonances identical with those listed above). Contin-
ued heating resulted in a complete loss of the paramagnetically
shifted peaks and to the formation of a complicated mixture
of diamagnetic products which were not further investigated
(1H NMR resonances in the Cp region at δ 5.24, 4.66, 4.46,
4.36, and 4.04).
(b) L ) P Me2P h . Equimolar quantities of Cp*MoCl2(PMe2-
Ph)2 and amalgamated Na (ca. 0.5%) were allowed to react
in THF (ca. 0.02 M) with 1H NMR monitoring by periodically
withdrawing an aliquot of the solution, evaporating it to
dryness, and redissolving it in the NMR solvent (C6D6). After
1-2 days the 1H NMR spectrum indicated the complete
conversion to the 16-electron Cp*MoCl(PMe2Ph)2 complex (4).
Solutions of this compound were used directly for the deriva-
tization reactions with CO, H2, and PMe3 (vide infra).
Th er m a l Tr ea tm en t of Cp *MoCl(P Me3)3. A sample of
Cp*MoCl(PMe3)3 (67 mg, 0.135 mmol) was dissolved in C6D6
(ca. 2 mL). The resulting solution was warmed to 60 °C in a
sealed NMR tube with periodical monitoring by 1H and 31P-
{1H} NMR. After 5.5 h, 1H and 31P{1H} NMR analyses showed
the presence of a mixture of starting material, free PMe3, and
compound 3. After a total of 17 h of heating a mixture similar
to that before was observed, but the resonances of 3 had
diminished in intensity and new resonances of the thermal
decomposition product of 3 (vide supra) were present as well
as an increase in the resonance at δ -2.3 for Cp*MoCl2(PMe3)2.
Rea ction betw een Cp *MoClL2 a n d CO. (a ) L ) P Me3.
F or m a tion of tr a n s-Cp *MoCl(P Me3)2(CO) (6). A solution
Syn th esis of Cp *MoCl(P Me3)2 (3). Cp*MoCl4 (0.511 g,
1.37 mmol) was added to a THF solution (40 mL) of amalgam-
ated Na (0.098 g, 4.26 mmol, in 9 g of Hg) and PMe3 (0.284
mL, 2.74 mmol). After it was stirred overnight, the resulting
yellow-brown solution was evaporated to dryness, the residue
extracted with heptane (25 mL), and the extract filtered
through Celite until the washings were colorless. The com-
pound could not be crystallized out of solutions of saturated
hydrocarbons, and these solutions were therefore used directly
for the derivatization reactions with CO, H2, and PMe3 (vide
infra). The concentration of Cp*MoCl(PMe3)2 was determined
by reacting an aliquot of the heptane solution with H2 to afford
Cp*MoCl(H)2(PMe3)2 quantitatively and gas-volumetrically
measuring the amount of gas absorbed. On the basis of this
procedure the yield was 41%. 1H NMR (C6D6, δ): 44.8 (br s,
w1/2 ) 105 Hz, 15H, C5Me5), 17.8 (br s, w1/2 ) 55 Hz, 18 H,
PMe3). µeff (Evans’ method) ) 2.94 µB. An alternative to
amalgamated Na is using Na sand (3 mol/mol of Cp*MoCl4)
and naphthalene (0.3-0.5 mol/mol of Cp*MoCl4).
Syn th esis of Cp *MoCl(P Me2P h )2 (4). In a procedure
identical with that described above for the preparation of 3,
(23) Poli, R.; Owens, B. E.; Krueger, S. T.; Rheingold, A. L.
Polyhedron 1992, 11, 2301-2312.
(24) Abugideiri, F.; Kelland, M. A.; Poli, R. Organometallics 1992,
11, 1311-1318.

4410 Organometallics, Vol. 15, No. 21, 1996
Ta ble 1. Cr ysta l Da ta for Com p ou n d s 1, 5, a n d 7
Abugideiri et al.
compd
formula
fw
CpMoCl(PMe2Ph)3, 1
29H38ClMoP3
610.89
Cp*MoCl(dppe), 5
C36H39ClMoP2
665.00
Cp*MoCl(CO)(PMe2Ph)2, 7
C27H37ClMoOP2
570.93
C
space group
a, Å
b, Å
P21/n
P21/n
P21/c
9.572(2)
13.461(2)
21.452(3)
97.84(2)
2738(2)
4
1.38
11.7902(8)
15.2863(11)
16.2122(11)
105.288(6)
2818.5(3)
4
11.1404(12)
17.628(2)
16.936(2)
107.922(9)
3164.7(5)
4
c, Å
â, deg
V, Å3
Z
d
calc, g/cm3
1.440
7.44
1.396
6.24
µ(Mo KR), cm-1
7.02
radiation (monochromated
Mo KR (λ ) 0.710 73 Å)
Mo KR (λ ) 0.710 73 Å)
Mo KR (λ ) 0.710 73 Å)
in incident beam)
temp, K
153
153
296
transmissn factors: max,
0.9686, 0.8644
0.9627, 0.8029
1.000, 0.958
min
Ra
0.0611
0.1067
0.0365
0.0812
0.0459
0.0551
Rwb
a
b
2
R ) ∑||Fo| - |Fc||/∑|Fo|. Rw ) [∑w(|Fo| - |Fc|)2/∑w|Fo| ]1/2; w ) 1/σ2(|Fo|).
of Cp*MoCl(PMe3)2 obtained in situ as described above from
0.660 mmol of Cp*MoCl2(PMe3)2 was evaporated to dryness,
and the residue was extracted in pentane (20 mL). The
resulting solution was filtered and then exposed to CO at 1
atm. The formation of a brown precipitate occurred im-
mediately, which was then filtered off. Spectroscopic inves-
tigations of the mother liquor indicated the presence of
compound 6. MS (CI, negative ions; m/e): 448 ([M]-, 100%),
372 ([M]- - PMe3, 93%). IR (pentane, cm-1): 1786. 1H NMR
(C6D6, δ): 1.66 (s, 15H, Cp*), 1.30 (vt, J PH ) 4 Hz, 18H, PMe3).
