Synthesis, structure, and hydride-deuteride exchange studies of CpMoH3(PMe2Ph)2 and theoretical studies of the CpMoH3(PMe3)2 model system

Article abstract of DOI:10.1021/om960360o

The synthesis and characterization of CpMoH₃(PMe₂Ph)₂,1, is described. Compound 1 is obtained from the reaction between CpMoCl₃, PMe₂Ph, and LiAlH₄, in 61% yield. Compound 1 has also been obtained from the reaction of CpMo(o-C₆H₄PMe₂)(PMe₂Ph)₂ with H₂. Characterization of 1 by ¹H- and ³¹P-NMR spectroscopies shows a high degree of fluxionality for the hydride atoms, even at low temperatures. A single-crystal X-ray structure indicates that the geometry is pseudo-octahedral, with a relative mer arrangement of the three H ligands and the two PMe₂Ph ligands occupying relative trans positions. This is in contrast with the only other previously reported structure of a Mo(IV) trihydride, Cp*MoH₃(dppe), which adopts a pseudo trigonal prismatic structure. The Mo-P (average 2.41(8) ?) and Mo-H distances (average 1.64(4) ?) are similar to those found in Cp*MoHs(dppe). The closest H-H distance is 1.79 ?, consistent with a classical hydride. The results of ab initio calculations for the CpMoH₃CPMe₃)₂ model in different configurations agree with the experimental observations and suggest that a mechanism of hydride exchange consisting of a Bailar twist, which interconverts pseudo-octahedral mer, trans and fac geometries, is possible. The process of hydride-deuteride exchange of 1 in C₆D₆ is also examined.

Full text of DOI:10.1021/om960360o

Organometallics 1997, 16, 1179-1185
1179
Syn th esis, Str u ctu r e, a n d Hyd r id e-Deu ter id e Exch a n ge
Stu d ies of Cp MoH3(P Me2P h )2 a n d Th eor etica l Stu d ies of
th e Cp MoH3(P Me3)2 Mod el System
Fatima Abugideiri, J ames C. Fettinger, Brett Pleune, and Rinaldo Poli*,†
Department of Chemistry and Biochemistry, University of Maryland,
College Park, Maryland 20742
Craig A. Bayse and Michael B. Hall*
Department of Chemistry, Texas A&M University, College Station, Texas 77843
Received May 14, 1996X
The synthesis and characterization of CpMoH3(PMe2Ph)2, 1, is described. Compound 1 is
obtained from the reaction between CpMoCl3, PMe2Ph, and LiAlH4, in 61% yield. Compound
1 has also been obtained from the reaction of CpMo(o-C6H4PMe2)(PMe2Ph)2 with H2.
Characterization of 1 by 1H- and 31P-NMR spectroscopies shows a high degree of fluxionality
for the hydride atoms, even at low temperatures. A single-crystal X-ray structure indicates
that the geometry is pseudo-octahedral, with a relative mer arrangement of the three H
ligands and the two PMe2Ph ligands occupying relative trans positions. This is in contrast
with the only other previously reported structure of a Mo(IV) trihydride, Cp*MoH3(dppe),
which adopts a pseudo trigonal prismatic structure. The Mo-P (average 2.41(8) Å) and
Mo-H distances (average 1.64(4) Å) are similar to those found in Cp*MoH3(dppe). The closest
H-H distance is 1.79 Å, consistent with a classical hydride. The results of ab initio
calculations for the CpMoH3(PMe3)2 model in different configurations agree with the
experimental observations and suggest that a mechanism of hydride exchange consisting of
a Bailar twist, which interconverts pseudo-octahedral mer, trans and fac geometries, is
possible. The process of hydride-deuteride exchange of 1 in C6D6 is also examined.
In tr od u ction
or protonation16-25 of these systems have been reported
in recent years. The (ring)MH3L2 polyhydride system
(M ) Mo or W) has been known since 197926 but has
not been the subject of detailed studies. In particular,
no structural information was reported until the recent
investigation of Cp*MoH3(dppe) in our laboratory,27
which was determined to be pseudotrigonal prismatic,
contrary to the expected pseudo-octahedral structure.
A mechanism of hydride scrambling was proposed based
on the interconversion between the pseudo trigonal
prismatic and pseudo-octahedral geometries. In order
Transition metal polyhydride systems have been the
focus of much recent attention. The determination of
classical and nonclassical hydrides,1-3 the mechanism
of hydride fluxionality,4 and other studies on oxidation5-15
† Current address: Laboratoire de Synthe`se et d’EÄ lectrosynthe`se
Organome´tallique, Faculte´ des Sciences “Gabriel”, 6, Boulevard Gab-
riel, 21100 Dijon, France. E-mail: Rinaldo.Poli@u-bourgogne.fr.
