Synthesis and characterization of dihydrogen and dihydride complexes of [CpMH2(diphosphine)]+ (M = Fe, Os)

Article abstract of DOI:10.1021/om960472f

Protonation of CpFeH(P-P) (P-P = dppe, dppp) with HBF₄ produced the molecular dihydrogen complexes [CpFe(H₂)(P-P)]BF₄. Protonation of CpOsH(P-P) (P-P = dppm, dppe, dppp) with HBF₄ at -78°C gave a mixture of cis-[CpOsH₂(P-P)]BF₄ and trans-[CpOsH₂(P-P)]BF₄. Depending on the size of the chelating rings, the thermodynamically stable structures of the protonated products at room temperature may be exclusively trans-[CpOsH₂(P-P)]-BF₄ or a mixture of cis-[CpOsH₂(P-P)]BF₄ and trans-[CpOsH₂(P-P)]BF₄. At room temperature in dichloromethane solutions, [CpOsH₂(dppm)]BF₄ exists as a mixture of cis and trans isomers in a ratio of 10:1, [CpOsH₂(dppe)]BF₄ exists as a mixture of cis and trans isomers in a ratio of 1:70, and [CpOsH₂(dppp)]BF₄ adopts only trans geometry.

Full text of DOI:10.1021/om960472f

Organometallics 1996, 15, 5039-5045
5039
Syn th esis a n d Ch a r a cter iza tion of Dih yd r ogen a n d
Dih yd r id e Com p lexes of [Cp MH2(d ip h osp h in e)]+ (M ) F e,
Os)
Guochen J ia,* Weng Sang Ng, and J unzhi Yao
Department of Chemistry, The Hong Kong University of Science & Technology,
Clear Water Bay, Kowloon, Hong Kong
Chak-Po Lau* and Yuzhong Chen
Department of Applied Biology & Chemical Technology, The Hong Kong Polytechnic
University, Hung Hom, Hong Kong
Received J une 12, 1996X
Protonation of CpFeH(P-P) (P-P ) dppe, dppp) with HBF4 produced the molecular
dihydrogen complexes [CpFe(H2)(P-P)]BF4. Protonation of CpOsH(P-P) (P-P ) dppm, dppe,
dppp) with HBF4 at -78 °C gave a mixture of cis-[CpOsH2(P-P)]BF4 and trans-[CpOsH2(P-
P)]BF4. Depending on the size of the chelating rings, the thermodynamically stable structures
of the protonated products at room temperature may be exclusively trans-[CpOsH2(P-P)]-
BF4 or a mixture of cis-[CpOsH2(P-P)]BF4 and trans-[CpOsH2(P-P)]BF4. At room temperature
in dichloromethane solutions, [CpOsH2(dppm)]BF4 exists as a mixture of cis and trans isomers
in a ratio of 10:1, [CpOsH2(dppe)]BF4 exists as a mixture of cis and trans isomers in a ratio
of 1:70, and [CpOsH2(dppp)]BF4 adopts only trans geometry.
In tr od u ction
C5R5)RuH2(PR3)2]+. Analogous complexes with chelat-
ing diphosphines can adopt either the dihydride form
[(η5-C5R5)MH2(P-P)]+ or the dihydrogen form [(η5-C5R5)M-
(H2)(P-P)]+ or a mixture of both, depending on the size
of the chelating ring and C5R5. For example, Simpson
et al. reported that protonation of CpRuH(dppm), CpRu-
H(dppe), and CpRuH(dppp) produced [CpRu(H2)(dppm)]+,
[CpRu(H2)(dppe)]+/[CpRuH2(dppe)]+, and [CpRuH2-
(dppp)]+, respectively.5b It is also interesting to note
that although Cp*RuH(dppm)8b-d and CpRuH(dmpe)7c
are more electron rich than CpRuH(PPh3)2,9 the proton-
ated products of these monohydride complexes are
[Cp*Ru(H2)(dppm)]+/[Cp*RuH2(dppm)]+, [CpRu(H2)-
(dmpe)]+/[CpRuH2(dmpe)]+, and [CpRuH2(PPh3)2]+, re-
spectively. It appears that the presence of some chelat-
ing ligands favors the dihydrogen form.
Compared to ruthenium, the chemistry of the iron and
osmium homologs is still underdeveloped. The reported
iron complexes of the formula [CpFeH2(L)(L′)]+ include
trans-[(C5R5)FeH2(dippe)]BPh4 (dippe ) (i-Pr)2PCH2-
CH2P(i-Pr)2; R ) H, Me),1 [Cp*Fe(H2)(dppe)]BF4 (which
isomerized to trans-[Cp*FeH2(dppe)]BF4 on warming),2
and [CpFe(H2)(CO)(PR3)]BAr′4 (PR3 ) PEt3, PPh3, Ar′
) 3,5-(CF3)2C6H3).3 The reported osmium complexes of
the formula [CpOsH2(L)(L′)]+ include [Cp*Os(H2)(CO)2]+/
[Cp*OsH2(CO)2]+, trans-[CpOsH2(CO)(P(i-Pr)3)]BF4, and
trans-[CpOsH2(PR3)2]+ ((PR3)2 ) (PPh3)2, (Ph2PMe)2,
(PPh3)(P(OEt)3)).9-12 To compare the structures and
properties of iron and osmium homologs with those of
ruthenium, and to study the effect of chelating ring size
on the structure and properties of [CpMH2(P-P)]+, we
have studied the protonation reactions of CpMH-
(diphosphine) (M ) Fe, Os).
Complexes of the formula [(η5-C5R5)MH2(L)(L′)]+ (M
) Fe,1-3 Ru,4-8 Os;9-12 (L)(L′) ) (CO)2, (CO)(PR3),
(PR3)2) have attracted considerable attention recently
for their structural, chemical, and physical properties.
Previous study on ruthenium complexes shows that [(η5-
C5R5)RuH2(L)(L′)]+ can adopt either the dihydride form
trans-[(η5-C5R5)RuH2(L)(L′)]+ or the dihydrogen form
[(η5-C5R5)Ru(H2)(L)(L′)]+, depending on the ligands
used. The dihydrogen form is adopted by the CO-
containing complexes [(η5-C5R5)Ru(H2)(CO)(PR3)]+ and
[(η5-C5Me5)Ru(H2)(CO)2]+. The dihydride form is usu-
ally observed for the monophosphine complexes [(η5-
X Abstract published in Advance ACS Abstracts, October 15, 1996.