31P{1H} NMR (C6D6, δ): 18.5.
(b) L ) P Me2P h . F or m a tion of tr a n s-Cp *MoCl(P Me2-
P h )2(CO) (7). A solution of Cp*MoCl(PMe2Ph)2 obtained in
situ as described above from 0.520 mmol of Cp*MoCl2(PMe2-
Ph)2 was evaporated to dryness, and the residue was extracted
in ether (20 mL). The resulting solution was filtered and then
exposed to CO at 1 atm. The solution immediately changed
color from yellow-brown to orange-brown. Cooling to -20 °C
over 1 week afforded 65 mg of compound 7 (27% yield), which
was isolated by decanting off the mother liquor and drying
under vacuum. Anal. Calcd for C27H37ClMoOP2: C, 56.80; H,
6.53; Cl, 6.21. Found: C, 56.4; H, 6.9; Cl, 6.9. IR (pentane,
cm-1): 1793. 1H NMR (C6D6, δ): 7.79 (m , 4 H, PMe2Ph), 7.08
(m, 6 H, PMe2Ph), 1.52 (s, 15H, Cp*), 1.44 (overlap of 2 vt, 12
H, PMe2Ph). 31P{1H} NMR (C6D6, δ): 22.0. A single crystal
for X-ray analysis of Cp*MoCl(PMe2Ph)2(CO) was obtained by
cooling a pentane solution to -80 °C.
Rea ction betw een Cp *MoClL2 a n d H2. (a ) L ) P Me3.
P r ep a r a tion of Cp *MoClH2(P Me3)2 (8). Cp*MoCl(PMe3)2-
(N2) (0.340 g, 0.761 mmol) was dissolved in heptane (20 mL)
and the solution exposed to an atmosphere of H2. After the
blue solution was heated to 45 °C for 1 h, the solution became
red. The absence of νN-N in the IR confirmed the complete
consumption of the starting material. The heptane solution
was concentrated to half the original volume and placed at
-80 °C overnight, precipitating a red-brown solid. The solid
was filtered and dried under vacuum. Yield: 0.140 g, 44%.
Anal. Calcd for C16H35ClMoP2: C, 45.67; H, 8.39. Found: C,
45.56; H, 8.39. 1H NMR (C6D6, δ): 1.76 (s, 15 H, C5Me5), 1.32
(d, 18 H, J PH ) 9 Hz, PMe3), -2.83 (t, 2H, J PH ) 50 Hz, MoH2).
31P{selective-1H} NMR (δ): 11.0 (t, J PH ) 38 Hz).
This compound was also generated by an alternative
method, i.e. addition of H2 to Cp*MoCl(PMe3)2, although it was
not isolated from this procedure. A solution of Cp*MoCl-
(PMe3)2 obtained in situ as described above from 0.44 mmol
of Cp*MoCl2(PMe3)2 was evaporated to dryness, and the
residue was extracted in heptane (20 mL) and filtered. The
resulting solution was then stirred at room temperature under
an atmosphere of H2, resulting in no apparent color change.
After 3 h, an aliquot of the solution was inspected by NMR
after evaporation to dryness and redissolution in C6D6, con-
firming the formation of compound 8 as the major product.
The 1H NMR also showed the presence of Cp*MoCl2(PMe3)2
(broad resonance at δ -2.3), which was present prior to the
reaction with H2.
(b) L ) P Me2P h . F or m a tion of Cp *MoClH2(P Me2P h )2
(9). A solution of Cp*MoCl(PMe2Ph)2 obtained in situ as
described above from 0.52 mmol of Cp*MoCl2(PMe2Ph)2 was
evaporated to dryness, and the residue was extracted in
heptane (20 mL) and the extract filtered. The resulting
solution was then stirred at room temperature under an
atmosphere of H2, resulting in no apparent color change. After
3 h, an aliquot of the solution was inspected by NMR after
evaporation to dryness and redissolution in C6D6, confirming
the formation of compound 9 as the major product. 1H NMR
(C6D6, δ): 1.71 (s, 15 H, C5Me5), 1.46 (d, 12 H, J PH ) 10 Hz,
PMe2Ph), -2.23 (t, 2H, J PH ) 50 Hz, MoH2). 31P{selective-
1H} NMR (δ): 21.8 (t, J PH ) 34 Hz). Other resonances in the
1H and 31P NMR spectra were due to free PMe2Ph and to other
unidentified diamagnetic and paramagnetic complexes.
X-r a y Cr ysta llogr a p h y. (a ) Cp MoCl(P Me2P h )3 (1). A
reddish purple crystal with dimensions 0.225 × 0.200 × 0.038
mm was placed and optically centered on the Enraf-Nonius
CAD-4 diffractometer. The parameters and crystal orientation
matrix were determined from 25 reflections in the range 17.5
< θ < 19.8° and confirmed with axial photographs. Data were
collected with ω/2θ scans over the range 2.25 < θ < 24.0°. No
decay correction was necessary. An absorption correction was
applied on the basis of crystal faces with transmission factors
ranging from 0.8644 to 0.9686. The uniquely determined
centrosymmetric monoclinic space group P21/n was indicated
by the systematic absences from the data and confirmed by
the successful completion of the structure. After data reduc-
tion and correction for Lorentz and polarization factors, direct
methods successfully located all the non-hydrogen atoms. All
hydrogen atoms were found from an initial difference-Fourier
map; however, hydrogen atoms attached to carbon atoms were
placed instead in calculated positions, with d(C-H3) ) 0.980
Å and UH ) 1.5U(parent) and with aromatic d(C-H) ) 0.950
Å and UH ) 1.2U(parent). All of the non-hydrogen atoms were
refined anisotropically by full-matrix least-squares cycles.