X Abstract published in Advance ACS Abstracts, February 15, 1997.
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S0276-7333(96)00360-3 CCC: $14.00 © 1997 American Chemical Society

1180 Organometallics, Vol. 16, No. 6, 1997
Abugideiri et al.
6H, J PH ) 9.8 Hz), -4.18 (d, 5H, J PH ) 52.0 Hz). 31P-NMR
(C6D6): δ 26.9 (s, PMe2Ph).
to establish whether the unexpected pseudo trigonal
prismatic geometry is the result of electronic or steric
factors, we have synthesized and crystallographically
characterized a less sterically hindered compound of this
genre, namely CpMoH3(PMe2Ph)2, 1. As will be shown
here, the geometry of this compound corresponds to the
originally anticipated pseudo-octahedral geometry. The
hypothesis of a pseudo trigonal prismatic-pseudo-
octahedral interconversion as a mechanism of hydride
scrambling will be examined in more detail and sup-
ported by theoretical calculations on model systems in
both geometries. We also report here on the slow H/D
Rea ction of Cp MoCl2(P Me2P h )2 w ith LiAlH4. F or m a -
tion of 1 a n d Cp MoH(P Me2P h )3 (3). CpMoCl2 (83 mg, 0.357
mmol) and 4 equiv of PMe2Ph (204 µL, 1.428 mmol) were added
to 2 mL of toluene. To this mixture, LiAlH4 (100 mg, 0.263
mmol) was added slowly, as a powder. Over the next 12 h,
the yellow-brown starting material slowly dissolved as the
solution color changed from red-brown to yellow-brown over a
grey precipitate. An aliquot of this solution was filtered, the
solvent was removed under reduced pressure, and the residue
was redissolved in C6D6 for 1H-NMR analysis, which showed
3 as the major product, as well as another broad triplet
resonance centered at δ -9.40 (J PH ) 22.0 Hz). The solution
was allowed to stir for 1 week, and another aliquot was taken
for 1H-NMR analysis, which showed 1 as the major product
with 3 and the broad triplet resonance as minor products. After
2 weeks, MeOH was added to the solution until gas evolution
ceased, followed by complete removal of the solvent under
reduced pressure. NMR spectroscopy showed a nearly 1:1
mixture of 1 and 2 as the only hydride products. The above
reaction was repeated using 2 equiv of PMe2Ph with similar
results. 1H-NMR of 3 (C6D6): δ 7.8-6.9 (m, 15H, Ph), 4.50 (s,
5H, Cp), 1.57 (d, 18H, Me, J PH ) 6.2 Hz), -7.68 (q, 1H, M-H,
J PH ) 50.9 Hz). 31P-NMR (C6D6): δ 34.7 (s, PMe2Ph).
Rea ction betw een Cp Mo(o-C6H4P Me2)(P Me2P h )2 a n d
H2. F or m a tion of 1 a n d 3. CpMo(o-C6H4PMe2)(PMe2Ph)2
(42 mg, 0.073 mmol) was dissolved in C6D6 (1 mL) and
introduced in a thin-walled 5 mm NMR tube. After one
freeze-pump-thaw cycle, the sample was exposed to H2 (1
atm) and the tube was flame sealed. This solution was kept
at room temperature and exposure to normal laboratory
fluorescent light for several days, with 1H- and 31P-NMR
monitoring. The 1H-NMR spectrum indicated the formation
of 3 (initially) and then 1, followed by H/D exchange at the
hydride positions on a longer time scale. Exposure of the
solution to UV light accelerated the rate of H/D exchange. Full
details are discussed in the Results section. 1H-NMR reso-
1
exchange for 1 in C6D6, which gives rise to distinct H-
NMR hydride resonances for all CpMoHnD3-n(PMe2Ph)2
species (n ) 3, 2, 1).
Exp er im en ta l Section
All manipulations were carried out under an inert atmo-
sphere of nitrogen or argon by the use of vacuum-line, Schlenk,
syringe, or drybox techniques. Solvents were dried by con-
ventional methods and distilled under nitrogen prior to use.
Deuterated solvents were dried over molecular sieves and
degassed by three freeze-pump-thaw cycles prior to use.
Methanol was degassed by three freeze-pump-thaw cycles
prior to use. 1H-, 2H-, and 31P{1H}-NMR measurements were
made on Bruker AF200, WP200, or AM400 spectrometers; the
peak positions are reported with positive shifts downfield of
TMS (1H, 2H), as calculated from the residual solvent peaks
(1H) or from external D2O (2H), and 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 reference.