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Exp er im en ta l Section
Microanalyses were performed by M-H-W Laboratories
(Phoenix, AZ). 1H and 31P{1H} NMR spectra were collected
S0276-7333(96)00472-4 CCC: $12.00 © 1996 American Chemical Society

5040 Organometallics, Vol. 15, No. 23, 1996
J ia et al.
on a J EOL EX-400 spectrometer (400 MHz) or a Bruker ARX-
300 spectrometer (300 MHz). 1H NMR chemical shifts are
relative to TMS, and 31P NMR chemical shifts are relative to
85% H3PO4.
[Cp F e(d p p e)(µ-d p p e)Cp F e(d p p e)](BP h 4)2. A mixture of
CpFeCl(dppe)‚CHCl3 (135 mg, 0.200 mmol), dppe (39.8 mg,
0.100 mmol), and NaBPh4 (130 mg, 0.38 mmol) in 20 mL of
CH2Cl2 was stirred at room temperature for 3 h. The insoluble
material was remove by filtration to give a red solution. The
solvent was then removed completely. The residue was
washed with methanol and water. The crude product was
purified by column chromatography on silica gel with CH2Cl2
as the eluent. Yield: 125 mg, 60%. 31P{1H}NMR (CD2Cl2):
δ 88.5 (d, J (PP) ) 43.3 Hz, η2-dppe)); 51.5-50.61 (m, µ-dppe).
1H NMR (CD2Cl2): δ 7.60-6.87 (m, 100 H, PPh2, BPh4), 4.11
(s, 10 H, Cp), 2.28 (m, 4 H, CH2), 1.59 (m, 4 H, CH2), 1.20 (m,
4 H, CH2). Anal. Calcd for C136H122B2P6Fe2: C, 78.70; H, 5.92.
Found: C, 78.58; H, 6.11.
All manipulations were carried out under nitrogen atmo-
sphere using standard Schlenk techniques. Solvents were
distilled under nitrogen from sodium-benzophenone (hexane,
diethyl ether, THF), sodium (benzene), or calcium hydride
(dichloromethane). The complexes CpFeCl(dppe),13 CpFeH-
(dppe),14 CpOsBr(dppm),15 CpOsBr(dppe),15 and CpOsBr-
(dppp)11a were prepared according to literature methods. The
procedure for CpFeCl(dppe)12 was also used to prepare CpFeCl-
(dppp). All other reagents were used as purchased from
Aldrich or Strem.
Cp F eH(d p p p ). A mixture of CpFeCl(dppp) (0.50 g, 0.88
mmol) and NaBH4 (0.50 g, 14 mmol) in THF (50 mL) was
stirred for 2 h to give a cloudy yellow solution. The solvent of
the reaction mixture was evaporated to give a brown residue.
The residue was extracted with hexane, and the insoluble
material was removed by filtration to give a yellow solution.
The solvent of the extract was evaporated to give an orange
yellow solid. Yield: 0.26 g, 55%. 31P{1H} NMR (C6D6): δ 75.1
Cp OsBr (P P h 3)2. An ampule of osmium tetraoxide (1 g,
3.93 mmol) was broken in a flask containing hydrobromic acid
(48%, 40 mL), and the red solution was refluxed for 2 h. The
solution was reduced to 5 mL and added to a stirred, boiling
solution of triphenylphosphine (6.3 g, 24 mmol) in methanol
(180 mL) to give a red precipitate. The solid was collected by
filtration, washed with methanol and benzene, and dried under
vacuum to give 4.0 g of red powder. A mixture of the red solid
(0.85 g), Zn (3.00 g, 45.9 mmol), and 0.9 mL of cyclopentadiene
in 50 mL of EtOH was stirred at room temperature for 12 h
to give a yellow solution with some yellow fine precipitate. The
unreacted zinc metal was separated, and the yellow solid was
collected by filtration and washed with hexane. The crude
product was purified by column chromatography on silica gel
with benzene as the eluent. The benzene was removed to give
1
(s). H NMR (C6D6): -15.75 (t, J (PH) ) 70.1 Hz, 1 H, FeH),
1.23-2.38 (m, 6 H, CH2), 4.11 (s, 5 H, Cp), 7.07-7.79 (m, 20
H, Ph). Anal. Calcd for C32H32P2Fe: C, 71.92; H, 6.04.
Found: C, 71.83; H, 5.96.
[Cp F e(H2)(d p p e)]BF 4. HBF4‚Et2O was added to a solution
of CpFeH(dppe) in CD2Cl2 in an NMR tube. The color of the
solution changed immediately from yellow to light brown.
NMR spectra were collected immediately. 31P{1H}NMR
1
0.58 g of yellow powder. 31P{1H} NMR (C6D6): δ -5.7 (s). H
1
(CD2Cl2): δ 94.5 (s). H NMR (CD2Cl2): δ 7.0-8.0 (m, Ph),
NMR (C6D6): δ 7.67-7.02 (m, 30 H, Ph), 4.45 (s, 5 H, Cp).
Anal. Calcd for C41H35BrP2Os: C, 57.28; H, 4.10; Br, 9.29.
Found: C, 57.27; H, 4.19; Br, 9.09.
4.5 (s, Cp), 2.8-1.2 (m, dppe), -12.5 (br, Fe(H2)). T1 (ms, 300
MHz): 17 (298 K), 13 (280 K), 5 (240), 5 (230 K), 7 (220 K). A
T1(min) value of 5 ms (at 235 K) was estimated from a plot of
T1 vs temperature.
Cp OsH(d p p m ). A mixture of CpOsBr(dppm) (0.50 g, 0.69
mmol) and Na (0.10 g, 4.3 mmol) in 80 mL of methanol was
refluxed for 4 days to give a pale yellow solution. The volume
of the solution was reduced, and a yellow powder precipitated
out. The solid was collected by filtration, washed with
methanol, and dried under vacuum. Yield: 0.34, 75%. 31P{1H}
NMR (CD2Cl2): δ -35.4 (s). 1H NMR (CD2Cl2): δ -14.79 (t,
J (PH) ) 24.5 Hz, 1 H, OsH), 4.32 (dt, J (PH) ) 11.1 Hz, J (HH)
) 14.7 Hz, 1 H, CH2), 6.26 (dtd, J (PH) ) 10.0 Hz, J (HH) )
14.7 Hz, J (HH) ) 2.5 Hz, 1 H, CH2), 4.87 (s, 5 H, Cp), 7.3-7.7
(m, 20 H, Ph). Anal. Calcd for C30H28P2Os: C,56.24; H, 4.41.
Found: C, 56.42; H, 4.62.