Crystal data are reported in Table 1, and selected bond
distances and angles are collected in Table 2.
(b) Cp *MoCl(d p p e) (5). A reddish purple crystal with
dimensions 0.375 × 0.375 × 0.050 mm was placed and optically
centered on the Enraf-Nonius CAD-4 diffractometer. The cell
parameters and crystal orientation matrix were determined
from 25 reflections in the range 16.0 < θ < 19.3°; these
constants were confirmed with axial photographs. Data were
collected (Mo KR) with ω/2θ scans over the range 2.25 < θ <
24.0°. No decay correction was necessary. Data were cor-

Four-Legged Piano Stool Mo(II) Compounds
Organometallics, Vol. 15, No. 21, 1996 4411
Ta ble 2. Selected In tr a m olecu la r Dista n ces (Å)
a n d An gles (d eg) for Cp MoCl(P Me2P h )3 (1)
Ta ble 4. Selected Bon d Dista n ces (Å) a n d An gles
(d eg) for Cp *MoCl(CO)(P Me2P h )2
Mo(1)-CNTa
Mo(1)-P(2)
Mo(1)-P(3)
1.974(9)
2.453(2)
2.459(2)
Mo(1)-P(1)
Mo(1)-Cl(1)
2.486(2)
2.585(2)
Mo-Cl
2.577(5)
2.479(2)
1.874(11)
2.342(8)
2.298(8)
2.377(9)
1.827(8)
1.830(8)
1.841(9)
1.114(15)
Mo-Cl(A)
Mo-P(2)
2.616(10)
2.469(2)
1.887(15)
2.304(8)
2.352(9)
2.017(9)
1.815(8)
1.833(8)
1.827(7)
1.104(17)
Mo-P(1)
Mo-C(O)
Mo-C(17)
Mo-C(19)
Mo-C(21)
P(1)-C(1)
P(1)-C(3)
P(2)-C(10)
C(O)-O
Mo-C(OA)
Mo-C(18)
Mo-C(20)
Mo-CNTa
P(1)-C(2)
P(2)-C(9)
P(2)-C(11)
C(OA)-OA
CNTa-Mo(1)-Cl(1) 109.9(2)
P(1)-Mo(1)-P(3)
P(2)-Mo(1)-P(3)
P(1)-Mo(1)-Cl(1)
P(2)-Mo(1)-Cl(1)
84.01(8)
85.20(8)
72.75(8)
77.79(8)
CNTa-Mo(1)-P(1)
CNTa-Mo(1)-P(2)
CNTa-Mo(1)-P(3)
P(1)-Mo(1)-P(2)
120.0(3)
114.7(3)
114.3(2)
123.72(8) P(3)-Mo(1)-Cl(1) 135.74(8)
aCNT is the center of gravity of atoms C(1)-C(5).
Cl-Mo-P(1)
79.7(1)
130.2(4)
76.0(2)
128.9(7)
116.6(1)
76.8(7)
76.6(4)
121.2(4)
111.9(8)
175(3)
Cl-Mo-P(2)
76.5(1)
111.9(4)
75.6(2)
119.1(4)
76.2(4)
122.1(4)
79.2(7)
117.8(5)
173.4(13)
Ta ble 3. Selected Bon d Dista n ces (Å) a n d An gles
(d eg) for Cp *MoCl(d p p e) (5)
Cl-Mo-C(O)
Cl-Mo-CNTa
Cl(A)-Mo-P(2)
Cl(A)-Mo-CNTa
P(1)-Mo-C(O)
P(1)-Mo-CNTa
P(2)-Mo-C(OA)
C(O)-Mo-CNTa
Mo-C(O)-O
Cl(A)-Mo-P(1)
Cl(A)-Mo-C(OA)
P(1)-Mo-P(2)
P(1)-Mo-C(OA)
P(2)-Mo-C(O)
P(2)-Mo-CNTa
C(OA)-Mo-CNTa
Mo-C(OA)-OA
Mo(1)-CNTa
Mo(1)-Cl(1)
1.998(5)
2.416(1)
Mo(1)-P(1)
Mo(1)-P(2)
2.428(1)
2.413(1)
CNTa-Mo(1)-Cl(1) 120.1(1) Cl(1)-Mo(1)-P(1) 83.59(4)
CNTa-Mo(1)-P(1)
CNTa-Mo(1)-P(2)
136.8(1) Cl(1)-Mo(1)-P(2) 92.94(5)
129.6(1) P(1)-Mo(1)-P(2) 78.71(4)
a
CNT is the center of gravity of atoms C(1)-C(5).
a
CNT ) centroid of atoms C(17)-C(21).
rected for Lorentz and polarization factors, and an absorption
correction was applied on the basis of crystal faces with
transmission factors ranging from 0.8029 to 0.9627. The
uniquely determined centrosymmetric monoclinic space group
P21/n was indicated by the systematic absences from the data
and confirmed by the successful completion of the structure.
