Heterolytic H2 Activation by Dihydrogen Complexes. Effects of the Ligand X in [M(X)H2{Ph2P(CH2)3PPh 2}2]n+ (M = Ru, Os; X = CO, Cl, H)

Article abstract of DOI:10.1021/ic9605579

Complexes of the general formula [MXH₂(dppp)₂]ⁿ⁺ (M = Ru, Os; X = H, Cl, CO; dppp = 1,3-bis-(diphenylphosphino)propane) have been prepared and characterized, and the effect of the donor/acceptor properties of X on their structure and acidity has been studied. The five-coordinate complexes [MCl(dppp)₂]⁺ (M = Ru (1a), Os (1b)) react with H₂ gas in CH₂Cl₂ to give the complexes [MCl(η²-H₂)(dPPP)₂]⁺ (M = Ru (2a), Os (2b)) containing elongated dihydrogen ligands. The molecular structure, of 2b has been determined by X-ray crystallography (monoclinic, space group P2₁/n with a = 13.314(7) A?, b = 18.63(2) A?, c = 23.20(2) A? β = 94.58(6)° and Z = 4). Chlorohydride [OsH(Cl)(dppp)2] (3b) reacts with H₂ gas in the presence of Na[BPh₄] forming [OsH₃(dppp)₂]⁺ (4b). Protonation of [OsH₂(dPpp)₂] (5b) with HBF₄·Et₂O also gives 4b. A combination of X-ray crystallography (monoclinic, space group P2₁/n with a = 13.392(3) A?, b = 25.306(7) A?, c = 21.247(7) A?, β = 91.15(2)° and Z = 4) and ¹H and ³¹P NMR studies indicate that 4b is a classical trihydride. Hydridocarbonyls [MH(CO)(dppp)₂]⁺ (M = Ru (6a), Os (6b)) are protonated by F₃CSO₃H in CD₂Cl₂ to yield [M(CO)(η²-H₂)(dPpp)₂]²⁺ (M = Ru (7a), Os (7b)), which were characterized in solution. 7a is stable only at low temperature. Compound 7b is a highly acidic dihydrogen complex with an estimated PK_a of -6.

Full text of DOI:10.1021/ic9605579

Inorg. Chem. 1997, 36, 711-720
711
Heterolytic H2 Activation by Dihydrogen Complexes. Effects of the Ligand X in
[M(X)H2{Ph2P(CH2)3PPh2}2]n+ (M ) Ru, Os; X ) CO, Cl, H)
Eliana Rocchini,† Antonio Mezzetti,*,‡ Heinz Ru1egger,‡ Urs Burckhardt,‡ Volker Gramlich,§
Alessandro Del Zotto,† Paolo Martinuzzi,† and Pierluigi Rigo†
Dipartimento di Scienze e Tecnologie Chimiche, Universita` di Udine, Via del Cotonificio 108,
I-33100 Udine, Italy, Laboratorium fu¨r Anorganische Chemie, ETH-Zentrum,
CH-8092 Zu¨rich, Switzerland, and Institut fu¨r Kristallographie und Petrographie, ETH-Zentrum,
CH-8092 Zu¨rich, Switzerland
ReceiVed May 16, 1996X
Complexes of the general formula [MXH2(dppp)2]n+ (M ) Ru, Os; X ) H, Cl, CO; dppp ) 1,3-bis-
(diphenylphosphino)propane) have been prepared and characterized, and the effect of the donor/acceptor properties
of X on their structure and acidity has been studied. The five-coordinate complexes [MCl(dppp)2]+ (M ) Ru
(1a), Os (1b)) react with H2 gas in CH2Cl2 to give the complexes [MCl(η2-H2)(dppp)2]+ (M ) Ru (2a), Os (2b))
containing elongated dihydrogen ligands. The molecular structure of 2b has been determined by X-ray
crystallography (monoclinic, space group P21/n with a ) 13.314(7) Å, b ) 18.63(2) Å, c ) 23.20(2) Å, â )
94.58(6)°, and Z ) 4). Chlorohydride [OsH(Cl)(dppp)2] (3b) reacts with H2 gas in the presence of Na[BPh4]
forming [OsH3(dppp)2]+ (4b). Protonation of [OsH2(dppp)2] (5b) with HBF4‚Et2O also gives 4b. A combination
of X-ray crystallography (monoclinic, space group P21/n with a ) 13.392(3) Å, b ) 25.306(7) Å, c ) 21.247(7)
Å, â ) 91.15(2)°, and Z ) 4) and 1H and 31P NMR studies indicate that 4b is a classical trihydride.
Hydridocarbonyls [MH(CO)(dppp)2]+ (M ) Ru (6a), Os (6b)) are protonated by F3CSO3H in CD2Cl2 to yield
[M(CO)(η2-H2)(dppp)2]2+ (M ) Ru (7a), Os (7b)), which were characterized in solution. 7a is stable only at
low temperature. Compound 7b is a highly acidic dihydrogen complex with an estimated pKa of -6.
Introduction
hydrogenation reactions.2a,3 The Brønsted acidity of B can be
expected to be of fundamental importance for stoichiometric
Dihydrogen complexes allow for the design of a simple,
fascinating pathway for the heterolytic activation of molecular
hydrogen by means of transition metal compounds. The overall
process can be sketched according to eq 1 as (a) the formation
of dihydrogen complex B (or of the classical hydride D) by
addition of H2 to the coordinatively unsaturated metal fragment
A, followed by (b) dissociation of H+ to give monohydride C.
and catalytic hydrogenations which are sensitive to the con-
centration of acid.4
The reactivity of formally unsaturated transition metal
complexes with H2 was first reported for [M(CO)3(η2-H2)L2]
(M ) Cr, Mo, W; L ) phosphine)2a,b and, more recently, for
[Re(CO)3(η2-H2)L2]+.5 Agostic interactions stabilize the un-
saturated, 16-electron fragments [M(CO)3L2]n+ (n ) 0 or 1). A
further stabilization mechanism for the five-coordinate fragment
A has been suggested recently for the case in which X is a
π-donor.6 Thus, when one hydride ligand is substituted by
halide in RuH6L2 and IrH5L2, the (formally) 16-electron species
Ru(X)H(η2-H2)L2 (X ) Cl, I)7a and Ir(Cl)H2L27b are stabilized
by X f M π-donation. Reversing this argument suggests that
“π-stabilized unsaturated compounds may readily bind H2, and
some of them might be dihydrogen compounds”.6b Actually,
five-coordinate complexes of the type [MCl(L2)2]n+ (M ) Ru
or Os, L2 ) diphosphine ligand)8,9c,e have been known to react
with molecular hydrogen to give trans-[MCl(η2-H2)(L2)2]n+
,
Crabtree and Lavin have recognized at an early stage that
dihydrogen complexes can act as Brønsted acids.1 This has led
to systematic investigations of ligand effects and to the discovery
of highly acidic dihydrogen complexes.2 Although conclusive
experimental evidence is still lacking, the reaction sequence A
f B f C is thought to play a key role in some homogeneous
which contain an elongated dihydrogen ligand.9
(3) (a) Crabtree, R. H. The Organometallic Chemistry of the Transition
Metals; Wiley: New York, 1988; p 198. (b) Ashby, M. T.; Halpern,
J. J. Am. Chem. Soc. 1991, 113, 589.
(4) See, for instance: (a) Bullock, R. M.; Song, J.-S.; Szalda, D. J.
Organometallics 1996, 15, 2504. (b) Mezzetti, A.; Tschumper, A.;
Consiglio, G. J. Chem. Soc., Dalton Trans. 1995, 49 and references
therein.
† Universita` di Udine.
‡ Laboratorium fu¨r Anorganische Chemie.
(5) Heinekey, D. M.; Schomber, B. M.; Radzewich, C. E. J. Am. Chem.
Soc. 1994, 116, 4515.
§ Institut fu¨r Kristallographie und Petrographie.
X Abstract published in AdVance ACS Abstracts, January 1, 1997.
(1) Crabtree, R. H.; Lavin, M. J. Chem. Soc., Chem. Commun. 1985, 794.
(2) (a) Jessop, P. G.; Morris, R. H. Coord. Chem. ReV. 1992, 121, 155.
(b) Heinekey, D. M.; Oldham, W. J. Chem. ReV. 1993, 93, 913. (c)
Morris, R. H. Inorg. Chem. 1992, 31, 1471. (d) Schlaf, M.; Lough,
A. J.; Morris, R. H. Organometallics 1993, 12, 3808. (e) Schlaf, M.;
Lough, A. J.; Maltby, P. A.; Morris, R. H. Organometallics 1996, 15,
2270. (f) Heinekey, D. M.; Luther, T. A. Inorg. Chem. 1996, 35, 4396.
(6) (a) Caulton, K. G. New J. Chem. 1994, 18, 25. (b) Johnson, T. J.;
Folting, K.; Streib, W. E.; Martin, J. D.; Huffman, J. C.; Jackson, S.
A.; Eisenstein, O.; Caulton, K. G. Inorg. Chem. 1995, 34, 488. (c)
Riehl, J.-F.; Jean, Y.; Eisenstein, O.; Pe´lissier, M. Organometallics
1992, 11, 729.
(7) (a) Chaudret, B.; Chung, G.; Eisenstein, O.; Jackson, S. A.; Lahoz, F.
J.; Lopez, J. A. J. Am. Chem. Soc. 1991, 113, 2314. (b) Mediati, M.;
Tachibana, G. N.; Jensen, C. M. Inorg. Chem. 1992, 31, 1827.
S0020-1669(96)00557-5 CCC: $14.00 © 1997 American Chemical Society

712 Inorganic Chemistry, Vol. 36, No. 4, 1997
Rocchini et al.
