Synthesis and Characterization of OsH2Cl[κN,κ O-(ON=CR2)](PiPr3)2 (CR2 = C(CH2)4CH2, R = CH3): Influence of the L2 Ligand on the Nature of the H2 Unit in OsH2 ClL2(PiPr3)2

Article abstract of DOI:10.1021/om9904192

The dihydride-dichloro complex O_sH₂CI₂(PⁱPr₃) ₂fe (1) reacts with cyclohexanone oxime and acetone oxime in the presence of Et₃N to give the dihydride derivatives O_sH₂CI{_kN,_kO- [ON=C(CH₂)₄CH₂]}(PⁱPr ₃)₂ (2) and O_sH₂CI{_kN,_kO-[ON=C(CH ₃)₂]}(PⁱPr₃)₂ (3), respectively. The structure of 2 has been determined by X-ray diffraction. The geometry around the osmium atom can be described as a distorted pentagonal bipyramid, with the triisopropylphosphine ligands occupying two relative trans positions. The remaining perpendicular plane is formed by the hydride ligands, the chlorine, and the oximate group, which acts with a bite angle of 36.6(1). In solution the hydride ligands of 2 and 3 undergo an intramolecular thermally activated site exchange process. The activation parameters of this process are ΔH^? = 11.9(±0.7) kcal mol-¹ and ΔS^? = -0.5(±1.4) cal mol^-1 for 2 and ΔH1 = 11.7(±0.8) kcal mol-¹ and ΔS^? = -0.8(±2.0) cal mol^-1 K^-1 for 3. To understand why complexes 2 and 3 are dihyride derivatives, while the previously reported complex O_sCl- {NH=C(Ph)C₆H₄}(η²-H₂)(P ⁱPr₃)₂ is an elongated dihydrogen compound, a quantitative theoretical analysis of the interaction between the H₂ moiety and the O_sClL₂(PH_{3 2}(L₂ = ON=CH₂, NH=CHCH=CH) complex fragments, along the oxidative addition pathway, is also reported.

Full text of DOI:10.1021/om9904192

4296
Organometallics 1999, 18, 4296-4303
Syn th esis a n d Ch a r a cter iza tion of
OsH2Cl[KN,KO-(ONdCR2)](P iP r 3)2 (CR2 ) C(CH2)4CH2, R )
CH3): In flu en ce of th e L2 Liga n d on th e Na tu r e of th e H2
Un it in OsH2ClL2(P iP r 3)2 (L2 ) ONdCR2, NHdC(P h )C6H4)
Com p lexes
Ricardo Castarlenas,† Miguel A. Esteruelas,*,† Enrique Gutie´rrez-Puebla,‡
Yves J ean,§ Agust´ı Lledo´s,*,| Marta Mart´ın,† and J aume Toma`s|
Departamento de Quı´mica Ino´rganica-Instituto de Ciencia de Materiales de Arago´n,
Universidad de Zaragoza-CSIC, 50009 Zaragoza, Spain, Instituto de Ciencia de Materiales de
Madrid, CSIC, Campus de Cantoblanco, 28049 Madrid, Spain, Laboratoire de Chimie
The´orique (ESA Q8077), Baˆtiment 490, Universite´ de Paris Sud, 91405 Orsay Cedex, France,
and Departament de Quı´mica, Universitat Auto`noma de Barcelona,
08193 Bellaterra, Barcelona, Spain
Received J une 1, 1999
The dihydride-dichloro complex OsH2Cl2(PiPr3)2 (1) reacts with cyclohexanone oxime and
acetone oxime in the presence of Et3N to give the dihydride derivatives OsH2Cl{κN,κO-
[ONdC(CH2)4CH2]}(PiPr3)2 (2) and OsH2Cl{κN,κO-[ONdC(CH3)2]}(PiPr3)2 (3), respectively.