[Cp F e(HD)(d p p e)]BF 4. This compound was prepared in
an NMR tube in CD2Cl2 by the reaction of CpFeH(dppe) with
DBF4. DBF4 was prepared in situ by mixing HBF4‚Et2O and
D2O in a ratio of 1:3 (v:v). 1H NMR (CD2Cl2): δ -12.5 (tt,
J (PH) ) 6.8 Hz, J (HD) ) 30.7 Hz, Fe(HD)).
[CpFe(H2)(dppp)]BF4. The procedure for [CpFe(H2)(dppe)]-
BF4 was followed exactly. 31P{1H}NMR (CD2Cl2): δ 54.8 (s).
1H NMR (CD2Cl2): δ 7.8-7.4 (m, Ph), 4.6 (s, Cp), 3.0-1.2 (m,
CH2), -12.0 (br, Fe(H2)). T1 (ms, 300 MHz): 18 (298 K), 10
(250 K), 7 (230 K), 7 (210 K), 9 (190 K). A T1(min) value of 7
ms (at 220 K) was estimated from a plot of T1 vs temperature.
[Cp F e(HD)(d p p p )]BF 4. This compound was prepared in
an NMR tube in CD2Cl2 by the reaction of CpFeH(dppp) with
DBF4. DBF4 was prepared in situ by mixing HBF4‚Et2O and
D2O in a ratio of 1:3 (v:v). 1H NMR (CD2Cl2): δ -12.0 (t, J (HD)
) 29.0 Hz, Fe(HD)).
[Cp OsH2(d p p m )]BF 4. To a solution of CpOsH(dppm) (0.20
g, 0.31 mmol) in 30 mL of Et2O was added 0.1 mL of HBF4‚
Et2O. The reaction mixture was stirred for 30 min to give a
white solid. The solid was collected by filtration, washed with
Et2O, and dried under vacuum. Yield: 0.16 g, 70%. 31P{1H}
NMR (CD2Cl2): δ -47.3 (s, cis-OsH2), -36.1 (s, trans-OsH2).
1H NMR (CD2Cl2): δ -11.42 (t, J (PH) ) 6.5 Hz, cis-OsH2),
-10.76 (t, J (PH) ) 31.8 Hz, trans-OsH2), 5.01 (dt, J (PH) )
12.0 Hz, J (HH) ) 15.8 Hz, CH2, cis-OsH2), 6.19 (dt, J (PH) )
11.7 Hz, J (HH) ) 15.8 Hz, CH2, cis-OsH2), 5.27 (s, Cp, cis-
OsH2), 5.53 (m, CH2, trans-OsH2), 5.68 (s, Cp, trans-OsH2),
7.3-7.8 (m, Ph), cis-OsH2/trans-OsH2 ) 10:1. T1 (ms, 300
MHz, cis-OsH2): 228 (270 K), 192 (250), 160 (230), 149 (210
K), 149 (200 K), 182 (190 K). T1 (ms, 300 MHz, trans-OsH2):
1210 (270 K), 1160 (250 K), 881 (230 K), 613 (210 K), 779 (200
K), 832 (190 K). Anal. Calcd for C30H29BF4P2Os: C, 49.46;
H, 4.01. Found: C, 49.49; H, 3.86.
[Cp F e(η2-d p p e)(η1-d p p e)]BP h 4. A mixture of CpFeCl-
(dppe)‚CHCl3 (135 mg, 0.200 mmol), dppe (159 mg, 0.400
mmol), and NaBPh4 (137 mg, 0.400 mmol) in 20 mL of CH2Cl2
was stirred at room temperature for 2 h. The insoluble
material was remove by filtration to give an orange red
solution. The solvent was then removed completely. The
residue was washed with hexane, benzene, and water and then
extracted with CH2Cl2. The solvent of the extract was removed
completely to give a brownish yellow solid. Yield: 212 mg,
86%. 31P{1H}NMR (CD2Cl2): δ 86.7 (d, J (PP) ) 45.2 Hz); 49.9
(dt, J (PP) ) 30.6, 45.0 Hz); -13.88 (d, J (PP) ) 30.6 Hz). 1H
NMR (CD2Cl2): δ 7.42-6.59 (m, 60 H, Ph), 4.45 (s, 5 H, Cp),
2.43 (m, 2 H, CH2), 2.09 (m, 2 H, CH2), 1.80 (m, 2 H, CH2),
1.40 (m, 2 H, CH2). Anal. Calcd for C81H73BP4Fe: C, 78.65;
H, 5.95. Found: C, 78.50; H, 5.94.
[Cp OsHD(d p p m )]BF 4. This compound was prepared ac-
cording to the same protocol, except that DBF4 was used
instead of HBF4‚Et2O. DBF4 was prepared in situ by mixing
HBF4‚Et2O and D2O in a ratio of 1:3 (v:v). 1H NMR (CD2Cl2):
δ -11.48 (tt, J (PH) ) 6.5 Hz, J (HD) ) 3.0 Hz, cis-Os(HD)),
-10.77 (t, J (PH) ) 31.5 Hz, trans-Os(HD)). 31P{1H} NMR
(CD2Cl2): δ -47.3 (s, cis-Os(HD), 298 K), -35.7 (s, trans-
Os(HD), 298 K); -48.2 (s, cis-Os(HD), 190 K), -36.2 (s, trans-
Os(HD), 190 K).
(13) Adams, R. D.; Davison, A.; Selegue, J . P. J . Am. Chem. Soc.
1979, 101, 7232.
(14) Mays, M. J .; Sears, P. L. J . Chem. Soc., Dalton Trans. 1973,
1873.
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Chem. 1979, 32, 1003.

[CpMH2(diphosphine)]+
Organometallics, Vol. 15, No. 23, 1996 5041
.
Cp OsH(d p p e). A mixture of CpOsBr(dppe) (0.50 g, 0.68
mmol) and Na (0.10 g, 4.3 mmol) in 80 mL of methanol was
refluxed for 4 h to give a pale yellow solution. The volume of
the solution was reduced, and a yellow powder precipitated
out. The solid was collected by filtration, washed with
methanol, and dried under vacuum. Yield: 0.38 g, 85%.
Protonation of CpFeH(dppe) with HBF4 Et2O at room
temperature in CD2Cl2 produced [CpFe(H2)(dppe)]BF4,
1. The 31P NMR spectrum of the dihydrogen complex
showed a singlet at 94.5 ppm for the dppe ligand. The
1H NMR spectrum of the dihydrogen complex showed
a broad hydride signal at -12.5 ppm. The existence of
the η2-H2 moiety in [CpFe(H2)(dppe)]BF4 was confirmed
by the variable-temperature T1 measurements and the
31P{1H} NMR (CD2Cl2): δ 50.3 (s). 1H NMR (CD2Cl2):
δ
-16.60 (t, J (PH) ) 27.3 Hz, 1 H, OsH), 2.02 (m, 2 H, CH2),
2.40 (m, 2 H, CH2), 4.56 (s, 5 H, Cp), 7.3-7.8 (m, 20 H, Ph).