Table 1. Low-temperature 31P NMR Dataa
δ (ppm)
With minor exceptions,10 X is either H or Cl in most
dihydrogen complexes of the type trans-[MX(η2-H2)L4]+ (M
) d6 metal ion, L ) phosphine ligand). However, a thorough
investigation of the related system trans-[OsL(η2-H2)(en)2]n+
(en ) 1,2-diaminoethane), with L chosen amongst a wide variety
of neutral and anionic ligands, has shown that the trans ligand
L deeply affects the H-H distance.11 Thus, we became
intrigued by the question of whether it is possible to choose an
ancillary ligand X which both stabilizes the five-coordinate
species A and imparts a high Brønsted acidity to the dihydrogen
complex B, in order to achieve the reaction sequence A f B
f C in eq 1.
2J(P,P′) (Hz)
T (K)
2a
2b
3ab
3b
4b
6a
6b
7a
7b
-4.1 (t), 18.5 (t)
-42.5 (t), -16.6 (t)
10.3 (t), 25.5 (t)
-27.9 (t), -14.9 (t)
3.3 (t), -25.2 (t)
15.3 (t), 20.5 (t)
31.6
22.3
41.3
29.5
22.6
35.9
24.9
24.6
17.9
183
193
173
173
193
173
173
173
173
-23.4 (t), -19.6 (t)
3.7 (t), 12.3 (t)
-32.3 (t), -22.1 (t)
a In CD2Cl2 solution. b 1H NMR (CD2Cl2, 173 K, 200 MHz): δ
-17.5 (tt, 1 H, RuH, J(P,H) ) 25.4, J(P′,H) ) 12.7 Hz).
Since only [RuH(η2-H2)(dppp)2]+ has been reported in the
dppp series,12 we prepared the dihydrogen complexes [MX(η2-
H2)(dppp)2]+ (M ) Ru or Os, dppp ) 1,3-bis(diphenylphos-
phino)propane) with different X ligands and measured their
Brønsted acidity. The reaction of the previously reported five-
coordinate complexes [MCl(dppp)2]+ (M ) Ru (1a), Os (1b))8a,b
with H2 gave dihydrogen complexes containing chloride, a
π-donor. Then, X was changed to hydride (a strong σ-donor)
and to a strong π-acceptor, such as carbon monoxide. Proto-
nation of the monohydrides C was used for the synthesis of
dihydrogen complexes containing CO. The first results of this
investigation are reported in the present paper.
Table 2. 1H NMR Data for [MXH2(dppp)2]n+ a
T1(min)
(ms)
d(H-H)
J(D,H)
J(P,H)
(Hz)
T
(K)
X
(Å)b
(Hz)c
2a
2b
4a
4b
7a
7b
Cl
Cl
H
H
CO
CO
8.2
23.7
5.5e
72
5.0
5.5
1.0
1.2
0.89
1.6f
0.85
0.89
24d
11
32
0
8
11
223
213
243
243
223
233
g
34h
32
a CD2Cl2 solution, 200 MHz. b Calculated using eq 6.9e c At room
temperature, unless otherwise stated. d Measured at 0 °C. e Literature
value: 6 ms at 400 MHz.8b f Nonbonded distance. g See text. h Mea-
sured at -80 °C.
Experimental Section
General Considerations. All manipulations involving solutions of
the complexes were performed under argon with use of Schlenk line
techniques. Solvents were purified by standard methods. All chemicals
used were of reagent grade or comparable purity. The ligand dppp,
RuCl3‚H2O, and (NH4)2OsCl6 were purchased from Aldrich; RuCl2-
(PPh3)3,13 [RuH(Cl)(dppp)2],14 and [RuH(η2-H2)(dppp)2][PF6]12a were
prepared according to the literature. Yields are based on the metal.
Infrared spectra were recorded on a Nicolet Magna 550 FT-IR
spectrophotometer. Microanalyses were performed by the Microana-
lytical Laboratory of the Dipartimento di Scienze e Tecnologie
Chimiche, Universita` di Udine. 1H and 31P{1H} NMR spectra were
obtained with Bruker AMX 500 or AC 200 spectrometers; solid state
31P{1H} CP-MAS NMR spectra were recorded on a Bruker AMX 400.
31P chemical shifts are relative to 85% H3PO4 and (NH4)H2PO4 for
solutions and solids, respectively. Inverse-gated decoupling was used
to record the 31P NMR spectra when their integration was required.
Spectral simulations were performed with PANIC (Bruker Spectrospin
AG). Low-temperature 31P NMR data are reported in Table 1, and
Preparation of [RuCl(dppp)2][PF6] (1a). An improved procedure
consisted of refluxing [RuCl2(PPh3)3] (1.00 g, 1.04 mmol), dppp (0.86
g, 2.09 mmol), and NH4[PF6] (0.51 g, 3.1 mmol) in 30 mL of EtOH
for 4 h. Recrystallization from CH2Cl2/iPrOH gave brown microcrystals
with properties consistent with those reported in the literature.8a
Yield: 1.00 g, 87%. 1H NMR (CD2Cl2, 200 MHz, 293 K): δ 8.0-
6.9 (m, PC6H5), 2.6 (br, 4 H, PCH2), 2.2 (br, 4 H, PCH2), 1.7 (br, 4 H,
PCH2CH2). 31P{1H} NMR (CD2Cl2, 81 MHz, 293 K): δ -1.6 (t),
43.2 (t, J(P,P′) ) 31.5 Hz), -143.9 (sept, J(P,F) ) 711 Hz).
Preparation of [OsCl(dppp)2][PF6] (1b). An improved procedure
consisted of refluxing (NH4)2OsCl6 (1.00 g, 2.28 mmol) and dppp (3.30
g, 8.00 mmol) in 50 mL of CH3O(CH2)2OH for 6 h. After the solution
was cooled, NH4[PF6] (1.12 g, 6.84 mmol) in 20 mL of EtOH was
added thereto. The brown precipitate was filtered off, vacuum dried,
and recrystallized from CH2Cl2/iPrOH. Yield: 1.75 g, 65%. Anal.
Calcd for C54H52ClF6OsP5: C, 54.25; H, 4.38. Found: C, 55.06; H,
4.54. Physical properties consistent with those reported in the
literature.8b 1H NMR (CD2Cl2, 200 MHz, 293 K): δ 8.0-6.8 (m,
PC6H5), 2.9 (br, 4 H, PCH2), 2.4 (br, 4 H, PCH2), 1.6 (br, 4 H,
PCH2CH2). 31P{1H} NMR (CD2Cl2, 81 MHz, 293 K): δ -23.0 (t),
-17.5 (t, J(P,P′) ) 16.7 Hz), -143.9 (sept, J(P,F) ) 711 Hz).
Reaction of 1a with H2 at Low Pressure. [RuCl(dppp)2][PF6] (25
mg, 0.045 mmol) was dissolved in 0.25 mL of CD2Cl2 in a Wilmad
NMR tube fitted with a pressure valve. The tube was connected
alternatively to a high vacuum or hydrogen by means of a manifold
and Swagelock fittings, and after three freeze-pump-thaw cycles, the
tube was filled with hydrogen. Two distinct experiments were run with
a H2 pressure of 1 and 2 atm. After the valve was closed, the tube
was rapidly warmed to room temperature and transferred into the NMR
magnet (T ) 23 °C). 1H and inverse-gated {1H}-decoupled 31P NMR
spectra were recorded over a period of 96 h. The H2 concentration
has been corrected for 25% NMR-silent parahydrogen.
Observation of trans-[RuCl(η2-H2)(dppp)2][PF6] (2a). Complex
1a (50 mg, 0.090 mmol) was dissolved in 0.5 mL of CD2Cl2 in an
NMR tube and transferred into a stainless steel autoclave, which was
closed, purged with H2 (50 atm), and pressurized to 300 atm. After
36 h at room temperature, the pressure was released. The sample,
whose color had turned pale yellow, was removed from the autoclave
and was immediately frozen in liquid nitrogen. Afterward, the sample
was kept at -25 °C. 1H NMR (CD2Cl2, 200 MHz, 293 K): δ 8.0-
6.9 (m, PC6H5), 2.8 (br, 4 H, PCH2), 2.2 (br, 4 H, PCH2), 1.7 (br, 4 H,
PCH2CH2), -10.6 (br, 2 H, RuH2). 31P{1H} NMR (CD2Cl2, 81 MHz,
293 K): δ 6.6 (br), -143.9 (sept, J(P,F) ) 711 Hz).
1
selected H NMR data (T1(min) and J(D,H)) in Table 2.
(8) (a) Bressan, M.; Rigo, P. Inorg. Chem. 1975, 14, 2286. (b) Bressan,
M.; Ettorre, R.; Rigo, P. Inorg. Chim. Acta 1977, 24, L57. (c) Batista,
A. A.; Centeno Cordeiro, L. A.; Oliva, G. Inorg. Chim. Acta 1993,
203, 185. (d) Mezzetti, A.; Del Zotto, A.; Rigo, P.; Bresciani Pahor,
N. J. Chem. Soc., Dalton Trans. 1989, 1045. (e) Mezzetti, A.; Del
Zotto, A.; Rigo, P. J. Chem. Soc., Dalton Trans. 1990, 2515.
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Steele, M. R. Inorg. Chem. 1989, 28, 4437. (b) Mezzetti, A.; Del Zotto,
A.; Rigo, P.; Farnetti, E. J. Chem. Soc., Dalton Trans. 1991, 1525.
(c) Burrell, A. K.; Bryan, J. C.; Kubas, G. J. J. Am. Chem. Soc. 1994,
116, 1575. (d) Chin, B.; Lough, A. J.; Morris, R. H.; Schweitzer, C.
T.; D’Agostino, C. Inorg. Chem. 1994, 33, 6278. (e) Maltby, P. A.;
Schlaf, M.; Steinbeck, M.; Lough, A. J.; Morris, R. H.; Klooster, W.
T.; Koetzle, T. F.; Srivastava, R. C. J. Am. Chem. Soc. 1996, 118,
5396.
(10) Jime´nez-Tenorio, M.; Puerta, M. C.; Valerga, P. J. Chem. Soc., Chem.
Commun. 1993, 1750.