The standard inversion-recovery-pulse sequence 180-τ-90
was used to determine T1. Values of T1 were obtained from
the slopes of linear plots of ln(2Ieq/(Ieq - Iτ)) vs τ, where Ieq is
the peak intensity at τ ) ∞. PMe2Ph (Strem Chemical Co.)
and LiAlH4 (Aldrich) were used without further purification.
CpMoCl328 and CpMo(o-C6H4PMe2)(PMe2Ph)229 were prepared
according to literature procedures.
nance for the Mo-H protons in 1-dn (δ, C6D6): -5.16 (t, J PH
)
40.5 Hz, d0); -5.20 (t, J PH ) 40.4 Hz, d1); -5.24 (t, J PH ) 39.7
Hz, d2). The C6D6 solvent was evaporated under reduced
2
pressure, and the residue was redissolved in C6H6 for H-NMR
Syn th esis of Cp MoH3(P Me2P h )2 (1). To a suspension of
810 mg of CpMoCl3 (3.03 mmol) in 50 mL of THF was added
1.045 mL of PMe2Ph (7.57 mmol). The suspension rapidly
dissolved and turned red-brown. One gram of LiAlH4 (24.0
mmol) was slowly added, as a powder, to the solution. The
solution immediately turned orange-yellow. After the addition
was complete, the mixture was stirred for 60 min. Methanol
(10 mL) was then added dropwise at 0 °C, resulting in vigorous
evolution of H2. After H2 evolution ceased, the solvent was
evaporated under vacuum and the resulting residue was
extracted into heptane (150 mL). The heptane solution was
filtered and concentrated to ca. 5 mL. The solution began to
develop an orange precipitate; the mixture was stored at -80
°C for 12 h. The solution was filtered, and the microcrystalline
precipitate was washed with cold (-80 °C) heptane. The solid
was dried in vacuo. Yield: 816 mg (61%). This crude solid
proved to be a mixture of compounds 1 and CpMoH5(PMe2Ph)
spectroscopy. Three broad resonances were observed in the
deuteride region of the 2H-NMR spectrum, centered at δ -5.07,
-5.29, and -5.50 ppm. This reaction was also repeated in
toluene-d8 with identical results, except that the rate of H/D
exchange was, in this case, dramatically reduced (see Results
section). The solution of 1 and 3 in toluene-d8 was used for
the T1 measurements.
X-r a y An a lysis for Com p ou n d 1. A reddish-purple
crystal with dimensions 0.38 × 0.25 × 0.23 mm was placed
and optically centered on the Enraf-Nonius CAD-4 diffracto-
meter. The crystal final cell parameters and crystal orienta-
tion matrix were determined from 25 reflections in the range
15.1° < θ < 19.1° and confirmed with axial photographs. The
data did not need correction for decay but were corrected for
absorption, on the basis of the variation in the intensity of
the ψ-scan of eight reflections (transmission factors ranging
from 0.5026-0.5524).
1
(2) by H-NMR: the pentahydride byproduct constituted less
than 12% of the material by NMR integration. Pure 1 was
obtained by recrystallization from a saturated solution of
heptane, affording orange-purple crystals. One of the crystals
obtained in this manner was used for the X-ray analysis.
1H-NMR of 1 (C6D6): δ 7.65-7.08 (m, 10H, Ph), 4.25 (s, 5H,
Cp), 1.62 (d, 12H, Me, J PH ) 7.6 Hz), -5.14 (t, 3H, M-H, J PH
) 40.5 Hz). 31P-NMR (C6D6): δ 36.3 (s, PMe2Ph). 1H-NMR of
2 (C6D6): δ 7.68-7.02 (m, 5H, Ph), 4.86 (s, 5H, Cp), 1.55 (d,
On the basis of systematic absences, the space group could
be either C2/c (No. 15) or Cc (No. 9). Intensity statistics
indicated the former, along with the cell contents requiring
eight asymmetric units. Direct methods resulted in the
successful location of the Mo and P atoms. The remaining non-
hydrogen atoms were found from an initial difference Fourier
map. After full-matrix least-squares refinement, all of the
hydrogen atoms were directly located. All of the hydrogen
atoms were freely refined isotropically, whereas the non-
hydrogen atoms were refined anisotropically. A final differ-
(28) Poli, R.; Kelland, M. A. J . Organomet. Chem. 1991, 419, 127-
136.
(29) Poli, R.; Krueger, S. T.; Abugideiri, F.; Haggerty, B. S.;
Rheingold, A. L. Organometallics 1991, 10, 3041-3046.
ence Fourier map was featureless with |∆r| e 0.437 e‚Å-3
,
indicating that the structure is both correct and complete.