Anal. Calcd for C31H30P2Os: C, 56.87; H, 4.62. Found: C,
56.69; H, 4.75.
1
observation of a large J (HD) value for the correspond-
ing isotopomer [CpFe(HD)(dppe)]BF4. The variable-
temperature T1 measurements gave a T1(min) of 5 ms
(300 MHz) for the broad hydride signal at -12.5 ppm
at ca. 235 K. Acidification of CpFeH(dppe) with 1 equiv
of DBF4 gave the η2-HD isotopomer, [CpFe(HD)(dppe)]-
BF4. The HD signal in the 1H NMR spectrum in CD2Cl2
was observed as a 1:1:1 triplet (1J (HD) ) 30.7 Hz) of
1:2:1 triplets (2J (PH) ) 6.8 Hz) at -12.5 ppm, after
nulling the η2-H2 peak. The observations of small
T1(min) (5 ms, 300 MHz) and large 1J (HD) (30.7 Hz)
clearly indicate that the molecular dihydrogen complex
1 was produced in the protonation reaction.16 Small
2J (P-H) coupling constants have been observed for a
few η2-HD complexes.17
[Cp OsH2(d p p e)]BF 4. The procedure for [CpOsH2(dppm)]-
BF4 was followed exactly, substituting CpOsH(dppe) for CpOsH-
(dppm). Yield: 0.16 g, 70%. 31P{1H} NMR (CD2Cl2): δ 24.9
(s, trans-OsH2), 45.6 (s, cis-OsH2). 1H NMR (CD2Cl2): δ -13.1
(t, J (PH) ) 32.5 Hz, trans-OsH2), -12.53 (t, J (PH) ) 7.6 Hz,
cis-OsH2), 2.58 (m, CH2), 4.95 (s, Cp of cis-OsH2), 5.56 (s, Cp
of trans-OsH2), 7.5 (m, Ph). cis-OsH2/trans-OsH2 ) 1:70. Anal.
Calcd for C31H31BF4P2Os: C, 50.14; H, 4.21. Found: C, 50.20;
H, 4.30.
Cp OsH(d p p p ). A mixture of CpOsBr(dppp) (0.50 g, 0.67
mmol) and Na (0.10 g, 4.3 mmol) in 80 mL of methanol was
refluxed for 4 h to give a pale yellow solution. The volume of
the solution was reduced, and a yellow powder precipitated
out. The solid was collected by filtration, washed with
methanol, and dried under vacuum. Yield: 0.36 g, 80%.
31P{1H} NMR (CD2Cl2): δ 1.2 (s). 1H NMR (CD2Cl2): δ -15.85
(t, J (PH) ) 28.2 Hz, 1 H, OsH), 1.5-3.3 (m, 6 H, CH2), 4.50 (s,
5 H, Cp), 7.2-7.5 (m, 20 H, Ph). Anal. Calcd for C32H32P2Os:
C, 57.47; H, 4.82. Found: C, 57.19; H, 5.04.
[Cp OsH2(d p p p )]BF 4. The procedure for [CpOsH2(dppm)]-
BF4 was followed exactly, substituting CpOsH(dppp) for
CpOsH(dppm). Yield: 0.18 g, 78%. 31P{1H} NMR (CD2Cl2):
δ -14.3 (s, cis-OsH2), -0.7 (s, trans-OsH2). 1H NMR
(CD2Cl2): δ -12.80 (t, J (PH) ) 31.7 Hz, trans-OsH2), -12.22
(t, J (PH) ) 6.8, cis-OsH2), 1.94 (m, CH2), 2.88 (m, CH2), 5.07
(s, Cp, cis-OsH2), 5.17 (s, Cp, trans-OsH2), 7.5 (m, Ph). cis-
OsH2/trans-OsH2 ) 1:3, if the protonation was carried out at
-78 °C. Anal. Calcd for C32H33BF4P2Os: C, 50.80; H, 4.40.
Found: C, 51.00; H, 4.16.
The molecular dihydrogen complex 1 is thermally
unstable in solution, and it decomposed at room tem-
perature after 1 day. Loss of H2 was also observed in
attempts to isolate it. Among the decomposed products,
two were identified as [CpFe(η2-dppe)(η1-dppe)]+, 3, and
P r oton a tion a t Low Tem p er a tu r e. Appropriate amounts
of CpMH(P-P) and CD2Cl2 were loaded into an NMR tube
under inert atmosphere. The NMR tube was capped with a
rubber septum and cooled to -78 °C. HBF4‚Et2O was added
via a microsyringe. The NMR tube was quickly inverted to
ensure complete mixing and then placed in an NMR probe
precooled to 200 K. 1H and 31P NMR spectra were collected.
[{CpFe(η2-dppe)}2(µ-dppe)]2+, 4, by comparing the NMR
spectra of [CpFe(η2-dppe)(η1-dppe)]BPh4 and [{CpFe(η2-
dppe)}2(µ-dppe)](BPh4)2. Complexes [CpFe(η2-dppe)(η1-
dppe)]BPh4 and [{CpFe(η2-dppe)}2(µ-dppe)](BPh4)2 could
be easily prepared by reactions of CpFeCl(dppe) with 1
and 0.5 equiv of dppe in the presence of excess NaBPh4,
respectively. They were characterized by 31P and 1H
NMR spectroscopy and elemental analysis.
Resu lts a n d Discu ssion
Protonation of CpFeH(dppp) with HBF4‚Et2O at room
temperature in CD2Cl2 produced the dihydrogen com-
plex [CpFe(H2)(dppp)]BF4, 2. The molecular dihydrogen
complex 2 is also unstable in solution, and it decom-
posed at room temperature after 1 day. The 31P NMR
spectrum of 2 in CD2Cl2 showed a singlet at 54.8 ppm
Ch a r a cter iza tion of [Cp F e(H2)(d ip h osp h in e)]-
BF 4 Com p lexes. The molecular dihydrogen complexes
[CpFe(H2)(diphosphine)]BF4 (diphosphine ) dppe (1),
dppp (2)) were prepared by protonation of the corre-
sponding hydride complexes in CD2Cl2 with HBF4‚Et2O
(eq 1). Due to their low stability, these complexes could
1
for the dppp ligand, and the H NMR spectrum showed
(16) (a) Crabtree, R. H. Angew. Chem., Int. Ed. Engl. 1993, 32, 789.