Direct methods successfully located all of the heavy atoms (Mo,
P, Cl) and many of the carbon atoms. All remaining non-
hydrogen atoms were found from an initial difference-Fourier
map and/or had their positions calculated. Hydrogen atoms
were treated as described in the previous section for compound
1. Crystal data are assembled in Table 1, and selected bond
distances and angles are collected in Table 3.
dppe (5)) were obtained by four different methods: (i)
by reduction of Cp*MoCl4 in the presence of L (eq 1),
(ii) by reduction of Cp*MoCl2L2 (eq 2), (iii) by reduction
of CpMoCl2 in the presence of L (eq 3) (this reaction
probably proceeds by initial coordination of the phos-
phine ligand to generate the 17-electron CpMoCl2L2
complexes, followed by reduction as for the correspond-
ing Cp* systems in eq 2), and (iv) by thermal ligand
dissociation from the 18-electron complexes CpMoCl-
(PMe2Ph)3 and Cp*MoCl(PMe3)3 (eq 4).
THF, Ar
(1)
8 Cp*MoClL2 + 3NaCl
Cp*MoCl4 + 3Na + 2L
(c) Cp *MoCl(CO)(P Me2P h )2 (7). A single crystal was
glued to the inside of a thin-walled glass capillary, which was
then flame-sealed under dinitrogen and mounted on the
diffractometer. The cell determination, data collection, and
reduction were carried out as described above for compound
5. The position of the Mo atom was obtained from the analysis
of the Patterson map, and the positions of all the other non-
hydrogen atoms were revealed by a subsequent DIRDIF run.
The structure was then refined by least-squares cycles to
convergence with all non-hydrogen atoms anisotropic. The
final difference-Fourier map revealed disorder with a minor
orientation of the two relative trans Cl and CO ligands in the
opposite configuration (e.g. the Cl of the minor species occupies
the same coordination position as the CO of the major species
and vice versa). Both major and minor orientations were
introduced at variable occupancies, which were refined inde-
pendently (Cl, x; ClA, 1 - x; C and O, y; CA and OA, 1 - y). In
order to avoid instability, the following distances were re-
strained with DFIX cards in SHELX76: Mo-Cl ) Mo-ClA )
2.58(5) Å; Mo-C(O) ) Mo-C(OA) ) 1.94(2) Å; C(O)-O )
C(OA)-OA ) 1.15(2) Å. Refinement converged to R ) 0.0544
with all atoms except ClA and the carbonyl C and O atoms
for both major and minor orientations anisotropic, giving x )
0.66 and y ) 0.74, confirming the correctness of the model.
Finally, x and y were constrained to have the same value as
dictated by stoichiometry and the hydrogen atoms were
included in calculated positions as described above for the
structure of 1. Crystal data are reported in Table 1, and
selected bond distances and angles are collected in Table 4.
L ) PMe3, PMe2Ph; L2 ) dppe
Cp*MoCl2L2 + Na THF, Ar8 Cp*MoClL2 + NaCl (2)
L ) PMe3, PMe2Ph
CpMoCl2 + Na + 2PMe2Ph THF, Ar8
CpMoCl(PMe2Ph)2 + NaCl (3)
∆, C6D6
(Ring)MoClL3
8 (Ring)MoClL2 + L
(4)
Ring ) Cp, L ) PMe2Ph; Ring ) Cp*, L ) PMe3
Compounds 2-4 could not be isolated in the crystal-
line state and were charcacterized by spectroscopy and
by chemical derivatization (vide infra). However, com-
pound 5 was isolated as a crystalline solid and charac-
terized by 1H NMR, elemental (C,H) analysis, magnetic
susceptibility, and X-ray crystallography.
1
The H NMR spectra for all of the 16-electron com-
plexes show broad paramagnetically shifted resonances.
1
Figure 1a shows the H NMR spectrum of compound 3.
The broad resonance (w1/2 ) 105 Hz) at 45 ppm is
assigned to the Cp* ligand, whereas the peak at 18 ppm
(w1/2 ) 55 Hz) is assigned to the PMe3 ligand. The
qualitative features of this spectrum are identical in
other NMR solvents (i.e. acetone-d6) or in the presence
of THF. As expected, the 16-electron complexes are
EPR-silent, since the two possible spin states are S ) 1
and S ) 0. Systems with more than one unpaired
electron are generally EPR-inactive due to large zero-
field splittings and fast electronic relaxations. Com-
Resu lts
Syn th esis a n d Sp ectr oscop ic Ch a r a cter iza tion
of (Rin g)MoClL2. Solutions of the paramagnetic, 16-
electron (Ring)MoClL2 complexes (Ring ) Cp, L ) PMe2-
Ph (2); Ring ) Cp*, L ) PMe3 (3), PMe2Ph (4), L2
)

4412 Organometallics, Vol. 15, No. 21, 1996
Abugideiri et al.
nances, one set being for the two rings and methylene
protons pointing toward the Cp* ligand and the other
set for those directed away from it. As a result, the dppe
ligand should exhibit a total of eight resonances in a
4:4, 4:4, 2:2, and 2:2 ratio for the two sets of ortho, meta,
para, and methylene protons, respectively. The reso-
nances assigned to the ortho protons are at 76 ppm (w1/2
) 50 Hz) and 15 ppm (w1/2 ) 90 Hz). It is interesting
to note the large difference in the shifts of these two
resonances, much larger than for all of the other types
of protons. This can be attributed to a large difference
in through-space dipolar interaction with the electronic
spins. The meta and para protons show much less
differences in chemical shifts, i.e. 3.0 and 2.1 ppm,
respectively.