Synthesis and Studies on CpMoH3(PMe2Ph)2
Organometallics, Vol. 16, No. 6, 1997 1181
carbon,36 and phosphorus.37 Calculations were performed
using the GAMESS-UK package38 on a SGI Power Challenge
at the Supercomputer Center of Texas A&M University and
on the SP2 at Cornell Theory Center.
Ta ble 1. Cr ysta l Da ta for Com p ou n d 1
mol form
C21H30MoP2
mol wt
440.33
temp
153(2) K
wavelength
cryst syst
space group
unit cell dimensions
0.710 73 Å
monoclinic
C2/c
Resu lts a n d Discu ssion
a ) 24.9623(12) Å, R ) 90°
b ) 7.4092(4) Å, â ) 113.687(5)°
c ) 24.423(2) Å, γ ) 90 °
4136.4(4) Å3, 8
Syn th esis a n d Sp ectr oscop ic Ch a r a cter iza tion .
Compound 1 has been synthesized from the reaction of
CpMoCl3 with LiAlH4 in THF in the presence of excess
PMe2Ph (eq 1), followed by methanolysis of the reaction
mixture. The reaction also gave rise to minor but
volume, Z
density (calcd)
abs coeff
1.414 Mg/m3
0.789 mm-1
F(000)
1824
cryst size
θ range for data collection
limiting indices
0.375 × 0.250 × 0.225 mm
CpMoCl3 + 2PMe2Ph + LiAlH4 (excess) f
2.89-24.99°
-29 e h e 29, 0 e k e 8,
-14 e 1 e 28
3680
3630 (R(int) ) 0.0137)
semi-empirical from ψ-scans
0.5524 and 0.5026
full-matrix least-squares on F2
3630/0/337
CpMoH3(PMe2Ph)2 (1)
no. of reflns collected
no. of independent reflns
abs corr
significant quantities of a byproduct, which is charac-
terized by a doublet hydride resonance at δ -4.18 (J PH
) 52.0 Hz). This is assigned to CpMoH5(PMe2Ph), 2,
by analogy with the resonance of Cp*MoH5(PMe3),
which was reported as a doublet at δ -3.17 (J PH ) 50.3
Hz).39 Compound 1, however, has been obtained pure
by low-temperature recrystallization from heptane.
Similar compounds, namely (η5-C5H4Pri)MoH3L2 (L )
PMe3, PMe2Ph, or L2 ) Pri PCH2CH2PPri ), have been
max and min transmission
refinement method
no. of data/restraints/
parameters
goodness-of-fit on F2
final R indices (I > 2σ(I))
1.316
R1 ) 0.0292, wR2 ) 0.0700
(3369 data)
R1 ) 0.0327, wR2 ) 0.0717
0.437 and -0.391 e‚Å-3
R indices (all data)
largest diff peak and hole
2
2
prepared from Mo(III) precursors, namely (η5-C5H4Pri)-
MoCl2L2, and LiAlH4.40 Since CpMoCl2(PMe2Ph)2 is
known,29 we also explored the utilization of such a
starting material by analogy with the strategy reported
by Green et al.40 However, although spectroscopic
monitoring shows that compound 1 also forms by this
route, a considerable amount of CpMoH(PMe2Ph)3, 3,
(quartet hydride resonance at δ -7.72, J PH ) 50.9 Hz,
cf. the resonance for the similar41 CpMoH(PMe3)3 at δ
-8.37, J PH ) 52.6 Hz) is also generated and the
separation of the two compounds is difficult. Longer
reaction times led to the disappearance of 3, but at the
same time, considerable amounts of compound 2 were
also formed. Overall, the utilization of CpMoCl3 as a
starting material has proven more effective in our
hands. Reaction 1 probably proceeds via the formation
of the adducts CpMoCl3(PMe2Ph)n (n ) 1, 2), as previ-
ously reported.42
Ta ble 2. Selected Bon d Dista n ces (Å) a n d An gles
(d eg) for Com p ou n d 1a
Mo(1)-C(4)
Mo(1)-C(5)
Mo(1)-C(3)
Mo(1)-C(1)
Mo(1)-C(2)
Mo(1)-CNT
2.249(3)
2.274(4)
2.302(3)
2.346(3)
2.370(4)
1.975(4)
Mo(1)-P(1)
Mo(1)-P(2)
Mo(1)-H(1A)
Mo(1)-H(1B)
Mo(1)-H(1C)
2.4038(8)
2.4095(8)
1.68(4)
1.65(4)
1.58(6)
P(1)-Mo(1)-P(2)
126.29(3) H(1A)-Mo(1)-H(1B) 136(2)
P(1)-Mo(1)-H(1A) 82.3(13) CNT-Mo(1)-P(1)
P(2)-Mo(1)-H(1A) 83.2(13) CNT-Mo(1)-P(2)
116.5(1)
117.0(1)
P(1)-Mo(1)-H(1B) 78(2)
P(2)-Mo(1)-H(1B) 78(2)
P(1)-Mo(1)-H(1C) 64(2)
P(2)-Mo(1)-H(1C) 63(2)
CNT-Mo(1)-H(1A) 111.4(14)
CNT-Mo(1)-H(1B) 112(2)
CNT-Mo(1)-H(1C) 178(2)
a
CNT ) Cp ring centroid.