(b) Heinekey, D. M.; Oldham, W. J ., J r. Chem. Rev. 1993, 93, 913. (c)
J essop, P. G.; Morris, R. H. Coord. Chem. Rev. 1992, 121, 155. (d)
Crabtree, R. H. Acc. Chem. Res. 1990, 23, 95. (e) Kubas, G. J . Acc.
Chem. Res. 1988, 21, 120. (f) Crabtree, R. H.; Hamilton, D. G. Adv.
Organomet. Chem. 1988, 28, 289.
(17) (a) Schlaf, M.; Lough, A. L.; Maltby, P. A.; Morris, R. H.
Organometallics, 1996, 15, 2270. (b) Chin, B.; Lough, A. J .; Morris, R.
H.; Schweitzer, C. T.; D’Agostino, C. Inorg. Chem. 1994, 33, 6278. (c)
Bianchini, C.; Marchi, A.; Marvelli, L.; Peruzzini, M.; Romerosa, A.;
Rossi, R.; Vacca, A. Organometallics 1995, 14, 3203. (d) Cotton, F. A.;
Luck, R. L. Inorg. Chem. 1989, 28, 2181. (e) Conroy-Lewis, F. M.;
Simpson, S. J . J . Chem. Soc., Chem. Commun. 1986, 506. (f) Conroy-
Lewis, F. M.; Simpson, S. J . J . Chem. Soc., Chem. Commun. 1987,
1675. (g) Chinn, M. S.; Heinekey, D. M. J . Am. Chem. Soc. 1987, 109,
5865. (h) J oshi, A. M.; J ames, B. R. J . Chem. Soc., Chem. Commun.
1989, 1785. (i) Mudalige, D. C.; Rettig, S. J .; J ames, B. R.; Cullen, W.
R. J . Chem. Soc., Chem. Commun. 1993, 830.
not be isolated as solids in analytically pure form and
so were characterized in situ. The starting hydride
complexes CpFeH(dppe)14 and CpFeH(dppp) were pre-
pared by the reaction of NaBH4 with the corresponding
chloride complexes CpFeCl(diphosphine).

5042 Organometallics, Vol. 15, No. 23, 1996
J ia et al.
a broad hydride signal at -12.0 ppm assignable to
Fe(H2). The classification of 2 as a dihydrogen complex
is based on variable-temperature T1 measurements and
in the presence of Zn. Reaction of the green compound
OsBr2(PPh3)3 with cyclopentadiene to give CpOsBr-
(PPh3)2 has been reported previously.21
1
the observation of large J (HD) coupling constants for
The bromide complexes CpOsBr(P-P) were converted
into the new hydride complexes CpOsH(P-P) (P-P )
dppm (5), dppe (6), and dppp (7)) by refluxing a mixture
of CpOsBr(P-P) and NaOMe in methanol (eq 2). It is
interesting to note that the time required to complete
the reaction is dependent on the diphosphines. It takes
several days refluxing in methanol to completely convert
CpOsBr(dppm) to CpOsH(dppm) and only about 4 h for
the dppe and dppp analogs. The hydride complexes
CpOsH(P-P) are pale yellow solids and were character-
ized by 1H and 31P NMR spectroscopic data and elemen-
the HD isotopomer. The variable-temperature T1 meas-
urements gave a T1(min) of 7 ms for the broad hydride
signal at ca. 220 K (300 MHz). The η2-HD iso-
topomer, [CpFe(HD)(dppp)]BF4, prepared by acidifica-
tion of CpFeH(dppe) with DBF4, showed a 1:1:1 triplet
1
(1J (HD) ) 29.0 Hz) centered at δ -12.0 ppm in the H
NMR spectrum after nulling the η2-H2 peak. Unlike
[CpFe(HD)(dppe)]BF4, the 2J (P-H) coupling was not
resolved for [CpFe(HD)(dppp)]BF4.
The fact that only dihydrogen complexes [CpFe(H2)-
(dppe)]+ and [CpFe(H2)(dppp)]+ were observed in solu-
tion is in sharp contrast to the protonation reactions of
CpRuH(dppe) and CpRuH(dppp), which produced [CpRu-
(H2)(dppe)]+/trans-[CpRuH2(dppe)]+ and trans-[CpRuH2-
(dppp)]+, respectively.5b This observation is consistent
with the general periodic trend in the relative stability
of dihydrogen and dihydride forms.16,18 There are a few
reported analogous iron complexes. The unstable di-
hydrogen complexes [CpFe(H2)(CO)(PR3)]BAr′4 (PR3 )
PEt3, PPh3, Ar′ ) 3,5-(CF3)2C6H3) were reported by
Brookhart et al. recently.3 It was reported by Lapinte
et al. that protonation of the more electron rich complex
Cp*FeH(dppe) at low temperature leads to the initial
formation of [Cp*Fe(H2)(dppe)]BF4, which isomerizes to
trans-[Cp*FeH2(dppe)]BF4 on warming.2 Reactions of
(C5R5)FeCl(dippe) (dippe ) (i-Pr)2PCH2CH2P(i-Pr)2; R
) H, Me) with hydrogen in the presence of NaBPh4
produced the dihydride complexes trans-[(C5R5)FeH2-
(dippe)]BPh4.1
1
tal analysis. In particular, the H NMR spectra of the
monohydride complexes displayed the characteristic
hydride signals as a triplet due to coupling to the two
equivalent phosphorus atoms.
P r ep a r a tion a n d Ch a r a cter iza tion of [Cp OsH2-
(d p p m )]BF 4. Protonation of CpOsH(dppm) with HBF4‚
Et2O at room temperature produced a white powder
analyzed as [CpOsH2(dppm)]BF4. NMR spectroscopic
data indicate that the product consists of a mixture of
two hydride species in a ratio of 10:1. The minor
hydride species can be formulated as trans-[CpOsH2-
(dppm)]BF4, 8A, and the major one cis-[CpOsH2(dppm)]-
BF4, 8B (eq 3).
P r ep a r a t ion of Mon oh yd r id o Osm iu m Com -
p lexes. To prepare [CpOsH2(P-P)]+, we require the
hydride complexes CpOsH(P-P). These monohydrido
complexes were prepared according to eq 2.