The preparation of 1 follows the procedure previously
reported for the preparation of the related CpMoCl-
(PMe3)3.22 The yield of this product is low, being
crystallized from Et2O at low temperatures. A large
amount of more soluble material remained in solution,
and no additional significant amount of 1 was recovered
upon further concentration and cooling. The more
soluble material was recovered by crystallization from
very concentrated ether solutions (see Experimental
Section). It consists of the paramagnetic compound
CpMoCl(PMe2Ph)2 (2), along with small amounts of the
hydride complex CpMoH(PMe2Ph)3 as an impurity. The
latter is recognized from the diagnostic hydride reso-
nance, split into a binomial quartet by coupling to the
1
three equivalent phosphorus nuclei, at δ -7.7 in the H
NMR spectrum. The 1H NMR spectrum of 2 (see Figure
1c), like the other 16-electron complexes described
above, shows paramagnetically shifted and broadened
resonances. The Cp ligand was so broadened and/or
contact-shifted that it could not be detected. The
analysis of the relative intensities and line widths led
to the reasonable assignment of the δ 16.6 (w1/2 ) 80
Hz) resonance to the PMe2Ph methyl protons and the δ
21.5 (w1/2 ) 79 Hz), 10.9 (w1/2 ) 25 Hz), and 0.85 (w1/2
) 42 Hz) resonances to the PMe2Ph ortho, para, and
meta protons, respectively. The thermal treatment of
1 in C6D6 for several hours produces an observable
quantity of the same paramagnetic product 2 (eq 4). The
proposed phosphine dissociation/association process must
be sufficiently slow on the NMR time scale because 1
and 2 are independently observed. The analogous
equilibrium between the 18-electron Cp*MoCl(PMe3)3
and the paramagnetic 3 is also quite slow.18 The exact
position of the equilibrium that is established by heating
the 18-electron tris(phosphine) compound could not be
determined due to the formation of Cp*MoCl2(PMe3)2
as well as Cp*MoH(PMe3)3, the thermal decomposition
products of 3.
F igu r e 1. 1H NMR of (a) Cp*MoCl(PMe3)2 (3), (b)
Cp*MoCl(dppe) (5), and (c) CpMoCl(PMe2Ph)2 (2). All
spectra were recorded in C6D6. The starred resonance
corresponds to C6D5H.
pound 4 also shows a broad and strongly shifted Cp*
1
resonance in the H NMR spectrum (at δ 71 with w1/2
) 230 Hz). The diastereotopic methyl protons of the
PMe2Ph ligands are obeserved at δ 16.3 and 15.3 (w1/2
) 115 Hz). The phenyl protons also show paramag-
netically shifted and broadened peaks: the ortho pro-
tons at δ 6.1 (w1/2 ) 150 Hz), the meta at δ 7.8 (w1/2
)
Ma gn et ic P r op er t ies. The paramagnetism ob-
served for the solutions of 2-5 (as indicated by the
paramagnetically shifted resonances in the 1H NMR)
suggests a spin triplet ground state. The solid-state
magnetic moment of 5 was determined to be 2.65 µB.
This is in excellent agreement with the theoretical spin-
only value of 2.83 µB. As stated above, the other 16-
electron species could not be crystallized. However, the
magnetic moment of 3 could be determined in solution
as µeff ) 2.93 µB by the Evans method. One of the
concerns with systems of this type is the presence of a
spin equilibrium. This occurs when the energy gap
26 Hz), and the para at δ 8.9 (w1/2 ) 22 Hz). The
assignment is based on relative line widths (the protons
closest to the paramagnetic metal center, i.e. ortho, have
the broader resonance) and relative intensity (the para
protons are half the intensity of the meta protons).
1
The analysis of the H NMR spectrum of compound
5 (see Figure 1b) also deserves some attention, espe-
cially with respect to the phenyl protons of the dppe
ligand. The broad resonance at 63 ppm (w1/2 ) 150 Hz)
is assigned to the Cp* ligand and compares well with
that found for 3 and 4. The symmetry of the molecule
dictates two equal sets of phenyl and methylene reso-

Four-Legged Piano Stool Mo(II) Compounds
Organometallics, Vol. 15, No. 21, 1996 4413
F igu r e 3. ORTEP diagram of CpMoCl(PMe2Ph)3 (1).
than the 16-electron precursors, allowing in some cases
isolation in the crystalline state. Compound 5 also
forms a CO adduct, as already described elsewhere.25
Cp*MoClL2 + CO THF8 Cp*MoClL2(CO)
L ) PMe3, PMe2Ph
(5)
Cp*MoClL2 + H2 THF8 Cp*MoCl(H)2L2
L ) PMe3, PMe2Ph
(6)
The CO adducts 6 and 7 were characterized by IR and
1H and 31P NMR spectroscopy, and by an X-ray analysis
of 7. The CO stretching vibrations are observed at 1786
cm-1 for 6 and 1793 cm-1 for 7, in agreement with the
greater basicity of the PMe3 ligand as compared to
PMe2Ph. Each compound shows a single phosphorous
resonance in the 31P NMR and a virtual triplet reso-
nance in the 1H NMR for the phosphine methyl protons,
indicating equivalent phosphines in a trans geometry.
This structural supposition is further confirmed by the
X-ray analysis of 7 (vide infra).
1
The H and 31P NMR properties of 8 and 9 are fully
consistent with the formation of a classical Mo(IV)
dihydride complex with rapidly exchanging H ligands.