Crystal data are reported in Table 1, and selected bond
distances and angles are collected in Table 2.
Yet another route to compound 1 is the reaction
between the ortho-metalated complex29 CpMo(o-C6H4-
PMe2)(PMe2Ph)2 and H2 shown in eqs 2 and 3. The
Th eor etica l Deta ils. Geometry optimizations of CpMoH3-
(PMe3)2 were performed at the restricted Hartree-Fock (RHF)
level of theory. Relative energies of the RHF geometries were
recalculated at the MP2 level. The metal basis set was
double-ê with a triple-ê function for the d orbitals. The (n +
1)s and (n + 1)p functions of the metal have been included
according to recent studies that demonstrate the importance
of these functions.30,31 The hydride ligands are represented
in a triple-ê basis set.32 The carbons and hydrogens of the Cp
ligand and phosphorus are represented by double-ê functions.33
Minimal basis sets (STO-3G)34 were used for the carbons and
hydrogens of the methyl groups in PMe3. Effective core
potentials represent the inner shells of the molybdenum,35
CpMo(o-C6H4PMe2)(PMe2Ph)2 + H2 f
CpMoH(PMe2Ph)3 (2)
CpMoH(PMe2Ph)3 + H2 a
CpMoH3(PMe2Ph)2 + PMe2Ph (3)
reaction affords compound 3 initially, but a slow,
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Chem. 1983, 4, 241. Collins, J . B.; Schleyer, P. van R.; Binkley, J . S.;
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M. L. H.; Hare, P. M.; Bashkin, J .; Derome, A. E.; Grebenik, P. D. J .
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E.; Rheingold, A. L. Organometallics 1993, 12, 1575-1582.

1182 Organometallics, Vol. 16, No. 6, 1997
Abugideiri et al.
F igu r e 2. ORTEP view of compound 1 with the numbering
scheme used. The ellipsoids are shown at the 30% prob-
ability level.
Ta ble 3. Lon gitu d in a l Rela xa tion Tim es, T1, for
Com p ou n d s 1 a n d 3 a t Va r iou s Tem p er a tu r es
T1 (ms)
T (K)
1
3
205
215
225
235
245
260
520
340
282
325
345
460
600
580
735
The conversion of the ortho-metalated CpMo(o-C6H4-
PMe2)(PMe2Ph)2 complex to the monohydride interme-
diate probably involves a preliminary loss of phosphine,
followed by oxidative addition of H2 and reductive
elimination to form the intermediate CpMoH(PMe2Ph)2,
as indicated in Scheme 1. This species can either add
PMe2Ph to form the CpMoH(PMe2Ph)3 species or oxi-
datively add H2 to afford 1.
Compound 1 has similar spectroscopic properties to
Cp*MoH3(dppe)27 and other previously reported trihy-
drides.40 Like all of these previously reported com-
pounds, the hydride resonance does not decoalesce upon
cooling to -80 °C. The nature of compound 1 as a
classical trihydride, rather than as a monohydride-
dihydrogen complex, is strongly indicated by the long
minimum longitudinal relaxation time, T1min, measured
(282 ms at 210 K and 400 MHz). By comparison, the
corresponding T1min value for the monohydride com-
pound 3 is 580 ms at 245 K and 400 MHz. Values of T1
for compounds 1 and 3 at various temperatures are
collected in Table 3.
F igu r e 1. 1H-NMR spectrum (hydride region) of a mixture
of compounds 1 and 3 in C6D6 obtained from reactions 2
and 3: (a) after 7 days at room temperature in a sealed
NMR tube, (b) after 13 days at room temperature in a
sealed NMR tube, (c) after 29 days at room temperature
in a sealed NMR tube, (d) after 13 days on a Schlenk line
under H2 (P ) 1 atm), (e) as in d, after 30 min of UV
irradiation.
temperature-dependent equilibrium with compound 1,
which involves loss of PMe2Ph, is established later. At
room temperature, the reaction eventually proceeds
quantitatively to 1, under an atmosphere of H2, but can
be reversed by warming. In a sealed NMR tube, the
reaction finally arrives at a mixture of 3 (minor) and 1
(major) because of the insufficient amount of H2 avail-
able; when this mixture is warmed to 70 °C overnight,
equilibrium 3 shifts back toward the left, as shown by
X-r a y Str u ctu r e. Known molecular structures for
compounds of the type CpMoX3L2 are pseudo-octahedral
with three possible arrangements of the ligands, fac (I),
mer,cis (II), and mer,trans (III). For instance, CpMoCl3-
1
an increased 3:1 ratio in the H-NMR spectrum. This
shows that equilibrium 3 corresponds to an exothermic
process. No significant formation of compound 2 is
observed under a H2 atmosphere at room temperature
over 2 weeks. After long reaction times, a H/D exchange
process with the solvent was also observed (vide infra).