(11) (a) Li, Z.-W.; Taube, H. J. Am. Chem. Soc. 1991, 113, 8946. (b)
Hasegawa, T.; Li, Z.-W.; Parkin, S.; Hope, H.; McMullan, R. K.;
Koetzle, T. F.; Taube, H. J. Am. Chem. Soc. 1992, 114, 2712. (c) Li,
Z.-W.; Taube, H. J. Am. Chem. Soc. 1994, 116, 4352.
(12) (a) Ashworth, T. V.; Singleton, E. J. Chem. Soc., Chem. Commun.
1976, 705. (b) Saburi, M.; Aoyagi, K.; Takahashi, T.; Uchida, Y.
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(13) Stephenson, T. A.; Wilkinson, G. J. Inorg. Nucl. Chem. 1966, 28,
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Heterocyclic H2 Activation by Dihydrogen Complexes
Inorganic Chemistry, Vol. 36, No. 4, 1997 713
Table 3. Selected Bond Distances (Å) and Angles (deg) for
[OsCl(η2-H2)(dppp)2][PF6]
Table 4. Selected Bond Distances (Å) and Angles (deg) for
[OsH3(dppp)2][PF6]
Os(1)-Cl(1)
Os(1)-P(1)
Os(1)-P(2)
2.437(4)
2.416(4)
2.428(4)
Os(1)-P(3)
Os(1)-P(4)
2.412(4)
2.416(4)
Os(1)-P(1)
Os(1)-P(2)
2.379(4)
2.356(4)
Os(1)-P(3)
Os(1)-P(4)
2.375(4)
2.358(4)
P(1)-Os(1)-P(2)
P(1)-Os(1)-P(3)
P(2)-Os(1)-P(3)
87.6(1)
136.6(1)
97.3(1)
P(1)-Os(1)-P(4)
P(2)-Os(1)-P(4)
P(3)-Os(1)-P(4)
95.6(1)
169.1(1)
87.6(1)
Cl(1)-Os(1)-P(1)
Cl(1)-Os(1)-P(2)
P(1)-Os(1)-P(2)
Cl(1)-Os(1)-P(3)
P(1)-Os(1)-P(3)
84.1(1)
93.4(1)
86.8(1)
84.5(1)
168.4(1)
P(2)-Os(1)-P(3)
Cl(1)-Os(1)-P(4)
P(1)-Os(1)-P(4)
P(2)-Os(1)-P(4)
P(3)-Os(1)-P(4)
96.1(1)
97.7(1)
93.4(1)
168.9(1)
85.9(1)
2.5 (br, 8 H, PCH2), 1.7 (br, 4 H, PCH2CH2), -6.4 (br quintet, 3 H,
OsH3, J(P,H) ) 8.4 Hz). 1H NMR (CD2Cl2, 200 MHz, 193 K): δ
-6.10 (BB′ of BB′XX′YY′, OsHB2, 2 H, J(P,H) ) 13.6, J(P,H) +
J(P′,H) ) 12.8 Hz), -7.41 (A of AXX′YY′, tt, OsHA, 1 H, OsH, J(P,H)
) 32.0, J(P′,H) ) 7.0 Hz). 31P{1H} NMR (CD2Cl2, 81 MHz, 293 K):
δ -12.9 (br).
Preparation of trans-[OsCl(η2-H2)(dppp)2][PF6] (2b). [OsCl-
(dppp)2][PF6] (1.0 g, 0.84 mmol) was dissolved in 30 mL of CH2Cl2
under hydrogen. The solution turned colorless within 2 h. Addition
i
of 40 mL of PrOH, followed by partial evaporation of the solution,
X-ray Structure Analysis of [OsH3(dppp)2][BPh4] (4b‚BPh4).
Colorless crystals of 4b‚BPh4 suitable for X-ray analysis were obtained
from CH2Cl2/iPrOH. Crystal data for C78H75BOsP4‚2CH2Cl2 (fw
1504.1): monoclinic, space group P21/n, with cell dimensions at 293
K of a ) 13.392(3) Å, b ) 25.306(7) Å, c ) 21.247(7) Å, â ) 91.15-
(2)°, and V ) 7199(3) Å3 with Z ) 4 and Dcalcd ) 1.388 Mg/m3, µ
(Mo KR) ) 20.52 cm-1 (graphite monochromated), λ ) 0.710 73 Å,
and F(000) ) 3060. The data were collected on a Syntex P21
diffractometer using the ω scan mode (2θ range ) 3.0-40.0°). Data
collection was as described above for 2b. No absorption correction
was applied. The structure was solved using Patterson methods. Of
the 3691 independent reflections with the index ranges 0 e h e 12,
-24 e k e 24, and -20 e l e 0, 2638 with F > 3.0 σ(F) were used
in the refinement. A total of 543 parameters were refined by full-
matrix least-squares using SHELXTL PLUS (data-to-parameter ratio
4.9:1, quantity minimized ∑w(Fo - Fc)2). All phenyl rings were refined
as rigid groups. For non-H atoms, anisotropic displacement parameters
were applied, except for [BPh4]- which was refined isotropically. The
contribution of the H atoms in idealized positions (riding model with
fixed isotropic U ) 0.080 Å2) was taken into account but not refined.
Two independent CH2Cl2 molecules were located in the final Fourier
map. Final residuals (observed data) were R ) 0.034 and RW ) 0.040,
GOF ) 0.69 (weighting scheme w-1 ) σ2(F) + 0.0019 F2) with
yielded a white product, which was filtered off and vacuum dried. The
1H NMR spectrum indicates that the microcrystalline product contains
1 molecule of iPrOH per mole of complex. Yield: 0.89 g, 84%. Anal.
Calcd for C57H62ClF6OOsP5: C, 54.44; H, 4.97. Found: C, 54.04; H,
5.01. 1H NMR (CD2Cl2, 200 MHz, 293 K): δ 7.6-6.9 (m, PC6H5),
3.1 (br, 4 H, PCH2), 2.4 (br, 4 H, PCH2), 1.6 (br, 4 H, PCH2CH2),
-10.0 (br, 2 H, OsH2). 31P{1H} NMR (CD2Cl2, 81 MHz, 293 K): δ
-30.0 (br), -143.9 (sept, J(P,F) ) 711 Hz).
X-ray Structure Analysis of [OsCl(η2-H2)(dppp)2][PF6] (2b).
Colorless crystals of 2b suitable for X-ray analysis were obtained from
CH2Cl2/iPrOH. Crystal data for C54H54ClF6OsP5‚2CH2Cl2 (fw )
1365.3): monoclinic, space group P21/n, cell dimensions at 293 K are
a ) 13.314(7) Å, b ) 18.63(2) Å, c ) 23.20(2) Å, â ) 94.58(6)°, and
V ) 5735(9) Å3 with Z ) 4 and Dcalcd ) 1.581 Mg/m3, µ (Mo KR) )
26.52 cm-1 (graphite monochromated), λ ) 0.710 73 Å, and F(000)
) 2728. The data were collected on a Syntex P21 diffractometer using
the ω scan mode (2θ range ) 3.0-40.0°) with variable scan speeds
(1.0-4.0°/min in ω) to ensure constant statistical precision on the
collected intensities. No absorption correction was applied. The
structure was solved using Patterson methods. Of the 4965 independent
reflections with the index ranges -12 e h e 8, 0 e k e 17, and 0 e
l e 22, 3982 with F > 4.0 σ(F) were used in the refinement. A total
of 653 parameters were refined by full-matrix least-squares using
SHELXTL PLUS (data-to-parameter ratio 6.1:1, quantity minimized
∑w(Fo - Fc)2), with anisotropic displacement parameters for all non-H
atoms. The contribution of the H atoms in idealized positions (riding
model with fixed isotropic U ) 0.080 Å2) was taken into account but
not refined. Two independent CH2Cl2 molecules were located in the
final Fourier map. Final residuals (observed data) were R ) 0.053
and RW ) 0.070, goodness-of-fit (GOF) ) 0.98 (weighting scheme
w-1 ) σ2(F) + 0.0050 F2). Maximum and minimum difference peaks
were 2.29 and -1.26 eÅ-3, respectively, largest and mean ∆/σ ) 0.306
and 0.028. Tables of atomic coordinates, anisotropic displacement
coefficients, and an extended list of interatomic distances and angles
are available as Supporting Information. Selected bond distances and
angles are reported in Table 3.
maximum and minimum difference peaks of 0.38 and -0.37 eÅ-3
,
respectively, largest and mean ∆/σ ) 0.715 and 0.055. Tables of atomic
coordinates, anisotropic displacement coefficients, and extended lists
of interatomic distances and angles are available as Supporting
Information. Selected bond distances and angles are reported in Table
4.
Preparation of cis-[OsH2(dppp)2] (5b). [OsCl(dppp)2][PF6] (0.41
g, 0.34 mmol) and EtONa (47 mg, 0.68 mmol) were dissolved in 20
mL of CH2Cl2 and stirred overnight at room temperature under H2 (1
atm). Addition of 20 mL of hexane and concentration of the solution
gave a white precipitate. Yield: 0.26 g, 75%. Recrystallization was
from CH2Cl2/hexane. Anal. Calcd for C54H54OsP4: C, 63.77; H, 5.35.
Found: C, 63.25; H, 5.36. IR (Nujol), cm-1: ν(OsH2) 1986, 1957 (s).
1H NMR (CD2Cl2, 200 MHz, 293 K): δ 8.2-6.6 (m, PC6H5), 2.9-2.3
(br, 8 H, PCH2), 2.0 (br, 4 H, PCH2CH2), -9.24 (m, 2 H, OsH2).
31P{1H} NMR (CD2Cl2, 81 MHz, 293 K): δ -9.4 (AA′ of AA′BB′),
-10.4 (BB′ of AA′BB′, J(A,B) ) 18.6, J(A,B′) ) 18.4, J(B,B′) )
21.7, J(A,A′) (trans) not refined).
Preparation of [OsH3(dppp)2][BF4] (4b‚BF4). A C6H6 solution
(30 mL) of cis-[OsH2(dppp)2] (0.40 g, 0.39 mmol) was treated with
85% HBF4‚Et2O (70 µL, 0.40 mmol). A white precipitate was formed,
which was filtered off and vacuum dried. Yield: 350 mg, 81%. Anal.