The structure of 2 has been determined by X-ray diffraction. The geometry around the
osmium atom can be described as a distorted pentagonal bipyramid, with the triisoprop-
ylphosphine ligands occupying two relative trans positions. The remaining perpendicular
plane is formed by the hydride ligands, the chlorine, and the oximate group, which acts
with a bite angle of 36.6(1)°. In solution the hydride ligands of 2 and 3 undergo an
intramolecular thermally activated site exchange process. The activation parameters of this
process are ∆Hq ) 11.9((0.7) kcal mol-1 and ∆Sq ) -0.5((1.4) cal mol-1 K-1 for 2 and ∆Hq
) 11.7((0.8) kcal mol-1 and ∆Sq ) -0.8((2.0) cal mol-1 K-1 for 3. To understand why
complexes 2 and 3 are dihyride derivatives, while the previously reported complex OsCl-
{NHdC(Ph)C6H4}(η2-H2)(PiPr3)2 is an elongated dihydrogen compound, a quantitative
theoretical analysis of the interaction between the H2 moiety and the OsClL2(PH3)2 (L2 )
ONdCH2, NHdCHCHdCH) complex fragments, along the oxidative addition pathway, is
also reported.
In tr od u ction
ever, theoretical calculations3-6 have suggested an
alternative description for this class of compounds.
Instead of as an intermediate species along the reaction
path, they may be better described as complexes con-
taining two hydrogen atoms moving freely in a large
region of the coordination sphere of the metal. High-
level ab initio calculations on the elongated dihydrogen
complexes [OsX(η2-H2)(NH3)4]+ (X ) CH3CO2, Cl),3 [Ru-
(η5-C5H5)(η2-H2)(H2PCH2CH2PH2)]+,4 trans-[OsCl(η2-
H2)(H2PCH2CH2PH2)2]+,5 and OsCl2(η2-H2)(NHdCH2)-
Elongated dihydrogen complexes1 are often seen as
arrested structures at intermediate stages on the reac-
tion path for the dihydrogen oxidative addition.2 How-
† Universidad de Zaragoza-CSIC.
‡ Instituto de Ciencia de Materiales de Madrid.
§ Universite´ de Paris Sud.
| Universitat Auto`noma de Barcelona.
(1) (a) Chinn, M. S.; Heinekey, D. M. J . Am. Chem. Soc. 1990, 112,
5166. (b) Brammer, L.; Howard, J . A. K.; J ohnson, O.; Koetzle, T. F.;
Spencer, J . L.; Stringer, A. M. J . Chem. Soc., Chem. Commun. 1991,
241. (c) Collman, J . P.; Wagenknecht, P. S.; Hembre, R. T.; Lewis, N.
S. J . Am. Chem. Soc. 1990, 112, 1294. (d) 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. (e) Hasegawa, T.;
Li, Z.; Parkin, S.; Hope, H.; McMullan, R. K.; Koetzle, T. F.; Taube, H.
J . Am. Chem. Soc. 1994, 116, 4352. (f) Klooster, W. T.; Koetzle, T. F.;
J ia, G.; Fong, T. P.; Morris, R. H.; Albinati, A. J . Am. Chem. Soc. 1994,
116, 7677. (g) Maltby, P. A.; Schlaf, M.; Steinbeck, M.; Tough, A. J .;
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155. (b) Heinekey, D. M.; Oldham, W. J ., J r. Chem. Rev. 1993, 93, 913.
6
(PH3)2 have shown that there is a very flat energy
profile for the stretch of the coordinated H-H bond: it
costs less than 2 kcal mol-1 to stretch an H-H bond
from 1.0 to 1.6 Å.
(3) Craw, J . S.; Backsay, G. B.; Hush, N. S. J . Am. Chem. Soc. 1994,
116, 5937.
(4) Gelabert, R.; Moreno, M.; Lluch, J . M.; Lledo´s, A. J . Am. Chem.
Soc. 1997, 119, 9840.