1
[P(OCH2)3CEt]2 has structure I,28 CpMoCl3(dppe) has
structure II,43 and CpMoCl3(PMe2Ph)2 has structure
Representative H-NMR spectra of this reaction in the
hydride region are shown in Figure 1.

Synthesis and Studies on CpMoH3(PMe2Ph)2
Organometallics, Vol. 16, No. 6, 1997 1183
Sch em e 1
Ta ble 4. Str u ctu r a l P a r a m eter s a t th e RHF Level
for Cp MoH3(P Me3)
Sch em e 2
I
IIa
IIIa
IVa
Mo-C
2.449
1.432
1.685
1.707
2.624
2.624
2.414
1.432
1.679
1.748
2.651
2.710
2.462
2.469
1.433
1.748
1.710
2.591
2.591
C-C
1.431
1.665
1.741
2.597
2.597
Mo-H1
Mo-H2,3
Mo-P1
Mo-P2
CNT-Mo-H1
CNT-Mo-H2,3
CNT-Mo-P1
CNT-Mo-P2
P1-Mo-P2
171.4
101.8
114.0
114.0
99.0
97.0
180.0b,c
112.2
109.9
109.9
140.0
106.9
110.0
120.2
120.2
113.8
0.0c
108.7
160.7
109.0
90.0
C1-CNT-Mo-H1
0.0c
0.0c
C1-CNT-Mo-H2,3 -80.2
C1-CNT-Mo-P1
C1-CNT-Mo-P2
(88.5 0.0,c 180.0c (150.6
86.9
-165.6 180.0
0.0b
89.2
-89.2
75.8
-75.8
a
b
Geometries were optimized in Cs symmetry. Ligand is axial
to the center of the Cp ring. c Parameter constrained in optimiza-
tion.
Ta ble 5. Rela tive En er gies of Cp MoH3(P Me3)2
(I-IV) fr om th e RHF Op tim ized Geom etr y
structure
RHF//RHF
MP2//RHF
I
II
III
IV
4.77
18.96
0.00
2.92
14.74
0.00
two hydrides in the title compound are much further
from the pseudo-equatorial plane (CNT-Mo-H, 111.4-
(14)° and 112(2)°) with respect to the corresponding
chlorides in the structure of CpMoCl3(PMe2Ph)2 (CNT-
Mo-Cl, 99.5(1)° and 104.5(1)°). Correspondingly, the
two phosphine ligands are distorted to a greater extent
in the trihydride structure (CNT-Mo-P, 116.5(1)° and
117.0(1)°) with respect to the trichloride structure
(105.8(1)° and 103.4(1)°). These phenomena are prob-
ably associated with relieved steric X‚‚‚X and X‚‚‚PMe2-
Ph repulsions upon changing Cl to H. The conformation
of the phosphine ligands is identical in the two struc-
tures, with the phenyl rings pointing toward the Cp
ring. The Mo-P distances in 1, however, compare
better with those in the pseudo trigonal prismatic
complex Cp*MoH3(dppe) (average 2.37(1) Å)27 than with
those in CpMoCl3(PMe2Ph)2 (average 2.554(1) Å).44
Thus, this structural parameter is more sensitive to the
Cl/H substitution than to the molecular geometry. The
average Mo-H distance of 1.64(4) Å in 1 compares with
the previously reported value of 1.59(5) for Cp*MoH3-
(dppe).27 The closest H‚‚‚H contact is 1.79 Å, further
confirming the nature of the compound as a classical
hydride.
11.23
11.66
III.44 However, the only structure so far reported for a
trihydride species having this stoichiometry, e.g.,
Cp*MoH3(dppe),27 is yet of a different and novel type,
e.g., based on the trigonal prismatic geometry, IV.