The complexes CpOsBr(P-P) were prepared by the
reactions of CpOsBr(PPh3)2 with appropriate diphos-
phines as reported previously.11a,15 To prepare the
starting material CpOsBr(PPh3)2, we initially attempted
to use the method developed by Bruce et al., which
involves the one-pot reaction of H2OsBr6, PPh3, and
cyclopentadiene in MeOH.19 However, the desired
complex CpOsBr(PPh3)2 could not always be obtained
in high yield in our hands, and often a significant
amount of an insoluble red solid was obtained. The
same red material could also be obtained by simply
refluxing H2OsBr6 with excess PPh3 in MeOH. Elemen-
tal analysis of the red solid indicated that its composi-
tion is close to that of OsBr4(PPh3)2.20 We found that
the red material could be easily converted to CpOsBr-
(PPh3)2 by its reaction with cyclopentadiene in MeOH
In the 31P NMR spectrum of the protonated product
in CD2Cl2, the dppm signals were observed as singlets
at -47.3 ppm for cis-[CpOsH2(dppm)]BF4 and at -36.1
ppm for trans-[CpOsH2(dppm)]BF4. In the 1H NMR
spectrum (in CD2Cl2), the hydride signal for trans-
[CpOsH2(dppm)]BF4 was observed at -10.76 ppm with
2J (PH) ) 31.8 Hz. The 2J (PH) coupling constant of 31.8
Hz is very similar to those observed for trans-
[CpOsH2(LL′)]+ ((LL′) ) (PPh3)2, (PPh2Me)2, (PPh3)-
(P(OMe)3), (CO)(P(i-Pr3)).9-11 The hydride signal for cis-
[CpOsH2(dppm)]BF4 was observed at -11.42 ppm with
2J (PH) of 6.5 Hz. Consistent with the structure assign-
ments, two methylene proton signals were observed at
5.01 and 6.19 ppm for cis-[CpOsH2(dppm)]BF4, and only
one methylene proton signal was observed at 5.53 ppm
1
for trans-[CpOsH2(dppm)]BF4 in the H NMR spectrum
in CD2Cl2.
(18) Lin, Z.; Hall, M. B. Coord. Chem. Rev. 1994, 135/ 136, 845 and
references therein.
2
The J (PH) coupling constant of 6.5 Hz observed for
(19) Bruce, M. I.; Windsor, N. J . Aust. J . Chem. 1977, 30, 1601.
(20) A number of complexes of the formula OsX4(PR3)2 (X ) Cl, Br)
have been reported. See for example: (a) Salmon, D. J .; Walton, R. A.
Inorg. Chem. 1978, 17, 2379. (b) Chatt, J .; Leigh, F. G. J .; Paske, R. J .
J . Chem. Soc. A 1969, 2239. (c) Chatt, J .; Mingos, D. M. P.; Paske, R.
J . J . Chem. Soc. A 1968, 2636.
the hydride signal of cis-[CpOsH2(dppm)]BF4 (at -11.42
ppm) is smaller than those normally observed for
(21) Blackmore, T.; Bruce, M. I.; Stone, F. G. A. J . Chem. Soc. A
1971, 2376.

[CpMH2(diphosphine)]+
Organometallics, Vol. 15, No. 23, 1996 5043
CpOsH(PR3)2 or [CpOsH2(PR3)2]+ and is close to those
observed for molecular dihydrogen complexes.16,17 Thus
there is a possibility that cis-[CpOsH2(dppm)]BF4 con-
tains a dihydrogen ligand. To clarify the structure of
[CpOsH2(dppm)]BF4, we have measured the T1 values
for the hydride signals and J (HD) coupling constants
for the HD isotopomers [CpOsHD(dppm)]BF4. The
variable T1 measurements at 300 MHz gave T1(min) of
ca. 150 ms for the hydride signal of cis-[CpOsH2(dppm)]-
BF4 at -11.42 ppm and ca. 610 ms for that of trans-
[CpOsH2(dppm)]BF4 at -10.76 ppm. Although the
T1(min) value for cis-[CpOsH2(dppm)]BF4 (150 ms) is
significantly smaller than that for trans-[CpOsH2(dppm)]-
BF4 (610 ms), the T1(min) value of 150 ms is too large
for molecular dihydrogen complexes. Molecular dihy-
drogen complexes usually have T1(min) values less than
50 ms at 250 MHz.16
proposed for H/H exchange in ReH2(CO)(NO)(PR3)2 (PR3
) PCy3, P(i-Pr)3, PMe3, P(O-i-Pr)3).25 Consistent with
the existence of the exchange process, a singlet dppm
signal was observed in the 31P NMR spectra of both cis-
[CpOsHD(dppm)]+ and cis-[CpOsH2(dppm)]+ in the
temperature range 298-190 K.
We have prepared the HD isotopomers [CpOsHD-
(dppm)]+ by protonation of CpOsH(dppm) with DBF4.
The small 2J (PH) coupling constant (6.5 Hz) observed
for cis-[CpOsH2(dppm)]BF4 could be related to the
1
In the H NMR spectrum of the isotopomers in CD2Cl2,
2
the hydride signal of cis-[CpOsHD(dppm)]BF4 appeared
exchange process and/or the fact that J (PH) are angu-
2
at -11.48 ppm with J (HD) ) 3.0 Hz and J (PH) ) 6.5
lar dependent and can have both positive and negative
values. Very small 2J (P-H) values have been reported
previously for classic hydride complexes. For example,
only about 3 Hz is observed for some RhH(CO)2(P2)
Hz. The J (HD) coupling was not resolved for the
hydride signal at -10.77 ppm due to trans-[CpOsHD-
(dppm)]BF4. The J (HD) value of 3.0 Hz is too small for
η2-HD complexes, which usually give 1J (HD) larger than
20 Hz.16 The observations of long T1(min) (>150 ms)
and small J (HD) for the HD isotopomers support the
classic dihydride formulation for both isomers of
[CpOsH2(dppm)]+. However, the alternative formula-
tion of cis-[CpOsH2(dppm)]+ as the elongated dihydro-
gen complex [CpOs(H‚‚H)(dppm)]+ cannot be ruled out
completely, especially in view of the observations of only
one hydride signal and small 2J (PH) and J (HD) values.
The observation of a J (HD) of 3.0 Hz for cis-[CpOsHD-
(dppm)]BF4 implies that there will be ca. 18 Hz coupling
for the two hydride ligands in cis-[CpOsH2(dppm)]BF4
when contribution from quantum mechanical exchange
is not considered. 2J (HH) coupling constants for classic
polyhydride complexes are usually less than 15 Hz.22
However, unusually large J (HD) coupling constants
have been observed for some metal hydride complexes
that can undergo quantum mechanical exchange.23 It
is also noted that the isotopomers of several osmium
complexes with elongated dihydrogen ligands also have
small J (HD) coupling constants,24 for example, [Os(HD)-
(NH3)4(acetone)]+ (J (HD) ) 4.0 Hz) and [Os(HD)(NH3)4-
(D2O)]+ (J (HD) ) 8.1 Hz).