Each compound shows a singlet for the Cp* resonance,
a doublet for the methyl resonances of the phosphine
ligand, and a triplet for the MoH2 resonances, indicating
coupling to two equivalent phosphines. The 31P{1H-
selective} NMR spectra show a single triplet resonance,
confirming the conclusion that, on the NMR time scale,
the two phosphorus nuclei are equivalent and are
coupled to two equivalent hydrides. The NMR proper-
ties of these chloro/dihydride complexes correspond
quite closely to those of (C5H4-i-Pr)MoClH2(PMe3)2,
which was shown to have a single hydride resonance
for the two rapidly exchanging inequivalent classical
hydride ligands at room temperature, which decoalesced
upon cooling to low temperature.26
F igu r e 2. Plot of δ versus T-1 for (a) Cp*MoCl(PMe3)2 (3)
and (b) Cp*MoCl(PMe2Ph)2 (4).
between the ground state and the excited state is small
enough to allow thermal population of the latter. In
order to probe for this possibility, variable-temperature
NMR experiments were carried out on compounds 3 and
4. As the temperature was lowered, the resonances for
the paramagnetic protons became broader and more
contact-shifted as expected. Figure 2 shows the depen-
dence of the chemical shift versus T-1. The observed
linear behavior indicates that the system follows Curie-
Weiss behavior and therefore excludes a significant
thermal population of an excited singlet state.
Rea ction of Cp *MoClL2 w ith CO a n d H2. Treat-
ment of solutions of compounds 3 and 4 with CO (eq 5)
gives rise to formation of the 18-electron CO adducts
Cp*MoCl(CO)L2 (L ) PMe3 (6), PMe2Ph (7)), and
treatment with H2 (eq 6) gives rise to the formation of
the products of H-H oxidative addition, the Mo(IV)
complexes Cp*MoClH2L2 (L ) PMe3 (8), PMe2Ph (9)).
These 18-electron H2 and CO adducts are less soluble
X-r a y An a lyses. The structures of compounds 1, 5,
and 7 have been determined by X-ray diffraction meth-
ods. Views of the three molecules are in Figures 3-5,
respectively. The geometry of 5 is a typical three-legged
piano stool; this is the first reported structure for a
(25) Fettinger, J . C.; Keogh, D. W.; Poli, R. J . Am. Chem. Soc. 1996,
118, 3617-3625.
(26) Grebenik, P. D.; Green, M. L. H.; Izquierdo, A.; Mtetwa, V. S.
B.; Prout, K. J . Chem. Soc., Dalton Trans. 1987, 9-19.

4414 Organometallics, Vol. 15, No. 21, 1996
Abugideiri et al.
donor), the only other being that of CpMoCl(triphos).27
The most interesting angular parameters are the angles
between the Mo-CNT vector and the other bonds. In
the structure of 1, the two relative trans donors Cl and
P2 have smaller angles than the other donors P1 and
P3. This follows the established pattern of angular
trans influence,29 which places the pair of ligands with
overall stronger σ-bonding ability at smaller angles in
order to maximize the interaction with the more favor-
able metal orbitals.30 This angular trans influence was
also observed in the structure of CpMoCl(triphos).27 The
Mo-Cl distance is significantly longer in compound 1
(2.58(1) Å) compared to CpMoCl(triphos) (2.526(2) Å),
but is in each case longer than for the Mo(III) complex
-
[CpMoCl(PMe3)3]+ (2.427(9) Å in the PF6 salt22 and
F igu r e 4. ORTEP diagram of Cp*MoCl(dppe) (5).
-
2.509(3) Å in the BF4 salt31 ), as expected. A longer
Mo-Cl bond length is also observed in 7 (2.577(5) Å),
this being comparable to the same distance in 1. Other
relevant four-legged piano stool Mo(II) complexes also
show longer Mo-Cl distances (e.g. 2.541(5) Å in Cp-
33
MoCl(CO)(dppe)32 and 2.542(9) Å in CpMoCl(CO)3 ).
The Mo-P distances are slightly shorter for 1 (average
2.46(1) Å) than for CpMoCl(triphos) (average 2.48(1) Å)
with the length for 7 (2.470(2) Å) falling directly in
between. A comparison with other literature values
(2.388(8) Å in (C5H4Me)MoI(CO)2[P(OMe3)3],34 2.50(1)
Å in CpMoI(CO)2(PBu3),35 2.532(6) Å in CpMoBr(CO)2-
(PPh3),36 2.473(3) Å in CpMo(COCH3)(CO)2(PPh3),37 and
2.47(3) Å in CpMoCl(dppe)(CO)32) suggests that the
Mo-P bonds tend to shorten when more phosphine and
fewer CO ligands are present in the coordination sphere
(consistent with diminished π-competition), and a fur-
ther shortening effect is also due to the presence of
chelate rings. A similar effect is also observable for the
M-C(O) bond length, which is shorter in compound 7
(1.88(2) Å) than the average distance in other com-
pounds with less donating ancillary ligands: 1.938(18)
Å in CpMoCl(dppe)(CO),32 1.94(6) Å in CpMoI(CO)2-
(PBu3),35 1.951(13) Å in CpMo(COCH3)(CO)2(PPh3),37
2.003(34) Å in (C5H4Me)MoI(CO)2[P(OMe3)3],34 and
2.091(14) Å in CpMoBr(CO)2(PPh3).36
F igu r e 5. ORTEP diagram of Cp*MoCl(CO)(PMe2Ph)2 (7).
Discu ssion
paramagnetic, 16-electron, organometallic Mo(II) com-
plex. The average L-M-L angle is 85.1(1)°, which
compares with the value of 84.7(2)° for the diamagnetic
CpW(CO)2(POCMe2CMe2O). The average CNT-Mo-L
angle (CNT is the Cp ring centroid), i.e. 129(1)°, is
significantly larger than for four-legged piano stool
compounds, i.e. 115(1)° for 1. One of the other expected
structural characteristics is a shorter Mo-Cl length
(2.416(1) Å) compared to 18-electron Mo(II) four-legged
species (e.g. 2.585(2) Å for 1 and 2.526(2) Å for CpMoCl-
(triphos) (triphos ) Ph2PCH2CH2P(Ph)CH2CH2PPh2)).27
A much less significant difference is observed for the
Mo-P lengths, those of 5 (average 2.420(1) Å) being only
slightly shorter than for 1 (average 2.466(2) Å).