Calcd for C54H55BF4OsP4: C, 58.70; H, 5.02. Found: C, 57.93; H,
4.96. Spectroscopic data were as given above.
Preparation of [RuH(CO)(dppp)2][BPh4] (6a). Na[BPh4] (62 mg,
0.18 mmol) in 5 mL of EtOH was added to [RuH(Cl)(dppp)2] (0.17 g,
0.18 mmol) in 20 mL of CH2Cl2. After the solution was stirred under
CO (1 atm) at room temperature for 1 h, 20 mL of iPrOH were added.
Concentration of the solution yielded a white precipitate, which was
filtered off, washed with iPrOH, and vacuum dried. Recrystallization
was from CH2Cl2/iPrOH. Yield: 184 mg, 80%. Anal. Calcd for
C79H73BOP4Ru: C, 74.47; H, 5.77. Found: C, 74.10; H, 5.73. IR
(CH2Cl2), cm-1: ν(CO) 1978 (s), ν(RuH) 2057 (s). 1H NMR (CD2-
Cl2, 200 MHz, 293 K): δ 7.8-6.7 (m, PC6H5), 2.3 (br, 4 H, PCH2),
Preparation of [OsH(Cl)(dppp)2] (3b). trans-[OsCl(η2-H2)(dppp)2]-
[PF6] (0.80 g, 0.67 mmol) and EtONa (46 mg, 0.67 mmol) were
suspended in 20 mL of acetone. After the solution was stirred at room
temperature for 2 h, the pale yellow product was filtered off, washed
with acetone, and vacuum dried. Recrystallization was from CH2Cl2/
hexane. Yield: 0.64 g, 90%. Anal. Calcd for C54H53ClOsP4: C, 61.68;
H, 5.08. Found: C, 60.87; H, 5.01. IR (Nujol), cm-1: ν(OsH) 2117
(s). 1H NMR (CD2Cl2, 200 MHz, 293 K): δ 7.5-6.8 (m, PC6H5), 2.9
(br, 4 H, PCH2), 2.4 (br, 4 H, PCH2), 1.5 (br, 4 H, PCH2CH2), -20.0
(quintet, 1 H, OsH, J(P,H) ) 15.5 Hz). 1H NMR (CD2Cl2, 200 MHz,
173 K): δ -19.7 (tt, 1 H, OsH, J(P,H) ) 17.8, J(P′,H) ) 8.9 Hz).
31P{1H} NMR (CD2Cl2, 81 MHz, 293 K): δ -21.9 (br).
Preparation of [OsH3(dppp)2][BPh4] (4b‚BPh4). Na[BPh4] (146
mg, 0.43 mmol) in 5 mL of EtOH was added to [OsH(Cl)(dppp)2] (0.30
g, 0.29 mmol) in 20 mL of CH2Cl2. After the solution was stirred
under H2 (1 atm) at room temperature for 1 h, 5 mL of EtOH were
added. Concentration of the solution yielded a white precipitate, which
was filtered off, washed with EtOH, and vacuum dried. Recrystalli-
zation was from CH2Cl2/iPrOH. Anal. Calcd for C78H75BOsP4: C,
70.05; H, 5.65. Found: C, 69.46; H, 5.61. IR (Nujol), cm-1: ν(OsH3)
1994 (s). 1H NMR (CD2Cl2, 200 MHz, 293 K): δ 7.4-6.8 (m, PC6H5),

714 Inorganic Chemistry, Vol. 36, No. 4, 1997
Table 5. Estimated pKa Valuesa
Rocchini et al.
6b. Complex 6b (20 mg, 15 µmol) was dissolved in 0.5 mL of
CD2Cl2, and CF3SO3H (14-24 µmol) was added via microsyringe. Keq
values were obtained from the concentrations of 6b and 7b, measured
as described above, assuming [CF3SO3-] ) [7b] and [CF3SO3H] )
[CF3SO3H]o - [7b], where [CF3SO3H]o is the total concentration of
acid added.
Keq
pKa
4.4
12.5
10.2
10.3
base
PPh3
NEt3
NEt3
NEt3
2a
2b
4a
4b
7a
7b
0.020(0.006
0.022 ( 0.008
4.6 ( 0.8
3.5 ( 1.2
b
-
-
ca. -6
-5.7
CF3SO3
CF3SO3
Results
5.8 ( 1.7
a On the pseudo-aqueous scale, see eqs 4 and 5. Keq values reported
H2 Addition onto [MCl(dppp)2]+. The reaction of the five-
coordinate species [MCl(dppp)2]+ (M ) Ru (1a), Os (1b)) with
with standard errors. b Not determined.
1
H2 (eq 2) was monitored by H and 31P NMR spectroscopy.
2.0 (br, 4 H, PCH2), 1.6 (br, 4 H, PCH2CH2), -5.1 (quintet, 1 H, RuH,
J(P,H) ) 20.4 Hz. 1H NMR (CD2Cl2, 200 MHz, 173 K): δ -5.2 (tt,
1 H, RuH, J(P,H) ) 26.8, J(P′,H) ) 13.5 Hz). 31P{1H} NMR (CD2-
Cl2, 81 MHz, 293 K): δ 18.1 (br s).
The dihydrogen complex trans-[RuCl(η2-H2)(dppp)2]+ (2a) is
slowly formed when a CD2Cl2 solution of 1a (0.090 mol L-1
)
is saturated with H2 (1 atm, 23 °C). The 1H NMR spectrum of
the reaction solution shows a broad signal for the dihydrogen
ligand at δ -10.6 and a sharp signal at δ 4.6 for free H2, while
a broad resonance at δ 6.6 appears and gains in intensity in the
31P NMR spectrum.
Preparation of [OsH(CO)(dppp)2][BPh4] (6b). Na[BPh4] (65 mg,
0.19 mmol) and [OsH(Cl)(dppp)2] (0.20 g, 0.19 mmol) were treated as
given for 6a. The crude reaction product refluxed in CH3OH for 8 h
gave the pure trans isomer. Yield: 200 mg, 77%. Anal. Calcd for
C79H73BOOsP4: C, 69.60; H, 5.40. Found: C, 69.12; H, 5.37. IR
(CH2Cl2), cm-1: ν(CO) 1972 (s), ν(RuH) 2055 (s). 1H NMR (CD2-
Cl2, 200 MHz, 293 K): δ 7.9-6.7 (m, PC6H5), 2.6 (br, 4 H, PCH2),
2.2 (br, 4 H, PCH2), 1.5 (br, 4 H, PCH2CH2), -6.1 (quintet, 1 H, OsH,
J(P,H) ) 19.2 Hz). 1H NMR (CD2Cl2, 200 MHz, 173 K): δ -6.3 (tt,
1 H, OsH, J(P,H) ) 24.5, J(P′,H) ) 12.5 Hz). 31P{1H} NMR (CD2-
Cl2, 81 MHz, 293 K): δ -21.8 (br s).
Observation of [Ru(CO)(η2-H2)(dppp)2]2+ (7a). trans-[RuH(CO)-
(dppp)2][BPh4] (20 mg, 16 µmol) was dissolved in 0.5 mL of CD2Cl2
under argon in an NMR tube; the solution was cooled to -40 °C, and
CF3SO3H (20 µL, 0.23 mmol) was added thereto by means of a syringe.
1H NMR (CD2Cl2, 200 MHz, 243 K): δ 9.1-6.5 (m, PC6H5), 2.5 (br,
8 H, PCH2), 1.5 (br, 4 H, PCH2CH2), -2.5 (br, 2 H, RuH2). 31P{1H}
NMR (CD2Cl2, 81 MHz, 243 K): δ 7.6 (br s).
The concentrations of 1a and 2a, obtained by integration of
the inverse-gated decoupled 31P NMR spectra, are constant after
48 h and give a 1a:2a molar ratio of 82:18. Besides 1a and
2a, a small amount of [RuH(η2-H2)(dppp)2]+ (4a, ca. 1% of
starting 1a) is present during the reaction. Integration of the
signals of bound and free dihydrogen in the 1H NMR spectrum
at equilibrium gives a value of 3.3 mmol L-1 for the concentra-
tion of free H2 (corrected for 25% NMR-silent parahydrogen),
yielding K ) 0.72 × 102 mol-1 L (at 23 °C). When the
experiment is repeated with an initial H2 pressure p(H2) of 2
atm, 37% of 2a is formed after reaching equilibrium (36 h).
Observation of [Os(CO)(η2-H2)(dppp)2]2+ (7b). trans-[OsH(CO)-
(dppp)2][BPh4] (20 mg, 15 µmol) was dissolved in 0.5 mL of CD2Cl2
under argon in an NMR tube, and CF3SO3H (20 µL, 0.23 mmol) was
added thereto by means of a syringe. 1H NMR (CD2Cl2, 200 MHz,
293 K): δ 8.0-6.6 (m, PC6H5), 2.8 (br, 8 H, PCH2), 2.0 (br, 4 H,
PCH2CH2), -3.0 (br, 2 H, OsH2). 31P{1H} NMR (CD2Cl2, 81 MHz,
293 K): δ -29.7 (br s).
Acidity Measurements. The equilibrium constant Keq of reaction
(4) was measured by 31P and 1H NMR in CD2Cl2 for all η2-H2
complexes, with the exception of 7a. The solvent was dried over
molecular sieves and saturated with argon or H2 before use. The
concentrations of [M(H)X(dppp)2] and [MX(η2-H2)(dppp)2]+ were
obtained by integration of the inverse-gated decoupled 31P spectra
recorded at 293 K and checked by integration of the 1H NMR spectra
(hydride part). At least three measurements were performed for each
complex. Details of the calculation of the equilibrium constants Keq
are given below. The Keq values are reported in Table 5, together with
their standard errors and with pKa values that were estimated assuming
pKa (NH3+) ) 10.8,2a pKa (Ph3PH+) ) 2.7,15a and pKa (CF3SO3H) )
-4.9, obtained by converting the literature value of 2.6 that is found
in acetonitrile15b onto the aqueous scale by means of pKa (H2O) ) pKa
(MeCN) - 7.5.2a
2a. Complex 3a (30 mg, 31 µmol) and variable amounts of Ph3-
PH+ (13-31 µmol) were dissolved in 0.5 mL of CD2Cl2. Integration
of the signals of 2a, 3a, Ph3P, and Ph3PH+ in the inverse-gated
decoupled 31P NMR spectrum of the solution gave Keq.