In order to understand whether the unusual struc-
tural preference of Cp*MoH3(dppe) is due to electronic
or steric factors, we have determined the structure of
the less sterically crowded compound 1. The molecular
geometry is shown in Figure 2. The three hydrides were
directly located from the difference Fourier synthesis
and refined without constraints. The geometry can be
described as a distorted octahedron with two phosphines
trans to one another, two hydrides trans to one another,
and the third hydride trans to the Cp ring, e.g., type
III, just like the trichloride analogue CpMoCl3(PMe2-
Ph)2.42 The two CpMoX3(PMe2Ph)2 (X ) H, Cl) com-
pounds experience significantly different distortions
from the ideal octahedral geometry. In particular, the
(43) Sta¨rker, K.; Curtis, M. D. Inorg. Chem. 1985, 24, 3006-3010.
(44) Abugideiri, F.; Keogh, D. W.; Poli, R. J . Chem. Soc., Chem.
Commun. 1994, 2317-2318.

1184 Organometallics, Vol. 16, No. 6, 1997
Abugideiri et al.
Sch em e 3
The adoption of a geometry related to III for com-
pound 1 strongly suggests that the previously reported
structure of Cp*MoH3(dppe)27 (e.g., IV) is sterically
enforced. However, neither structure III, structure IV,
nor the other possible structures I and II would lead to
equivalent hydride ligands. Thus, the measurement of
a single hydride resonance for all known compounds of
this stoichiometry can only be attributed to high flux-
ionality. We have previously proposed that the mech-
anism of hydride scrambling in this class of compounds
could take place via a facile interconversion between the
pseudo-octahedral and the pseudo trigonal bipyramidal
structure.27 Furthermore, recent calculations45 on
CpOsH5 show that the hydride scambling occurs by a
trigonal twist. Theoretical calculations, the results of
which are shown in the next paragraph, support this
proposal for the complexes studied here.
Th eor etica l Ca lcu la tion s. The geometries of the
four structures (I-IV) were optimized with X ) H and
L ) PMe3 at the RHF levels maintaining Cs symmetry,
except for I (C1). Each of the structures were con-
strained to keep one of the hydride ligands eclipsed with
one of the carbons in the Cp ring. Additionally, the
phosphine ligands were constrained to possess a C3
rotational axis and the Cp ring is kept planar. The
geometric parameters are provided in Table 4, and the
relative energies are listed in Table 5.
second-order J ahn-Teller (PSOJ T) effect46 in which
these predominately σ-bound ligands bend in order to
increase their overlap with the d orbitals of the
metal.45,47,48 Thus, the ligands are not cis or trans in
the usual sense of a ligand being at right angles or
directly across each other. However, that nomenclature
is utilized here in reference to the relative dihedral
angle with the centroid of the Cp ring. Relative dihedral
angles of close to 90° are referred to as cis and relative
dihedral angles of close to 180° are referred to as trans.
A major factor in the stability of structures I-III is
the proximity of the phosphine ligands. Structure III
has the lowest energy conformation because the large
P-Mo-P bond angle minimizes the steric crowding of
the PMe3 ligands. Thus, this structure is the low-energy
conformer in both the calculation and experiment.
Overall, the theoretical geometry fits well with the
experiment. However, since RHF calculations predict
bond lengths that are longer than expected, the large
difference in P-Mo-P bond angles between theory and
experiment can be attributed to the lengthened Mo-
Cp distance, which would allow the bond angle to open
up.
Structure I might be expected to have the next lowest
energy as the equatorial phosphine ligands can adjust
their dihedral angle (P-Mo-CNT-C) so that they are
farther apart. Interactions of the methyl groups on the
phosphines with the Cp ring also destabilize this
The hydride and phosphine ligands in these com-
plexes are bent away from the Cp ring at larger angles
than might be expected. This effect is due to a pseudo-
(46) Albright, T. A.; Burdett, J . K.; Whangbo, M. Orbital Interactions
in Chemistry; J ohn Wiley: New York, 1985.
(45) Bayse, C. A.; Couty, M.; Hall, M. B. J . Am. Chem. Soc. 1996,
118, 8916.
(47) Lin, Z.; Hall, M. B. Organometallics 1993, 12, 4046-4050.
(48) Bayse, C. A.; Hall, M. B. Inorg. Chim. Acta, in press.

Synthesis and Studies on CpMoH3(PMe2Ph)2
Organometallics, Vol. 16, No. 6, 1997 1185
structure, with the destabilization more pronounced if
Cp* is involved. Structure II is the most unstable as
the PSOJ T effects discussed above force a P-Mo-P
angle that would be significantly smaller than 90° if the
axial phosphine remained stationary. The optimization
compensates for this by moving the axial phosphine 20°
off the axis to a final P-Mo-P angle of 90°. The tri-
gonal prismatic structure IV roughly corresponds to the
previously synthesized Cp*MoH3(dppe), where the
P-Mo-P angle is larger and the CNT-Mo-P angles
are smaller than the experimental values, as there are
no constraints due to a ethylene linkage between the
phosphines. The high relative energy of this conformer
is due to the small P-Mo-P angle and to the stability
of the pseudo-octahedral geometry for this type of
complex.45
addition of D2 to CpMoH(PMe2Ph)2. HD and D2 are
available from the reductive elimination processes of
1-d1 and 1-d2, respectively. A similar process was also
described for (η5-C5H4Pri)MoH3L2 (L ) PMe3 or L2
)
dmpe).41 However, H/D exchange in C6D6 was observed
only upon UV irradiation for those systems, and the
measurement of an isotopic shift for the 1H-NMR
spectra of (η5-C5H4Pri)MoHnD3-nL2 was not reported.