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Soc. 1998, 120, 8168.
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I. Inorg. Chem. 1998, 37, 5033.
10.1021/om9904192 CCC: $18.00 © 1999 American Chemical Society
Publication on Web 09/21/1999

Ligand Influence on H2 in Osmium Complexes
Organometallics, Vol. 18, No. 21, 1999 4297
Sch em e 1
It can be expected that this particular behavior of the
H2 moiety could be modified by subtle electronic and
geometrical changes in the transition-metal fragment.
Experimental7a,b and theoretical7b,c studies have shown
that the dihidrogen or dihydride character of the H2 unit
is very dependent on the nature of the ligand disposed
trans to the two hydrogen atoms. Indeed, for this type
of system, the strength of the metal-hydrogen interac-
tion and the behavior of the hydrogen atoms in solution
is strongly dependent on the ligand disposed cis with
regard to the H2 unit, as is illustrated by the OsH2X-
{NHdC(Ph)C6H4}(PiPr3)2 (X ) H, Cl, Br, I) complexes.8
In solution, the hydride ligands of the trihydride deriva-
tive undergo two thermally activated exchange pro-
cesses, which occur at different rates. The faster one
involves the hydride located trans to the phenyl group.
The replacement of this hydride by chloride, bromide,
and iodide produces significant disruptions in the
interaction between the other two hydrogens and the
osmium atom. Thus, in the halogenated compounds
acetate affords the saturated seven-coordinate osmium-
(IV)-dihydride complexes OsH2Cl(κ2-O2CCH3)(PiPr3)2
and OsH2{κ1-OC(O)CH3}(κ2-O2CCH3)(PiPr3)2, which in
the solid state have structures similar to that of the
starting material that can be also described as square
antiprisms but with only one missing vertex.12
OsX{NHdC(Ph)C6H4}(η2-H2)(PiPr3)2 (X ) Cl, Br, I), the
H2 unit forms elongated dihydrogen ligands, which in
solution show restricted rotational motions.
In this paper, we report the synthesis and character-
ization of the saturated compounds OsH2Cl{κN,κO-
(ONdCR2)}(PiPr3)2 (CR2 ) C(CH2)4CH2, R ) CH3).
Despite having a structure similar to that of the
previously mentioned elongated dihydrogen complex
The dihydride or elongated dihydrogen nature of the
H2 unit of these systems depends not only on the ligand
located in the cis position but also on other factors, as
revealed by the chemistry of the unsaturated six-
coordinate osmium(IV) dihydride OsH2Cl2(PiPr3)2 in the
presence of nucleophilic reagents. Substitution of a
chloride anion in this complex by [EtOCS2]- and
[CH3COS]- produces saturated six-coordinate com-
pounds, where a dihydride-elongated dihydrogen trans-
formation has occurred.9 Similarly, the addition of 2,2′-
biimidazole (H2bim) leads to the saturated cationic
elongated dihydrogen [OsCl(η2-H2)(H2bim)(PiPr3)2]Cl,
which by deprotonation of the H2bim ligand with NaBH4
yields the neutral species Os(Hbim)Cl(η2-H2)(PiPr3)2.
Addition of pyrazole to OsH2Cl2(PiPr3)2 affords the
trans-dichloro species OsCl2(η2-H2)(Hpz)(PiPr3)2, which
is transformed into its cis-dichloro isomer by stirring
in hexane at 60 °C.10
OsCl{NHdC(Ph)C6H4}(η2-H2)(PiPr3)2,8 these complexes
turn out to be dihydride derivatives. To understand the
different natures of both types of complexes, we present
also a quantitative theoretical analysis of the interaction
between dihydrogen and OsClL2(PH3)2 (L2 ) ONdCH2,
NHdCHCHdCH) complex fragments along the oxida-
tive addition pathway.