2b. Complex 2b (20 mg, 17 µmol) was dissolved in 0.5 mL of
CD2Cl2 and variable amounts of Et3N (22-66 µmol) were added via
microsyringe. Integration of the inverse-gated decoupled 31P NMR
spectrum of the solution gave the concentrations of 2b and 3b, from
which Keq values were calculated assuming [Et3NH+] ) [3b] and [Et3N]
) [Et3N]o - [3b], where [Et3N]o is the total concentration of base added.
4a,b. Complexes 4a and 4b (24 µmol) were dissolved in 0.5 mL of
CD2Cl2, and Et3N (7-22 µmol) was added via microsyringe. Keq values
were obtained as described for 2b.
The equilibrium concentration of dihydrogen is 8.1 mmol L-1
,
yielding K ) 0.75 × 102 mol-1 L (at 23 °C), in good agreement
with the value obtained for p(H2) ) 1 atm.
As the relative intensity of free H2 in solution increases during
the time required for reaching the equilibrium position, while
no change is observed thereafter, the transfer of dihydrogen from
the gas phase to the solution must occur at a rate comparable
to that of the reaction of 1 f 2. Thus, no meaningful kinetic
information can be derived from these data. However, pre-
saturation experiments indicate that the exchange between free
and coordinated H2 is slow compared to their longitudinal
relaxation processes, as no magnetization transfer is observed
at 22 °C in CD2Cl2 by irradiating at the resonance of either
free or coordinated H2.
Complex 1a does not react in the solid state under 300 atm
of H2 at room temperature during 48 h. However, 2a is
quantitatively formed when a CD2Cl2 solution of 1a (0.090 mol
L-1) is kept under 300 atm of H2 for 24 h. When the H2
pressure is released to 1 atm, 2a slowly dissociates H2 at 22
°C, reaching the same equilibrium position that is obtained when
starting from 1a and saturating the CD2Cl2 solution with H2.
Dihydrogen dissociation is inhibited at low temperature: 2a can
be stored for days at -78 °C without changes. Even at 0 °C,
2a does not decompose significantly over the time needed for
the NMR measurements. Thus, 2a was fully characterized in
1
solution by means of variable-temperature H and 31P NMR
(15) (a) Allman, T.; Goel, R. G. Can. J. Chem. 1982, 60, 716. (b) Fujinaga,
T.; Sakamoto, I. J. Electroanal. Chem. 1977, 85, 185.
spectroscopy (Tables 1 and 2).

Heterocyclic H2 Activation by Dihydrogen Complexes
Inorganic Chemistry, Vol. 36, No. 4, 1997 715
CH2Cl2 molecules as the solvent of crystallization. The H atoms
of the dihydrogen ligand were not located. However, assuming
that dihydrogen occupies the sixth coordination position trans
to Cl, the coordination of the complex cation can be described
as distorted octahedral, with pseudo-C2 symmetry about the Os-
Cl vector (Figure 1). Both trans P-Os-P angles are closed
down to values about 168°. The arrangement of the four P
atoms is distorted in a tetrahedral fashion from the ideal
coplanarity, with P(2) and P(4) pushed away from Cl and P(1)
and P(3) bent toward it. The Os atom lies in the P(1)-P(2)-
P(3)-P(4) least-squares plane (-0.005 Å), while P(1), P(2),
P(3), and P(4) are symmetrically displaced by 0.237, -0.231,
0.233, and -0.240 Å, respectively. The Os-Cl vector ap-
proximately bisects the P(1)-Os(1)-P(3) and P(2)-Os(1)-
P(4) angles and forms an angle of 2° with the normal to the
P(1)-P(2)-P(3)-P(4) least-squares plane. Both chelate rings
adopt a twist conformation.
The Os-Cl distance (2.437(4) Å) lies at the upper extremity
of the range found for six-coordinate Os(II) complexes,16
suggesting that the (η2-H2) ligand exerts a relatively high trans
influence (see below). The Os-P bond lengths are scattered in
a narrow range (Table 3), with an average value of 2.418 Å.
Four phenyl rings form a pocket around the Cl atom, with short
separations (3.30-3.39 Å) between Cl and the ortho phenyl
carbons C(18), C(20), C(32), and C(44), while the nonbonded
distances between the phenyl rings are normal.
Figure 1. ORTEP view of the [OsCl(η2-H2)(dppp)2]+ cation 2b.
1
The H NMR spectra recorded over the range from -90 to
0 °C show that the dihydrogen signal at δ -10.6 broadens as
the temperature is lowered. The T1 of this signal reaches a
minimum value of 8.2 ms at -50 °C (200 MHz). The deuterio
derivative [RuCl(η2-HD)(dppp)2]+ (2a-d1) was prepared by
reaction of [RuH(Cl)(dppp)2]14 (3a) with the stoichiometric
amount of D2SO4 in CD2Cl2 at 0 °C. The low-frequency part
of the 1H NMR spectrum (CD2Cl2, 0 °C) of 2a-d1 consists of a
multiplet showing well-resolved couplings both to the D and P
nuclei that can be simulated with a 1J(D,H) ) 24.4 Hz (2J(P,H)
) 7.5 Hz).
H2 Addition to [OsH(dppp)2]+. The five-coordinate hydride
[RuH(dppp)2]+ is known to react with H2 (1 atm) to give [RuH-
(η2-H2)(dppp)2]+ (4a).12 A H-H distance of about 0.72 Å has
been estimated on the basis of the T1(min) of 6 ms (400 MHz,
1
-30 °C) and the J(D,H) of 32 Hz.12b We find a slightly
different T1(min) of 5.5 ms (CD2Cl2, 200 MHz, 213 K),
corresponding to d(H-H) ) 0.80 Å (rapidly spinning), while
the literature J(D,H) value of 32 Hz gives d(H-H) ) 0.89 Å.9e
Thus, we attempted the preparation of the osmium analog
[OsH(dppp)2]+ in order to investigate its reactivity with
hydrogen. The six-coordinate [OsH(Cl)(dppp)2] (3b) was
obtained by deprotonation of 2b with EtONa (see below).
Although the CH2Cl2 solutions of 3b turn dark red in the
presence of solid Na[BPh4] as a chloride scavenger, suggesting
that a five-coordinate species is actually formed, attempts to
isolate pure [OsH(dppp)2]+ were unsuccessful. However, when
the same reaction was repeated under H2 gas (1 atm), the
solution remained colorless and a white complex of composition
[OsH3(dppp)2][BPh4] (4b) was isolated. The same complex
cation was obtained by protonation of [OsH2(dppp)2] (5b) with
HBF4‚Et2O in CH2Cl2 and was isolated as its [BF4]- salt [OsH3-
(dppp)2][BF4].
The osmium analog [OsCl(dppp)2]+ (1b) reacts quantitatively
under H2 (1 atm, 22 °C, CH2Cl2 solution) within 2 h to give
trans-[OsCl(η2-H2)(dppp)2]+ (2b), which was isolated as its
[PF6]- salt. Replacing dihydrogen with argon reverses the
reaction, and 1b is the only species observed by 31P NMR after
24 h. However, 2b is stable toward loss of H2 in the solid state
for several days at room temperature, even under vacuum. The
1H NMR signal of the dihydrogen ligand is a broad quintet
centered at δ -9.97 [2J(P,H) ) 11 Hz], which broadens further
at low temperature. The T1 reaches a minimum value of 23.7
ms at -60 °C (200 MHz). The deuterio derivative [OsCl(η2-
HD)(dppp)2]+ (2b-d1) was prepared in various degrees of
deuteration either by reacting [OsHCl(dppp)] (3b) with sto-
ichiometric amounts of F3CSO3D or by reacting 1b with HD
1
gas, prepared by reacting LiAlH4 with D2O. A low J(D,H)
value of 11 Hz was obtained by direct observation of the 1:1:1
triplet in the partially relaxed, 31P-decoupled 1H NMR spectrum.
An identical value was obtained by simulation of the 31P-coupled
X-ray Structure of [OsH3(dppp)2][BPh4]. An ORTEP view
of the [OsH3(dppp)2]+ cation is shown in Figure 2, and selected
bond distances and angles are reported in Table 4. The crystal
contains discrete [OsH3(dppp)2]+ cations and [BPh4]- anions,
with normal nonbonded interactions, together with two CH2-
Cl2 molecules as the solvent of crystallization. The positions
of the P atoms in the complex cation are in good agreement
with a pentagonal bipyramidal structure in which the P(2), P(4)
pair approximately occupies the apical positions. The P(1)-
P(3) vector is approximately perpendicular to the P(2)-P(4)
one (85.2°). It appears reasonable to assume that the equatorial
plane is occupied by P(1) and P(3), together with the three
undetected hydride ligands. The value of 136.6° found for the
P(1)-Os-P(3) angle is not far from the ideal value (144°), as
well as the trans P(2)-Os-P(4) angle of 169.1° from 180°.
2
spectrum of 2b-d1, using J(P,H) ) 11 Hz.
The room temperature 31P NMR spectra of both 2a and 2b
in CD2Cl2 exhibit a broad signal at δ 6.6 and -30.0, respec-
tively, which splits up and resolves into a multiplet, consistent
with an AA′XX′ system, upon lowering the temperature (Table
1). This indicates that the symmetry of these (formally)
octahedral complexes is lowered by some degree of distortion
(see below). Interchange of the nonequivalent P atoms is
relatively fast at room temperature on the NMR time scale, as
indicated by the single, broad signal, while at low temperature
the process is frozen out.