Small upfield shifts upon H/D isotopic substitution are
typical of polyhydride compounds, while the absence of
a temperature dependence on the ∆δ between different
isotopes indicates the absence of an isotopic perturba-
tion of degeneracy,49,50 in further agreement with the
classical nature of the trihydride molecule.
Con clu sion s
In light of recent studies on the CpOsH5 system,45
a
Compound 1 is only the second compound in the class
of complexes (ring)MoH3L2 (ring ) Cp or substituted
derivative) that has been determined structurally. Its
structure is based on the octahedron with trans,mer
stereochemistry (III), unlike the previously structurally
characterized Cp*MoH3(dppe) which adopts a trigonal
prismatic structure (IV). Both of these hydrides are
highly fluxional in solution, even at -80 °C. Theoretical
calculations on the CpMoH3(PMe3)2 model system in-
dicate that the energy difference between the two
geometries is small, but suggest that the hydride
scrambling mechanism for CpMoH3(PMe2Ph)2 may pro-
ceed through the alternative fac stereochemistry (I), via
a pseudo-Bailar twist.
trigonal twist mechanism is likely for the exchange in
this system (Scheme 2). Since structure II is very high
in energy, it is most likely that the exchange route
would proceed through structures I and III by various
trigonal prismatic transition states. Further theoretical
work is being performed on these systems and will be
presented at a later date.
H/D Exch a n ge in C6D6. As mentioned above, the
1H-NMR monitoring of the reaction between the ortho-
metalated complex CpMo(o-C6H4PMe2)(PMe2Ph)2 and
H2 in C6D6, which generated compound 1, provided
evidence for solvent C-D bond activation and H/D
exchange. The development of both CpMoH2D(PMe2-
Ph)2 (δ ) 5.20, J PH ) 40.4 Hz) and CpMoHD2(PMe2-
Ph)2 (δ ) -5.24, J PH ) 39.7 Hz) as flanking triplets to
the triplet hydride resonance of 1 is shown in Figure 1.
Compound 1-d2 accumulates to observable amounts only
upon UV irradiation of the solution. The irradiation
also induces phosphine loss from 1 with formation of
the pentahydride complex 3, as evidenced by the doublet
centered at δ -4.2, and the formation of yet another
bisphosphine complex, characterized by a broad triplet
at δ -4.7, which remains currently unassigned. The
growth of the C6D5H resonance occurs simultaneously
with the growth of the triplets due to the partially
deuterated species. Further evidence for H/D exchange
was observed in the 2H-NMR spectrum of the above
mixture.
Ack n ow led gm en t. For the experimental part of this
work, funding by the Department of Energy (Grant No.
DEFG059ER14230 to R.P.) is gratefully acknowledged.
C.A.B. and M.B.H. would like to thank the National
Science Foundation (Grant No. CHE 94-23271) and the
Robert A. Welch Foundation (Grant No. A-648) for their
support. They also would like to thank the Supercom-
puter Facility at Texas A&M University for use of their
SGI Power Challenge and Cornell Theory Center for
time on the IBM SP2.
Su p p or tin g In for m a tion Ava ila ble: For CpMoH3(PMe2-
Ph)2, tables of crystal data and refinement parameters,
fractional atomic coordinates, bond distances and angles,
anisotropic thermal parameters, and calculated H-atom coor-
dinates (8 pages). Ordering information is available on any
current masthead page.
These phenomena can be accounted for by reductive
elimination of H2 from 1 to generate CpMo(PMe2Ph)2H,
which then engages in oxidative addition/reductive
elimination processes with the deuterated solvent as
shown in Scheme 3. Compound 1-d1 is obtained directly
by oxidative addition of H2 to the 16-electron CpMoD-
(PMe2Ph)2 intermediate, whereas 1-d2 can be obtained
by addition of HD to the same intermediate or by
OM960360O
(49) Heinekey, D. M.; Oldham, W. J ., J r. J . Am. Chem. Soc. 1994,
116, 3137-3138.
(50) Paneque, M.; Poveda, M. L.; Taboada, S. J . Am. Chem. Soc.
1994, 116, 4519-4520.