Half-sandwich 18-electron complexes of Mo(II) con-
stitute a very extensive class of compounds.38,39 Related
complexes with a 16-electron configuration are rare, and
most, especially when they contain one or more CO
ligands, have only been observed as transients in
chemical reactions. For instance, the CpMoX(CO)2 (X
) Cl, CH3) fragments are obtained by photolytic CO
ejection from CpMoX(CO)3, but the primary photoprod-
uct rapidly reacts with N2, C2H4, CO, and other sub-
(30) Lin, Z.; Hall, M. B. Organometallics 1993, 12, 19-23.
(31) Fettinger, J . C.; Kraatz, H.-B.; Poli, R.; Rheingold, A. L. Acta
Crystallogr., Sect. C 1995, C51, 364-367.
(32) Bush, M. A.; Hardy, A. D. U.; Manojlovic-Muir, L.; Sim, G. A.
J . Chem. Soc. A 1971, 1003-1009.
(33) Chaiwaise, S.; Fenn, R. H. Acta Crystallogr., Sect. B 1968, B24,
525-5219.
Molecular geometries of 1 and 7 are based on the
“four-legged piano stool”, which is ubiquitous for com-
pounds of the CpML4 type.28,29 The structure of 1 is
one of the first structures for a fully phosphine-
substituted CpMoXL3 complex (X ) any one-electron
(34) Hardy, A. D. U.; Sim, G. A. J . Chem. Soc., Dalton Trans. 1972,
1900-1903.
(35) Fenn, R. H.; Cross, J . H. J . Chem. Soc. A 1971, 3312-3315.
(36) Sim, G. A.; Sime, J . G.; Woodhouse, D. I.; Knox, G. R. Acta
Crystallogr., Sect. B 1979, B35, 2403-2406.
(37) Churchill, M. R.; Fennessey, J . P. Inorg. Chem. 1968, 7, 953-
959.
(27) Cole, A. A.; Mattamana, S. P.; Poli, R. Polyhedron 1996, 15,
2351-2361.
(38) Wilkinson, G.; Stone, F. G. A.; Abel, E. W. Comprehensive
Organometallic Chemistry; Pergamon: Oxford, U.K., 1982.
(39) Wilkinson, G.; Stone, F. G. A.; Abel, E. W. Comprehensive
Organometallic Chemistry II; Pergamon: Oxford, U.K., 1995.
(28) Kuba´cek, P.; Hoffmann, R.; Havlas, Z. Organometallics 1982,
1, 180-188.
(29) Poli, R. Organometallics 1990, 9, 1892-1900.

Four-Legged Piano Stool Mo(II) Compounds
Organometallics, Vol. 15, No. 21, 1996 4415
causing contraction of the metal-based orbitals and
therefore a greater pairing energy. The spin state
difference between the 6-coordinate, 14-electron deriva-
tives trans-TiX2(dmpe)2 (S ) 1 for X ) Cl; S ) 0 for X
) CH3)46 has analogously been attributed to smaller
pairing energies in the dimethyl compound because of
the lower electronegativity of CH3 relative to Cl.47
These Cp*MoX(PMe3)2 systems are isolobal with the
known trans-octahedral MoCl2(PMe3)4,48 which shows
4
the expected triplet ground state for the pseudo-t2g
configuration.
It is important to realize that, although each Mo-Cl
π interaction is rendered less effective by the antibond-
ing electron with respect to the single Mo-PR2 π-inter-
action, the overall Mo-Cl π-donation is still worth 2
electrons and contributes to the energetic stabilization
of the molecule. This is most clearly indicated by the
significant contraction of the Mo-Cl bond on going from
the 18-electron 1 and 7 to the 16-electron 5 (see Results).
For this reason, we represented the Mo-Cl π-interac-
tions as two single-pointed arrows in Figure 6a.
The 16-electron complexes (Ring)MoClL2 react with
2-electron-donor ligands, i.e. CO and H2, to afford stable
saturated 18-electron complexes. It has also been
shown earlier that 3 reacts with N2 to afford an
18-electron dinitrogen adduct.18 These reactions involve
a spin state change from S ) 1 to S ) 0. Evidently, the
energetic gain associated with the formation of the new
bonds more than compensates for the cost of pairing the
electrons and for the loss of Mo-Cl π-stabilization. It
is expected that the strength of the new M-L bonds will
affect the overall enthalpic picture. Indeed, the stronger
Mo-CO bond results in a quantitative carbonylation
reaction, whereas N2 leads to an equilibrium mixture
of starting material and dinitrogen adduct18 and weaker
donor molecules such as noble gases, hydrocarbons,
acetone, and THF do not react. The energetics of these
adduct formations have been investigated theoretically
and are reported in a separate contribution, which also
addresses the effect of this spin state change on the
reaction kinetics.49
A final point of consideration is related to the different
behavior of Mo and W. Baker et al. have reported that
the reduction of Cp*WCl4 with 3 equiv of Na in the
presence of PMe3 (a procedure identical with that in eq
1 for the synthesis of 3) leads to the compound Cp*W-
(H)(Cl)(PMe3)(η2-CH2PMe2), featuring a metallated PMe3
ligand.20 The formation of this product probably in-
volves a C-H oxidative addition process on a 16-electron
Cp*WCl(PMe3)2 intermediate, i.e. identical with the
stable Mo product 3 (see Scheme 1). On the other hand,
compound 3 fails to afford the corresponding product
of PMe3 metalation. Rather, upon warming, it decom-
poses, probably via a radical mechanism, to produce
Cp*MoCl2(PMe3)2 and Cp*MoH(PMe3)3 as the only
identified products by NMR and EPR.