X-ray Structure of [OsCl(η2-H2)(dppp)2][PF6]. An ORTEP
view of the [OsCl(η2-H2)(dppp)2]+ cation is shown in Figure
1, and selected bond distances and angles are reported in Table
3. The crystal contains discrete [OsCl(η2-H2)(dppp)2]+ and
[PF6]-, with normal nonbonded interactions, together with two
(16) Orpen, A. G.; Brammer, L.; Allen, F. H.; Kennard, O.; Watson, D.
G.; Taylor, R. J. Chem. Soc., Dalton Trans. 1989, S1.

716 Inorganic Chemistry, Vol. 36, No. 4, 1997
Rocchini et al.
Figure 2. ORTEP view of the [OsH3(dppp)2]+ cation 4b.
The Os-P distances are significantly shorter in 4b than those
in 2b, which probably reflects both the different formal oxidation
states and the steric crowding in the two complexes. The axial
Os-P distances in 4b are significantly longer (ca. 5 × σ) than
the equatorial ones, as observed in [OsH(η2-O2)(dcpe)2]+ (dcpe
) 1,2-bis(dicyclohexylphosphino)ethane), which can also be
described as a pentagonal bipyramid.17 The dppp bite angles
P(1)-Os-P(2) and P(3)-Os-P(4) have the same value (87.6°)
and are similar to those found in 2b (86.8 and 85.9°). Both
chelate rings adopt a fairly regular chair conformation. As
already observed for 2b, the overall structural features of 4b,
such as the conformations of the chelate and phenyl rings,
exhibit a pseudo-C2 symmetry. The nonbonded distances
between the phenyl rings are normal.
31P and 1H NMR Spectra of 4b. The 31P CP-MAS spectrum
of 4b (Figure 3a) as well as the two-dimensional spin diffusion
(Figure 3b) and two-dimensional 31P CP-MAS COSY (Figure
3c) experiments indicate that the observed spectrum is fully
consistent with the mentioned crystal structure and allowed the
apical and equatorial phosphorus sites within the complex to
be assigned.
The low-temperature solution 31P NMR spectrum (AA′XX′
spin system) indicates that the complex has C2 symmetry. The
agreement between the phosphorus chemical shifts and the
isotropic shifts observed in the solid suggest that the overall
structural features are maintained in the solution phase. Increas-
ing the measuring temperature leads to the broadening of the
resonances, and eventually coalescence to a single resonance
is observed.
The hydride part of the 1H NMR spectrum at 20 °C consists
of a quintet, representing three protons, centered at δ ) -6.35,
which resolves into two multiplets with a relative intensity of
2:1 as the temperature is lowered. Selective heteronuclear Pa-
and Pe-decoupling experiments indicate that these two hydrides
are not mutually exchanging at low temperature and that they
are in a planar arrangement with the two equatorial phosphorus
ligands. The T1 values of HA and HB, investigated in the
temperature interval from 20 to -90 °C, reach a minimum
before line shape decoalescence occurs. As 1/T1 values still
average at temperatures well below this point,2a individual T1-
(min) values for HA and HB could not be determined. However,
the observed T1(min) of 72 ms (200 MHz, -30 °C) suggests
Figure 3. 31P Solid-state NMR spectroscopy for [OsH3(dppp)2][BPh4]:
(a) isotropic region of the conventional CP-MAS spectrum (162 MHz,
νrot ) 10 kHz, τcontact ) 2 ms). The four lines around δ ≈ -27 represent
2
a resolved AB spin system with J(31P,31P) ) 190(5) Hz. (b) 31P spin
diffusion experiment (EXSY) recorded under the same conditions as
for a with a mixing time τmix ) 500 ms. (c) 31P COSY-45 experiment
showing mutual J correlations among the apical and the equatorial spins,
respectively.
that the HB-HB′ distance is ca. 1.6 Å, calculated for a dipolar
relaxation process involving only the two HB spins without
corrections for other contributions. Accordingly, no H-D
coupling is found either in 4b-d1, prepared by the reaction of
5b with F3CCO2D, or in 4b-d2, obtained from 3b by reaction
with D2 in a CD2Cl2 solution in the presence of Na[BPh4].
The positions of the three hydride ligands were determined
by a combination of 31P-1H and 13C-1H correlations and
analysis of the vicinal coupling constants and close contacts as
obtained from rotating-frame Overhauser enhancement spec-
troscopy (ROESY, Figure 4). The hydride HA and the two
isochronous hydrides HB are located on opposite sides of
osmium, as no ROE is observed between HA and HB and there
are close contacts between HB and the phenyl groups Ea, Aa,
and Ae. These facts, together with the nature of the spin systems
involved, indicate that all three hydride ligands are located
within or close to the equatorial plane of a pentagonal bipyramid
(Figure 5).
[M(CO)(η2-H2)(dppp)2]2+ (7a,b). The hydrido carbonyls
[MH(CO)(dppp)2]+ (M ) Ru (6a), Os (6b)) were prepared by
(17) Mezzetti, A.; Zangrando, E.; Del Zotto, A.; Rigo, P. J. Chem. Soc.,
Chem. Commun. 1994, 1597.

Heterocyclic H2 Activation by Dihydrogen Complexes
Inorganic Chemistry, Vol. 36, No. 4, 1997 717
Figure 5. Model showing the locations of the HA (top) and HB (bottom)
within the pentagonal bipyramidal coordination sphere of Os in [OsH3-
(dppp)2]+ (4b). Hydrides and protons which are spatially close to these
are shown in a shaded fashion. For clarity, one of the dppp ligands is
contoured.
Figure 4. Section of the two-dimensional ROESY spectrum (500 MHz,
193 K, τmix ) 120 ms) for [OsH3(dppp)2]+ (4b) used for locating the
two types of hydride ligands.
reaction of 3a,b with carbon monoxide in a CH2Cl2 solution
and were precipitated as their [BPh4]- salts by addition of
ethanolic Na[BPh4]. The osmium analog forms mixtures of cis-
and trans-(6b) under these conditions, as already observed for
CO addition onto analogous Os(II) five-coordinate species.8e
Quantitative conversion to trans-(6b) is achieved by refluxing
the isomer mixture in MeOH for 8 h.
The ruthenium hydrido carbonyl 6a does not react with
HBF4‚Et2O (pKa ) -2.4 on the aqueous scale)18 in anhydrous
CD2Cl2, even in the presence of a 5-fold excess of acid.
However, 6a reacts immediately with excess CF3SO3H at room
temperature. Gas evolution is observed from the solution,
apparently arising from the decomposition of an unstable
dihydrogen complex (eq 3).
between -40 and -60 °C. The T1 values reach a minimum of
5.0 ms at -50 °C, and a J(D,H) value of 34.2 Hz is found for
trans-[Ru(CO)(η2-DH)(dppp)2]2+, obtained by protonation of
6a with excess CF3SO3D (CD2Cl2 at -80 °C). The single broad
signal at δ 7.6 (br s) that is observed in the 31P{1H} NMR
spectrum of 6a at -30 °C resolves into an AA′XX′ system at
low temperature (Table 1).
The osmium derivative 6b does not react with either
HBF4‚Et2O (1:1 or 1:3 molar ratio) or a stoichiometric amount
of CF3SO3H in CD2Cl2 solution. However, [Os(CO)(η2-H2)-
(dppp)2]2+ (7b) is quantitatively formed in the presence of excess
triflic acid. Complex 7b is stable at room temperature under
both Ar and H2 but decomposes upon attempts to isolate it in
the solid state. The (η2-H2) NMR signal is a broad, featureless
hump centered at δ -3.0 (CD2Cl2, 200 MHz, 20 °C) with a
minimum T1 value of 5.5 ms at -40 °C. Protonation of 6b
with 12 equiv of F3CSO3D in CD2Cl2 gives a 1J(D,H) value of
32.0 Hz. The room temperature 31P{1H} NMR spectrum of
7b shows a single broad signal at δ -29.7, which resolves into
an AA′XX′ system at low temperature.
Acidity of [MX(η2-H2)(dppp)2]+. The pKa values of the
dihydrogen complexes in CD2Cl2 were estimated by 1H and 31
P
NMR by studying the proton transfer reaction between the
dihydrogen complex (or the corresponding hydride) and a
suitable secondary internal standard (eq 4):
The 31P NMR spectrum indicates that 6a is converted into a
series of products: among the unidentified species, trans-[RuCl-
(CO)(dppp)2]+ was observed. However, the dicationic dihy-
drogen complex [Ru(CO)(η2-H2)(dppp)2]2+ (7a) was trapped by
protonation of 6a with excess CF3SO3H at -40 °C in CD2Cl2
[MX(η2-H2)(dppp)2]n+ + B h
[M(H)X(dppp)2](n-1)+ + BH+ (4)
1
and was characterized in solution by low-temperature H and
31P NMR. Reproducible 1H NMR T1 data were collected
Investigation involving the stable dihydrogen complexes
indicates that these equilibria are established during the time
(18) Perdoncin, G.; Scorrano, G. J. Am. Chem. Soc. 1977, 99, 6983.

718 Inorganic Chemistry, Vol. 36, No. 4, 1997
Rocchini et al.