Resu lts a n d Discu ssion
1. Syn th esis a n d Ch a r a cter iza tion of OsH 2Cl-
{KN,KO-(ONdCR2)}(P iP r 3)2. Treatment at room tem-
perature of OsH2Cl2(PiPr3)2 (1) with the stoichiometric
amount of cyclohexanone oxime or, alternatively, ac-
etone oxime, in the presence of 5 equiv of Et3N, gives
the complexes OsH2Cl{κN,κO-[ONdC(CH2)4CH2]}(Pi-
Pr3)2 (2) and OsH2Cl{κN,κO-[ONdC(CH3)2]}(PiPr3)2 (3),
respectively, which were isolated as yellow (2) and
orange (3) solids in 82% (2) and 86% (3) yields, according
to Scheme 1.
The above-mentioned dihydride-elongated dihydro-
gen transformations involve the electronic saturation
of the osmium atom along with a rearrangement of its
coordination polyhedron, which changes from square
antiprism with two missing vertexes in the starting
compound11 to octahedron in the elongated dihydrogen
derivatives.9,10 However, it does not appear that there
exists a direct relationship between the electronic
saturation of the metallic center and the change in the
coordination polyhedron. As a matter of fact, the sub-
stitution of the chloride anions of OsH2Cl2(PiPr3)2 by
Complexes 2 and 3 were characterized by MS, el-
1
emental analysis, and IR and H, 31P{1H}, and 13C{1H}
NMR spectroscopy. Complex 2 was further character-
ized by an X-ray crystallographic study. A view of the
molecular geometry of this compound is shown in Figure
1. Selected bond distances and angles are listed in Table
1.
The hydrogen atoms H(1) and H(2) were located in
the difference Fourier maps and refined as isotropic
atoms together with the rest of the non-hydrogen atoms
of the structure, giving Os-H(1) and Os-H(2) distances
of 1.68(5) and 1.57(5) Å, respectively, and a H(1)-H(2)
separation of 1.77(7) Å. These values suggest that the
H2 unit gives rise to two hydride ligands. In agreement
with this, the IR spectra of 2 and 3 show two absorptions
(7) (a) Li, Z.-W.; Taube, H. J . Am. Chem. Soc. 1991, 113, 8946. (b)
Bautistas, M. T.; Cappellani, E. P.; Drouin, S. D.; Morris, R. H.;
Schweitzer, C. T.; Sella, A.; Zubkowski, J . J . Am. Chem. Soc. 1991,
113, 4876. (c) Craw, J . S.; Bacskay, G. B.; Hush, N. S. J . Am. Chem.
Soc. 1994, 116, 5937. (d) Dapprich, S.; Frenking, G. Organometallics
1996, 15, 4547.
(8) Barea, G.; Esteruelas, M. A.; Lledo´s, A.; Lo´pez, A. M.; On˜ate,
E.; Tolosa, J . I. Organometallics 1998, 17, 4065.
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3793.
(10) Esteruelas, M. A.; Lahoz, F. J .; Oro, L. A.; On˜ate, E.; Ruiz, N.
Inorg. Chem. 1994, 33, 787.
(11) Aracama, M.; Esteruelas, M. A.; Lahoz, F. J .; Lo´pez, J . A.;
Meyer, U.; Oro, L. A.; Werner, H. Inorg. Chem. 1991, 30, 288.
(12) Crochet, P.; Esteruelas, M. A.; Lo´pez, A. M.; Mart´ınez, M.-P.;
Oliva´n, M.; On˜ate, E.; Ruiz, N. Organometallics 1998, 17, 4500.

4298 Organometallics, Vol. 18, No. 21, 1999
Castarlenas et al.
F igu r e 2. B3LYP optimized structures of the minimum
4 (a) and the transition state 4-TS (b).
formed on the OsH2Cl{κN,κO-(ONdCH2)}(PH3)2 (4)
model system. The structure obtained in this way is
shown in Figure 2a, and the main geometrical param-
eters are collected in Table 1.