F igu r e 6. Qualitative MO diagram for the M-X π inter-
action: (a) interation with the double-sided Cl; (b) interac-
tion with the single-sided PR2.
strates to afford saturated adducts.40-42 Rare examples
of stable 16-electron complexes are based on the general
formula (Ring)MoXL2, where Ring is Cp or a substituted
derivative, L is CO or phosphine, and X is an efficient
π-donor group such as a phosphido or arsenido lig-
and.20,43-45 Consequently, all these complexes are ef-
fectively electronically saturated and are diamagnetic.
Nevertheless, some of them readily add ligands to form
18-electron adducts. The complexes 2-5 reported in
this contribution are the first organometallics of Mo(II)
to be stable as spin triplet ground states for a 16-
electron configuration.
It is interesting to compare the spin triplet compound
Cp*MoCl(PMe3)2 with the previouly reported spin sin-
glet compounds Cp*Mo(PR2)(PMe3)2 (R ) Ph, cyclo-
hexyl).20 This spin state difference can be rationalized
on the basis of differences in frontier orbital gaps and
pairing energies. Considering these complexes in a
similar light as low-valent organometallics with covalent
bonding, the open-shell configuration can gain stability
through π-bonding. Chlorine is a double-sided π-donor
and is therefore capable of interacting with and raising
the energy of two of the frontier metal orbitals (see
Figure 6a). Consequently, the gap between the two
highest orbitals is small. Conversely, the phosphide is
a single-sided π-donor and is only able to raise the
energy of one metal orbital (Figure 6b), resulting in a
larger energy gap between the two highest metal-based
orbitals. From the point of view of pairing energies, it
has to be recognized that the chlorine atom has a higher
electronegativity than the PR2 function; thus, a greater
effective positive charge on the metal center is expected,
This different behavior may be attributed either to a
greater kinetic barrier to the C-H oxidative addition
process in the Mo case or to the greater thermodynamic
(40) Hooker, R. H.; Mahmoud, K. A.; Rest, A. J . J . Chem. Soc.,
Dalton Trans. 1990, 1231-1241.
(41) Hill, R. H.; Becalska, A.; Chiem, N. Orgtanometallics 1991, 10,
2104-2109.
(42) Virrels, I. G.; George, M. W.; J ohnson, F. P. A.; Turner, J . J .;
Westwell, J . R. Organometallics 1995, 14, 5203-5208.
(43) Luksza, M.; Kimmer, S.; Malisch, W. Angew. Chem., Int. Ed.
Engl. 1983, 22, 416-417.
(44) Gross, E.; J o¨rg, K.; Fiederling, K.; Go¨ttlein, A.; Malisch, W.;
Boese, R. Angew. Chem., Int. Ed. Engl. 1984, 23, 738-739.
(45) Malisch, W.; Pfister, H. Organometallics 1995, 14, 4443-4445.
(46) Morris, R. J .; Girolami, G. S. Inorg. Chem. 1990, 29, 4167-
4169.
(47) Simpson, C. Q., II; Hall, M. B.; Guest, M. F. J . Am. Chem. Soc.
1991, 113, 2898-2903.
(48) Carmona, E.; Mar´ın, J . M.; Poveda, M. L.; Atwood, J . L.; Rogers,
R. D. Polyhedron 1983, 2, 185-193.
(49) Keogh, D. W.; Poli, R. J . Am. Chem. Soc., submitted for
publication.

4416 Organometallics, Vol. 15, No. 21, 1996
Abugideiri et al.
Sch em e 1
complex would provide thermodynamic stabilization
with respect to the formation of a diamagnetic 18-
electron LnM(L′) adduct, it would not provide any
stabilization with respect to the formation of a diamag-
netic 18-electron LnM(X)(Y) oxidative-addition product.
The spin state of the hypothetical 16-electron Cp*WCl-
(PMe3)2, let alone the magnitude of the singlet-triplet
gap, is not known; therefore, the relevance of the spin
state change to the different behavior of Mo and W in
Scheme 1 remains undetermined. However, a more
favored oxidative-addition process for the W system
could also derive from an increased strength of the W-H
and W-CH2 bonds with respect to the same bonds for
Mo. It is an experimental fact that oxidative-addition
processes are more favored for 5d relative to 4d systems,
even when there is no spin state change involved in
either process (e.g. addition to square-planar Rh(I) and
Ir(I) Vaska-type complexes).
stability of the 16-electron system with respect to the
product of PMe3 metalation in the case of Mo. The effect
of the spin state change on the thermodynamic picture
for an oxidative-addition process should be opposite to
that of a ligand addition (see Introduction), because the
ideal removal of the X and Y radicals from an 18-
electron LnM(X)(Y) complex leaves the metal with two
unpaired electrons (triplet state). Siegbahn has ex-
plained in this manner the computational results of a
lower thermodynamic gain for the oxidative addition of
CH4 to CpRh(CO) with respect to CpIr(CO), since the
spin triplet starting complex is less stable by 5.9 kcal/
mol for Rh and more stable by 0.3 kcal/mol for Ir.5
Thus, while a triplet configuration for a 16-electron MLn
Ack n ow led gm en t. We are grateful to the National
Science Foundation (Grant No. CHE-9508521) for sup-
port of this work.
Su p p or tin g In for m a tion Ava ila ble: For compounds 1,
5, and 7, tables of crystal data and refinement parameters,
fractional atomic coordinates, bond distances and angles,
anisotropic thermal parameters, and H atom coordinates (27
pages). Ordering information is given on any current mast-
head page.
OM960331D