1
of mixing; no significant changes are observed by H and 31P
NMR after 24 h in the solutions. The aqueous pKa value of
the dihydrogen complex can be estimated by means of eq 5
from the equilibrium constant Keq of eq 4 and the pKa of the
base B, extrapolated to the aqueous scale, as suggested by
Kristja´nsdo´ttir and Norton:19
basicity arguments and the structural analogies, as both [OsH-
(η2-H2)(dppe)2]+ and [OsH(η2-H2)(depe)2]+ (depe ) 1,2-bis-
(diethylphosphino)ethane) are dihydrogen complexes.21
A
possible explanation may invoke the flexibility of the diphos-
phine backbone, accounted for by the larger bite angle of dppp,
which is close to the “ideal” value of 90° (average values are
89.0, 86.4, and 87.6° in 1,8c 2b, and 4b, respectively). This is
significantly larger than that for both dppe and depe in [OsH-
(η2-H2)(P-P)2]+, where the average values are 82.2 and 82.9°,
respectively.21 The flexibility of the diphosphine backbone
might allow a pair of trans P atoms to bend away from the
octahedral geometry toward a seven-coordinate trihydride
structure.22 Accordingly, the ligand 1,1-bis(diphenylphosphino)-
ferrocene (dppf) favors the formation of the classical trihydride
[RuH3(dppf)2]+, in which the dppf ligand adopts an average
bite angle of 106.4°.23 Calculations taking into account both
the steric and electronic energies in [RuH3(diop)2]+ have shown
that the next low-energy isomer (after the absolute minimum
relative to trans-[RuH(η2-H2)(diop)]+) is a classical trihydride
structurally related to 4b and [RuH3(dppf)2]+.24
pKa(η2-H2) ) pKeq + pKa(HB+)
(5)
The standard errors observed for Keq’s are small compared
to the uncertainty related to the conversion of the pKa values of
the secondary standards onto the aqueous scale ((1). In view
of the wide pKa range observed, bases different than phosphines
had to be used in most cases. Protonation studies of chloro-
hydride 3a with Ph3PH+ in CD2Cl2 indicates that the decom-
position of the dihydrogen species 2a is slow compared to the
acid-base reaction; detectable amounts of five-coordinate 1a
(3% of starting 3a) are formed after 5 min. In agreement with
the estimated pKa value of 4.4 (on the pseudo-aqueous scale),
3a is not protonated by stoichiometric amounts of Et3NH+. The
pKa values of 2b and 4a,b were obtained by deprotonation with
Et3N (Table 5). As expected on the basis of the measured pKa
value of 10.3, 4b is protonated quantitatively by 1 equiv of
[HPPh3]+ (pKa ) 2.7).
Although both of the hydrido carbonyl complexes 6a,b are
protonated by excess CF3SO3H, quantitative NMR measure-
ments for 7a were not performed due to its instability at room
temperature. However, the fact that 7a is formed quantitatively
only in the presence of excess CF3SO3H suggests a pKa value
of ca. -6 or lower. In view of the limited reliability of the
pKa value reported for CF3SO3H, the pKa value of -5.7 for 7b
should be regarded as a rough estimation.
Dihydrogen complexes are observed in the osmium series
when X is a weaker σ-donor than hydride, as the electron density
at the metal is lowered. Thus, a change from classical hydrides
to an elongated dihydrogen complex is observed on going from
hydride to Cl, as in [M(H)3(dppe)2] (M ) Tc,9c Re21a,25) Vs
[MCl(η2-H2)(dppe)2]+ (M ) Tc,9c Re26) and in [ReH3(PMePh2)4]
Vs [ReCl(η2-H2)(PMePh2)4]+.27 When X is a strong π-acceptor,
such as CO, the dn (M) orbitals are further depleted and “short”
dihydrogen ligands are observed for both metals (see below).
H-H Distances in the (η2-H2) Complexes. The T1 (min)
values of the dihydrogen ligands and the 1J(D,H) values in the
analogous deuterated complexes are summarized in Table 2.
Equation 6,9e recently developed from the correlation between
the J(D,H) and d(H-H) values observed earlier,2b gives values
in substantial agreement with those from the procedure of
Hamilton and Crabtree that was based on T1 (min) values28 and
avoids the choice between different models (“rapid spinning”
or “slow rotation”). The d(H-H) values in Table 2 span nearly
the whole range known. The d(H-H) value of 1.0 Å calculated
for 2a (J(D,H) ) 24 Hz) is near that of 1.10 Å found by neutron
diffraction for [Ru(H‚‚‚H)(C5Me5)(dppm)] (J(D,H) ) 21 Hz).29
For 2b, the J(D,H) and d(H-H) values of 11 Hz and 1.2 Å are
similar to those of trans-[Os(η1-O2CCH3)(η2-H2)(en)2]+ (J(D,H)
) 9 Hz, d(H-H) ) 1.34 Å by neutron diffraction), which lie
at the low extremity of the range found for dihydrogen
complexes.11c
Discussion
NMR Spectroscopy. The available solution NMR spectro-
scopic data, as well as structural evidence for 2b, suggest that
the dynamic process observed for 2, 3, 6, and 7 is related to the
distortions of their pseudo-trans-octahedral structure, with two
nonequivalent pairs of trans P atoms exchanging positions with
inversion of the chelate ring conformation. The slow-exchange
spectra observed at low temperatures (AA′XX′ spin systems,
Table 1) indicate that the overall symmetry of these molecules
is C2, in accordance with the X-ray data of 2b.
The pentagonal bipyramidal structure of 4b cannot be
discriminated from that of the six-coordinate species 2, 3, 6,
and 7 on the basis of the 31P NMR solution spectra, as all show
similar temperature dependence and AA′XX′ spin systems at
low temperature. However, the number of trans coupling
constants (g100 Hz) observed in the solid state 31P NMR
spectrum should be diagnostic of either structure. Unfortunately,
the mutually trans-oriented P atoms in 2b are isochronous,
which is probably due to the presence of a pseudo-C2 symmetry
even in the solid state.
d(H-H) ) -0.0167J(D,H) + 1.42
(6)
(21) (a) Earl, K. A.; Jia, G.; Maltby, P. A.; Morris, R. H. J. Am. Chem.
Soc. 1991, 113, 3027. (b) Farrar, D. H.; Maltby, P. A.; Morris, R. H.
Acta Cryst. (C), 1992, 48, 28. Earl, K. A.; (c) Morris, R. H.; Sawyer,
J. F. Acta Crystallogr., Sect. C 1989, 45, 1137.
(22) Van Der Sluys, L. S.; Eckert, J.; Eisenstein, O.; Hall, J. H.; Huffman,
J. C.; Jackson, S. A.; Koetzle, T. F.; Kubas, G. J.; Vergamini, P. J.;
Caulton, K. G. J. Am. Chem. Soc. 1990, 112, 4831.
(23) Saburi, M.; Aoyagi, K.; Kodama, T.; Takahashi, T.; Uchida, T. Chem.
Lett. 1990, 1909.
(24) Maseras, F.; Koga, N.; Morokuma, K. Organometallics 1994, 13, 4008.
(25) Albano, V. G.; Bellon, P. L. J. Organomet. Chem. 1972, 37, 151.
(26) Kohli, M.; Lewis, D. J.; Luck, R. L.; Silverton, J. V.; Sylla, K. Inorg.
Chem. 1994, 33, 879.
Dihydride Ws Dihydrogen Preference. The trend found on
going from Ru to Os in the series [MH(H2)(dppp)2]+, where
4a is a dihydrogen complex and 4b a classical trihydride,
parallels that found for [RuH(η2-H2)(dcpe)2]+ Vs [OsH3-
(dcpe)2]+ 9b and for [RuH(I)(η2-H2)(PCy3)2]7a Vs [Os(H)3Cl-
(PiPr3)2].20 However, the classical nature of the three hydrides
in 4b contradicts the expectations based on both the phosphine
(27) (a) Cotton, F. A.; Luck, R. L. Inorg. Chem. 1989, 28, 2181. (b) Ibid.
1991, 30, 767.
(19) Kristja´nsdo´ttir, S. S.; Norton, J. R. In Transition Metal Hydrides;
Dedieu, A., Ed.; VCH: Weinheim, FRG, 1992; Chapter 9, pp 324-
334.
(20) Gusev, D. G.; Kuhlmann, R.; Sini, G.; Eisenstein, O.; Caulton, K. G.
J. Am. Chem. Soc. 1994, 116, 2685.
(28) Hamilton, D. G.; Crabtree, R. H. J. Am. Chem. Soc. 1988, 110, 4126.
Bautista, M. T.; Earl, K. A.; Maltby, P. A.; Morris, R. H.; Schweitzer,
C. T.; Sella, A. J. Am. Chem. Soc. 1988, 110, 7031.
(29) Klooster, W. T.; Koetzle, T. F.; Jia, G.; Fong, T. P.; Morris, R. H.;
Albinati, A. J. Am. Chem. Soc. 1994, 116, 7677 and references therein.

Heterocyclic H2 Activation by Dihydrogen Complexes
Inorganic Chemistry, Vol. 36, No. 4, 1997 719
The effects of X on d(H-H) can be discussed qualitatively
in terms of the σ (η2-H2) f dn (M) and dn (M) f σ* (η2-H2)
bonding components. The high trans influence of hydride
weakens the former and, synergistically, also the latter com-
ponent. In the case of CO, the π-acid ligand not only exerts a
high trans influence, but also stabilizes the dn (M) orbitals and
further reduces their overlap with the σ* (η2-H2) orbital. Both
effects strengthen the H-H bonding and reduce the M(η2-H2)
interaction, giving rise to (η2-H2) complexes with short H-H
distances. Density functional studies of J(D,H) predict a short
d(H-H) in trans-[OsX(η2-H2)(NH3)4]n+ when X is a π-acid
(CN-), but the experimental value is not known.30
When X is a π-donor, the low trans influence of Cl- favors
the σ (η2-H2) f dn (M) donation, while the four-electron desta-
bilization between the filled dπ orbital of the metal and the filled
pπ orbital of the halide31 results in a higher energy of the dn
orbitals and increased dπ (M) f σ* (η2-H2) back-bonding. Both
effects weaken the H-H bond and increase the M(η2-H2)
interaction, and elongated dihydrogen complexes are formed.
Interestingly, in 2b, two trans P atoms are bent away from the
dihydrogen ligand, like in [Re(Cl)(η2-H2)(PMePh2)4].27b This
structural feature is known to enhance dπ (M) f σ* (η2-H2)
back-bonding.22,24 Independent evidence for dihydrogen being
a π-acceptor11c,32 comes from the long Os-Cl distance observed
in 2b.
(dppp)2]2+ fragment is a powerful Lewis acid that abstracts Cl
from CD2Cl2. In the case of Os, the dihydrogen complex B is
relatively stable, probably because the metal is more electron
rich than Ru and the high-energy 16-electron fragment A
resulting from H2 dissociation is not stabilized by dimerization
or agostic interactions.