The theoretical results agree well with those obtained
from the X-ray diffraction experiment. The Os-H (1.610
Å) distances and the H(1)-H(2) separation (1.680 Å) are
statistically identical with those experimentally ob-
tained and corroborate the dihydride nature of the H2
unit in these compounds.
F igu r e 1. Molecular diagram for OsH2Cl{κN,κO-
[ONdC(CH2)4CH2]}(PiPr3)2 (2). Thermal ellipsoids are
shown at 50% probability.
Ta ble 1. Selected Bon d Len gth s (Å) a n d An gles
(d eg) of OsH2Cl{KN,KO-[ONdC(CH2)4CH2]}(P iP r 3)2
(2) a n d B3LYP Op tim ized P a r a m eter s of
OsH2Cl[KN,KO-(OdNCH2)](P H3)2 (4)
2
4
2
4
Assuming that the H2 and the ONdCR2 units both
occupy two sites in the coordination sphere of the metal,
the geometry around the osmium atom of 2 can be
rationalized as a very distorted pentagonal bipyramid
with the two phosphorus atoms of the triisopropylphos-
phine ligands occupying axial positions (P(1)-Os-P(2)
) 171.53(4)°) and the hydrides, the chlorine, and the
oximate group located in the equatorial plane. The
distortion of the coordination polyhedron is mainly due
to the oximate group, which acts with a bite angle of
36.61(12)°. Similar values have been found in related
compounds.16 If the oximate group is considered as a
monodentate ligand, the structure of the complex can
be described as a slightly distorted octahedron.
In agreement with the structure shown in Figure 1,
the 31P{1H} NMR spectrum of 2 contains a singlet at
12.3 ppm. In the 13C{1H} NMR spectrum, the most
noticeable resonance is a singlet at 139.4 ppm, corre-
sponding to the CdN carbon atom of the oximate ligand.
Os-Cl
2.3922(12) 2.438 P(2)-Os-O
2.3798(11) 2.349 P(1)-Os-N
93.04(8) 91.8
93.51(10) 92.2
2.3767(12) 2.349 P(1)-Os-H(1) 90(2)
Os-P(1)
Os-P(2)
Os-O
90.3
2.321 P(1)-Os-H(2) 87.1(14) 88.3
2.014 P(2)-Os-N 92.55(11) 92.2
1.610 P(2)-Os-H(1) 84.6(14) 90.3
1.610 P(2)-Os-H(2) 85(1) 88.3
1.296 O-N-C(1)
1.291 O-Os-N
2.250(4)
1.978(4)
1.68(5)
Os-N
Os-H(1)
Os-H(2)
O-N
1.57(5)
1.353(4)
1.281(6)
90.16(4)
89.39(4)
124.3(4) 126.7
36.61(12) 33.9
118.2(2) 115.6
N-C(1)
Cl-Os-P(1)
Cl-Os-P(2)
Cl-Os-O
Cl-Os-N
Cl-Os-H(1)
Cl-Os-H(2)
88.4 O-Os-H(1)
88.4 O-Os-H(2)
175(2)
178.5
62.9
100.69(8) 101.7 H(1)-Os-H(2) 66(2)
137.30(10) 135.6 Os-N-C(1)
152.9(3) 147.2
141(2)
142.6 Os-O-N
79.7 Os-N-O
60.7(2)
82.7(2)
82(2)
60.0
86.1
81.7
75.3(14)
P(1)-Os-P(2) 171.53(4) 175.7 N-Os-H(1)
P(1)-Os-O 95.35(8) 91.8 N-Os-H(2)
147.4(14) 144.6
at 2220 and 2157 cm-1 (2) and 2189 and 2165 cm-1 (3)
corresponding to the ν(Os-H) vibrations, while absorp-
tions in the typical region of dihydrogen ligands (3100-
2400 cm-1 2a
are not observed.