2
complexes (P2 ) diphosphites)26 and J (PH) of 0 Hz have
been noted for complexes like CpRuH3(PR3).27 Caulton
et al. have noted that 2J (PH) coupling constants are
usually small when the P-M-H angles are in the range
90-140° and change signs at ca. 75-80°.28 The small
2J (PH) (6.5 Hz) observed for cis-[CpOsH2(dppm)2]BF4
could be related to P-Os-H angles in this complex.
Alternatively, the small coupling constant could be due
2
to the average of 2J (PH)cis and J (PH)trans, which are of
opposite signs, involving the process shown in eq 4.
Formation of the dihydride complex [CpOsH2(dppm)]+
from the protonation reaction of CpOsH(dppm) is not
surprising, as even the less electron rich CO-containing
complex trans-[CpOsH2(CO)(P(i-Pr)3)]+ is a classic di-
hydride complex.10 In contrast, protonation of CpRuH-
(CO)(PR3)7a,d and CpRuH(dppm)5b at room temperature
gave the molecular dihydrogen complexes [CpRu(H2)-
(CO)(PR3)]BF4 and [CpRu(H2)dppm)]BF4, respectively.
Protonation of the more electron-rich ruthenium com-
plex Cp*RuH(dppm) with HBF4 led to a mixture of
trans-[Cp*RuH2(dppm)]BF4 and [Cp*Ru(H2)(dppm)]BF4
in a ratio of 1:2 in solution.8b-d
The observation that cis-[CpOsH2(dppm)]+ is more
stable than trans-[CpOsH2(dppm)]+ is rather unusual,
as dihydride complexes of the type [(η5-C5R5)MH2-
(PR3)2]x usually adopt trans geometry.1,2,4,5b,6,7c,9,11,29 The
large size of osmium and the small bite angle of dppm
are the most likely factors contributing to the stability
of cis-[CpOsH2(dppm)]+.
The two phosphorus atoms and the two hydrides in
cis-[CpOsH2(dppm)]+ are chemical equivalent but are
magnetically inequivalent. Thus a triplet signal is not
1
normally expected in the H NMR for the two hydride
ligands of cis-[CpOsH2(dppm)]BF4. However, the hy-
dride signal of cis-[CpOsH2(dppm)]BF4 was observed as
a triplet in the temperature range 298-230 K. The
observation of a triplet hydride signal for cis-[CpOsH2-
(dppm)]BF4 is likely due to fast exchange of the two
hydrides, which could involve the dihydrogen interme-
diate shown in eq 4. Similar mechanism has been
P r oton a tion Rea ction s of Cp OsH(P -P ) (P -P )
d p p e a n d d p p p ) a t Room Tem p er a tu r e. Protona-
(25) Bakhmutov, V.; Burgi, T.; Burger, P.; Ruppli, U.; Berke, H.
Organometallics 1994, 13, 4203.
(26) See for example: Buisman, G. H. B.; Vos, E. J .; Kamer, P. C.
J .; von Leeuwen, P. W. N. M. J . Chem. Soc., Dalton Trans. 1995, 409
and references therein.
(27) Arliguie, T.; Boreder, C.; Chaudret, B.; Devillers, J .; Poilblanc,
R. Organometallics 1989, 8, 1308.
(28) Gusev, D. G.; Kuhlman, R.; Rambo, J . R.; Berke, H.; Eisenstein,
O.; Caulton, K. G. J . Am. Chem. Soc. 1995, 117, 281.
(29) See for example: (a) J ones, W. D.; Maguire, J . A. Organo-
metallics 1987, 6, 1301. (b) Herrmann, W. A.; Theiler, H. G.; Herdtweck,
E.; Kiprof, P. J . Organomet. Chem. 1989, 367, 291.
(22) Moore, D. S.; Robinson, S. D. Chem. Soc. Rev. 1983, 12, 415.
(23) Heinekey, D. M.; Millar, J . M.; Koetzle, T. F.; Payne, N. G.;
Zilm, K. W. J . Am. Chem. Soc. 1990, 112, 909 and references therein.
(24) (a) Li, Z. W.; Taube, H. J . Am. Chem. Soc. 1991, 113, 8946. (b)
Li, Z. W.; Taube, H. J . Am. Chem. Soc. 1994, 116, 9506. (c) Hasegawa,
T.; Koetzle, T. J . Li, Z.; Parkin, S.; McMullan, R.; Koetzle, T. F.; Taube,
H. J . Am. Chem. Soc. 1994, 116, 4352. (d) Bacskay, G. B.; Bytheway,
I.; Hush, N. S. J . Am. Chem. Soc. 1996, 118, 3755.

5044 Organometallics, Vol. 15, No. 23, 1996
J ia et al.
tion of CpOsH(dppe) with HBF4‚Et2O at room temper-
ature produced a mixture of trans-[CpOsH2(dppe)]BF4,
9A, and cis-[CpOsH2(dppe)]BF4, 9B, in a ratio of 70:1
(eq 5). In the 1H NMR spectrum in CD2Cl2, trans-
that protonation of (C5R5)RuH(L2) (R ) H, Me; L )
phosphine) at low temperature appears always to give
the molecular dihydrogen complexes [(C5R5)Ru(H2)(L2)]+
which may isomerize into thermodynamically more
stable dihydrido complexes [(C5R5)RuH2(L2)]+.7c,8c Pro-
tonation of Cp*FeH(dppe) at -80 °C also initially
produced the molecular dihydrogen complex [Cp*Fe(H2)-
(dppe)]+, which converted to the dihydride complex
trans-[Cp*FeH2(dppe)]+ upon warming.2 However,
Brookhart and his co-worker recently reported that
protonation of CpFeH(CO)(PEt3) with H(OEt2)BAr′4 at
-78 °C in CD2Cl2 containing water initially produced
the dihydride complex trans-[CpFeH2(CO)(PEt3)]+, which
isomerized into the molecular dihydrogen complex
[CpFeH2(CO)(PEt3)]+ upon being warmed to -20 °C.3
We have studied the protonation reactions of CpFeH-
(P-P) (P-P ) dppe, dppp) and CpOsH(P-P) (P-P ) dppm,
dppe, and dppp) at low temperature to determine the
initial kinetic protonation products. Protonation of
CpFeH(P-P) (P-P ) dppe, dppp) at -78 °C produced the
dihydrogen complexes [CpFe(H2)(P-P)]+, as observed at
room temperature.