The intermediate stability of the chloro derivatives 2a,b is
not obvious at first, as the elongation of the dihydrogen ligand
is indicative of a strong M(η2-H2) bond. However, the Keq data
available for 2a show that longer H-H bonds do not necessarily
go along with increased thermodynamic stability. This is
probably due to the stabilization of the 16-electron fragment
by the π-donor. Accordingly, the presence of one or more
π-donors in an octahedral, nonclassical polyhydride complex
of a d6 metal ion is known to induce dissociation of one
dihydrogen ligand by the combined effects of the four-electron
destabilization mentioned above31 and the stabilization of the
16-electron fragment by means of Cl f M π-donation.6 Thus,
if fragment A is a π-stabilized five-coordinate complex, the
position of equilibrium a in eq 1 depends on the effectiveness
of the π-donation from X to the metal. As π-stabilized
unsaturated compounds have been used as hydrogenation
catalyst precursors, the reactivity of such species toward
dihydrogen is a theme of particular interest.
Complex 1a is known to be a Y-shaped complex, which is
not stabilized by any agostic interaction; the shortest Ru-
C(phenyl) nonbonded contacts are 3.42 and 3.51 Å.8c The Ru-
Cl bond length is expected to be diagnostic of π-bonding
between the 16-electron fragment and the π-donor chloride.6a
Indeed, the Ru-Cl distance in 1a (2.371(5) Å) is significantly
shorter than the average of 2.435(16)Å found for 34 values in
17 six-coordinate complexes of the type cis- or trans-[RuCl2L4]
(L ) phosphine ligand), as retrieved from the Cambridge
Structural Database after discarding eight values of four
complexes of the type cis-[RuCl2(PR3)4] (R ) alkyl). The Ru-
Cl distance is shorter in 1a than in [RuCl(dcpe)2]+ and [RuCl-
(dppe)2]+ (2.386(3) and 2.395(2) Å, respectively). This suggests
that if it is accepted that the 16-electron species [RuCl(P-P)2]+
(P-P ) dppp, dcpe, dppe) are π-stabilized to some extent, then
this effect is largest in 1a. Accordingly, among the complexes
of the type [RuCl(P-P)2]+, only 1a does not react quantitatively
with H2 under ambient conditions.
Effect of X on the Stability of the (η2-H2) Complexes.
Cationic Ru and Os dihydrogen complexes containing CO as a
π-acid coligand are relatively rare.2a,d,f,4a The same can be said
for dicationic dihydrogen complexes.2f,11,33 Morris’ additive
ligand approach allows one to predict the instability of 7a, a
dicationic dihydrogen complex containing a strong π-acid.2a
Also, the protonation of species of the type [MH2(CO)L3] (M
) Ru, Os; L ) P-donor ligand) with strong acids does not yield
stable dihydrogen complexes.34 Thus, it is rather surprising that
7b is stable under ambient conditions. Although a trans
arrangement of dihydrogen and carbonyl ligands is common in
neutral d6 Mo and W complexes1 and is documented in cationic
rhenium(I) complexes,5 it is still unprecedented, to the best of
our knowledge, in dicationic species of Ru and Os. In addition
to several neutral (dihydrogen) osmium carbonyls,1,35 a cis
arrangement of CO and η2-H2 ligands has been found for [Os-
(CO)(η2-H2)(PPh3)2(2-pyridinethiol)]+ and [Os(CO)(η2-H2)-
1
(PPh3)2L2]2+ (L ) /2bpy or PPh3).2d,f
As the presence of a π-donor is responsible for both the
elongation of the H-H bond and the stabilization of the five-
coordinate dissociation product A, the position of equilibrium
a in eq 2 ultimately depends on the balance between these two
effects. For 1a, the stabilization of the five-coordinate A appears
to overrun the effect of the increased interaction between the
metal center and the dihydrogen ligand, while in 1b, the contrary
seems to be the case. Finally, the increasing stability on going
from Ru to Os, a general feature of dihydrogen complexes, is
thought to be related to the increase in the strength of the M(η2-
H2) bonding on going down the group.36
The effects of the ancillary ligand X on the σ and π bonding
components account for the trend in the stability of the M(η2-
H2) bond in the ruthenium complexes, which increases in the
order CO < Cl < H. The lower stability with CO than with H
reflects the weakening of the dπ (M) f σ* (η2-H2) bonding
component due to the π-acid. Dissociation of dihydrogen from
7a gives a highly reactive 16-electron species: the [Ru(CO)-
(30) Backsay, G. B.; Bytheway, I.; Hush, N. S. J. Am. Chem. Soc. 1996,
118, 3753.
Effect of X on the Acidity. The complexes of the type trans-
[MX(η2-H2)(dppp)2]+ (M ) Ru, Os; X ) H, Cl, CO) allow
one to monitor the effect of X on the acidity. Admittedly, the
comparison of the pKa data for the sequence X ) H, Cl, CO is
complicated by the fact that 4b is a classical trihydride. For
the Ru complexes, the acidity of the dihydrogen ligand increases
in the sequence H < Cl < CO, with a trend similar to that in
the series [Os(X)(η2-H2)(dppe)2]n+ (X ) H, CH3CN, Cl, Br).2e
The most striking feature is the wide range of pKa values,
spanning 18 pKa units! This can be qualitatively explained on
(31) Albinati, A.; Bakhmutov, V. I.; Caulton, K. G.; Clot, E.; Eckert, J.;
Eisenstein, O.; Gusev, D. G.; Grushin, V. V.; Hauger, B. E.; Klooster,
W. T.; Koetzle, T. F.; McMullan, R. K.; O’Loughlin, T. J.; Pe´lissier,
M.; Ricci, J. S.; Sigalas, M. P.; Vymenits, A. B. J. Am. Chem. Soc.
1993, 115, 7300.
(32) Morris, R. H.; Schlaf, M. Inorg. Chem. 1994, 33, 1725.
(33) Smith, K. T.; Tilset, M.; Kuhlmann, R.; Caulton, K. G. J. Am. Chem.
Soc. 1995, 117, 9473.
(34) (a) Siedle, A. R.; Newmark, R. A.; Pignolet, L. H. Inorg. Chem. 1986,
25, 3412. (b) Blosser, P. W.; Gallucci, J. C.; Wojcicki, A. Inorg. Chem.
1992, 31, 2376.
(35) (a) Gusev, D. G.; Vymenits, A. B.; Bakhmutov, V. I. Inorg. Chem.
1992, 31, 1. (b) Buil, M. L.; Esteruelas, M. A.; Lahoz, F. J.; On˜ate,
E.; Oro, L. A. J. Am. Chem. Soc. 1995, 117, 3619. (c) Esteruelas, M.
A.; Lahoz, F. J.; On˜ate, E.; Oro, L. A.; Valero, C.; Zeier, B. J. Am.
Chem. Soc. 1995, 117, 7935 and references therein.
(36) Cappellani, E. P.; Drouin, S. D.; Jia, G.; Maltby, P. A.; Morris, R.
H.; Schweitzer, C. T. J. Am. Chem. Soc. 1994, 116, 3375.

720 Inorganic Chemistry, Vol. 36, No. 4, 1997
Rocchini et al.
the basis of the effect of X on the relative energies of A, B,
and C in eq 1.
Concluding Remarks
The properties of the dihydrogen complexes trans-[MX(η2-
H2)(dppp)2]+ can be tuned over the whole d(H-H) and pKa
ranges yet observed, taking advantage of the sensitivity of the
H-H distance and Brønsted acidity to the electronic structure
of the ancillary ligand X. The rational exploitation of this
phenomenon may allow for the synthesis of novel five-
coordinate species of the type [MX(dppp)2]+ and of the
corresponding dihydrogen complexes trans-[MX(η2-H2)L4]+ for
the efficient heterolytic activation of dihydrogen. Also, the
present data suggest that the η2-H2 Vs dihydride preference is
driven by the steric properties of the diphosphine ligand as well
as by its basicity.
The low acidity of [MH(η2-H2)L4]+ results from the combi-
nation of the strong H-H bond of B discussed above and the
energetically unfavorable trans arrangement of the hydride
ligands in C. By contrast, a π-accepting X stabilizes C relative
to both B and A. This is due to the competition for the dn (M)
electrons between X and dihydrogen in B and to the electron-
deficient nature of A. The result is that B is a strong acid when
X ) CO. The effect of CO on the acidity of dihydrogen
complexes is not restricted to trans geometries, as both [Os-
(CO)(η2-H2)(PPh3)2(2-pyridinethiol)]+ and [Os(CO)(η2-H2)-
(bpy)(PPh3)2]2+ are highly acidic.2d,f
The presence of a π-donor trans to dihydrogen increases the
dπ (M) f σ* (η2-H2) back bonding and stabilizes the dihydrogen
complex B relative to the six-coordinate hydride C, as in the
latter no π-acid relieves the four-electron dπ (M)-pπ (Cl)
destabilization. However, the energy of C is much lower with
X ) Cl than with H, in view of the energetically favorable trans
arrangement of Cl and hydride. The result is a pKa value for
2a intermediate between those of 4a and 7a. Loss of the
dihydrogen ligand according to (a) in eq 1 is also favored due
to the formation of a π-stabilized five-coordinate complex.
Finally, the decreasing acidity on going from 2a to 2b probably
reflects an increase in ∆HBDE for the M(η2-H2) bonding going
down the group.2a,19,36
Acknowledgment. The authors are grateful to Professor R.
H. Morris for helpful discussion and for making avalaible his
results prior to publication. E.R. gratefully acknowledges the
Regione Autonoma Friuli-Venezia Giulia for a fellowship.
Supporting Information Available: Tables of atomic coordinates
and equivalent isotropic displacement coefficients, anisotropic thermal
parameters, full listings of bond distances and angles, and H atom
coordinates (13 pages). Ordering information is given on any current
masthead page.
IC9605579