Protonation of CpOsH(dppm) with HBF4 at -78 °C
produced a mixture of cis-[CpOsH2(dppm)]BF4 and
trans-[CpOsH2(dppm)]BF4 in a ratio of 24:1. When the
solution was warmed to room temperature, the ratio of
cis-[CpOsH2(dppm)]BF4 to trans-[CpOsH2(dppm)]BF4
changed to 10:1. Protonation of CpOsH(dppe) with
HBF4 at -78 °C produced cis-[CpOsH2(dppe)]BF4 and
trans-[CpOsH2(dppe)]BF4 in a ratio of 1:3. When the
solution was warmed to room temperature, the ratio of
cis-[CpOsH2(dppe)]BF4 to trans-[CpOsH2(dppe)]BF4
changed to 1:70. Protonation of CpOsH(dppp) with
HBF4 at -78 °C produced a mixture of cis-[CpOsH2-
(dppp)]BF4 and trans-[CpOsH2(dppp)]BF4 in a ratio of
1:3. cis-[CpOsH2(dppp)]BF4 is unstable and isomerized
to trans-[CpOsH2(dppp)]BF4 on warming to room tem-
perature. Since more cis isomers were observed when
the protonation was carried out at low temperature, it
is likely that the Os-H was preferentially protonated
in the reaction of HBF4 with CpOsH(P-P), at least at
low temperature.
[CpOsH2(dppe)]BF4 displayed a triplet hydride signal
at -13.11 ppm with J (PH) of 32.5 Hz, and cis-[CpOsH2-
(dppe)]BF4 displayed a triplet hydride signal at -12.53
ppm with 2J (PH) of only 7.6 Hz. These coupling
constants are similar to those observed for the isomers
of [CpOsH2(dppm)]BF4.
For comparison, it has been reported that protonation
of CpRuH(dppe) produced a mixture of [CpRu(H2)-
(dppe)]+ and trans-[CpRuH2(dppe)]+ in a ratio of 2:1.5b
Reaction of [Cp*Ru(dppe)]+ with H2 only led to the
formation of trans-[Cp*RuH2(dppe)]+.30 The existence
of a small amount of cis-[CpOsH2(dppe)]+ in equilibrium
with trans-[CpOsH2(dppe)]+ in solution (not observed
for ruthenium complexes) could be attributed to the
relatively large size of osmium. As expected, the ratio
of cis to trans isomers of [CpOsH2(P-P)]+ increases from
1:70 for [CpOsH2(dppe)]+ to 9:1 for [CpOsH2(dppm)]+,
as dppm has a smaller bite angle.
2
The observation of cis-[CpOsH2(dppe)]+ in solution at
room temperature promoted us to prepare [CpOsH2-
(dppp)]+, which contains a more flexible diphosphine,
in order to see if cis-[CpOsH2(dppp)]+ is also a thermo-
dynamically stable complex. When CpOsH(dppp) in
Et2O was protonated with HBF4‚Et2O at room temper-
ature, a white powder of [CpOsH2(dppp)]BF4, 10, was
produced. The NMR spectra collected immediately after
dissolving the powder in CD2Cl2 showed signals of both
trans-[CpOsH2(dppp)]BF4 (10A, major) and cis-[CpOsH2-
(dppp)]BF4 (10B, trace) (eq 6). However cis-[CpOsH2-
(dppp)]BF4 is unstable and isomerized to trans-[CpOsH2-
(dppp)]BF4 in solution within 10 min, as confirmed by
the disappearance of the signals of cis-[CpOsH2(dppp)]-
1
BF4 in NMR experiments. Complexes 10A,B show H
NMR data similar to those of 8 and 9.
P r oton a tion Rea ction s a t Low Tem p er a tu r e.
The observation of cis-[CpOsH2(dppp)]BF4 isomerizing
to trans-[CpOsH2(dppp)]BF4 at room temperature raises
the question of the sites of protonation for the mono-
hydride complexes. In principle, both the metal center
and hydride can be protonated to give the initial
protonation products, which can then isomerize to the
final thermodynamic products. It has been reported
Con clu sion . This study on the protonation reactions
of CpMH(P-P) (M ) Fe, Os), together with previous
studies on ruthenium complexes, shows that the ther-
modynamically stable structures of [CpMH2(P-P)]+ are
dependent on the size of the chelating rings and metals.
The molecular dihydrogen form is more stable for the
iron complexes [CpFe(H2)(P-P)]BF4 (P-P ) dppe, dppp).
Analogous ruthenium complexes can adopt either the
(30) Kirchner, K.; Mauthner, K.; Mereiter, K.; Schmid, R. J . Chem.
Soc., Chem. Commun. 1993, 893.

[CpMH2(diphosphine)]+
Organometallics, Vol. 15, No. 23, 1996 5045
dihydride form trans-[(η5-C5R5)RuH2(P-P)]+ or the di-
hydrogen form [(η5-C5R5)Ru(H2)(P-P)]+ or a mixture of
both, depending on the size of the chelating rings and
C5R5. Due to its electron-rich nature, the dihydride
form is more stable for the osmium analogs [CpOsH2-
(P-P)]+. These observations support the general trend
in relative stability of dihydrogen and dihydride
forms.16,18
isomers in a ratio of 1:70, and [CpOsH2(dppp)]BF4
adopts only trans geometry. The isolation and observa-
tion of cis-[CpOsH2(P-P)]+ (P-P ) dppm, dppe) is rather
unusual, as dihydride complexes of the type [(η5-C5R5)-
MH2(PR3)2]x usually adopt trans geometry.1,2,4,5b,6,7c,9,11,29
The relatively large size of osmium and small bite
angles of dppm and dppe are the most likely factors
contributing to the stability of cis-[CpOsH2(P-P)]+.
Depending on the size of the chelating rings, the
thermodynamically stable structures of CpOsH2(P-P)]-
BF4 at room temperature may be exclusively trans-
[CpOsH2(P-P)]BF4 or a mixture of cis-[CpOsH2(P-P)]BF4
and trans-[CpOsH2(P-P)]BF4. At room temperature in
dichloromethane solution, [CpOsH2(dppm)]BF4 exists as
a mixture of cis and trans isomers in a ratio of 10:1,
[CpOsH2(dppe)]BF4 exists as a mixture of cis and trans
Ack n ow led gm en t. The authors acknowledge finan-
cial support from the Hong Kong Research Grants
Council (Grant No HKP91/94P) and the Hong Kong
University of Science and Technology. We thank Dr.
Lucy Hyatt for proofreading the manuscript.
OM960472F