)
In general, the M-H distances obtained from X-ray
diffraction data are imprecise.13 However, ab initio
calculations have been shown to provide useful accurate
data for the hydrogen positions in both classical poly-
hydride14 and dihydrogen complexes.15 Therefore, to
corroborate the dihydride nature of the H2 unit of these
compounds, a B3LYP geometry optimization was per-
(15) (a) Dapprich, S.; Frenking, G. Angew. Chem., Int. Ed. Engl.
1995, 24, 354. (b) Bakhmutov, V. I.; Bertra´n, J .; Esteruelas, M. A.;
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J . 1996, 2, 1815. (c) Maseras, F.; Lledo´s, A.; Costas, M.; Poblet, J . M.
Organometallics 1996, 15, 2947. (d) Gelabert, R.; Moreno, M.; Lluch,
J . M.; Lledo´s, A. Organometallics 1997, 16, 3805. (e) Albe´niz, M. J .;
Esteruelas, M. A.; Lledo´s, A.; Maseras, F.; On˜ate, E.; Oro, L. A.; Sola,
E.; Zeier, B. J . Chem. Soc., Dalton Trans. 1997, 181.
(16) (a) Dickman, M. H.; Doedens, R. J . Inorg. Chem. 1982, 21, 682.
(b) Porter, L. C.; Doedens, R. J . Acta Crystallogr., Sect. C 1985, 41,
838. (c) J aitner, P.; Huber, W.; Gieren, A.; Betz, H. J . Organomet.
Chem. 1986, 311, 379. (d) Pizzotti, M.; Porta, F.; Cenini, S.; Demartin,
F.; Masciocchi, N. J . Organomet. Chem. 1987, 330, 265. (e) Stella, S.;
Floriani, C.; Chiesi-Villa, A.; Guastini, C. J . Chem. Soc., Dalton Trans.
1988, 545. (f) Sharp, P. R.; Hoard, D. W.; Barnes, C. L. J . Am. Chem.
Soc. 1990, 112, 2024. (g) Hoard, D. W.; Sharp, P. R. Inorg. Chem. 1993,
32, 612. (h) Ang, H. G.; Kwik, W. L.; Ong, K. K. J . Fluorine Chem.
1993, 60, 43. (i) Vogel, S.; Huttner, G.; Zsolnai, L.; Emmerich, C. Z.
Naturforsch., B 1993, 48, 353. (j) Werner, H.; Daniel, T.; Knaup, W.;
Nu¨rnberg, O. J . Organomet. Chem. 1993, 462, 309. (k) Skoog, S. J .;
Campbell, J . P.; Gladfelter, W. L. Organometallics 1994, 13, 4137. (l)
Kukushkin, V. Y.; Tudela, D.; Pombeiro, A. J . L. Coord. Chem. Rev.
1996, 156, 333. (m) Skoog, S. J .; Gladfelter, W. L. J . Am. Chem. Soc.
1997, 119, 11049.
(13) Zhao, D.; Bau, R. Inorg. Chim. Acta 1998, 269, 162.
(14) (a) Lin, Z.; Hall, M. B. Inorg. Chem. 1991, 30, 2569. (b) Lin, Z.;
Hall, M. B. Coord. Chem. Rev. 1994, 135, 845. (c) Esteruelas, M. A.;
J ean, Y.; Lledo´s, A.; Oro, L. A.; Ruiz, N.; Volatron, F. Inorg. Chem.
1994, 33, 3609. (d) Buil, M. L.; Espinet, P.; Esteruelas, M. A.; Lahoz,
F. J .; Lledo´s, A.; Mart´ınez-Ilarduya, J . M.; Maseras, F.; Modrego, J .;
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Ligand Influence on H2 in Osmium Complexes
Organometallics, Vol. 18, No. 21, 1999 4299
The 31P{1H} and 13C{1H} NMR spectra are temperature-
1
invariant. However, the H NMR spectrum is temper-
ature-dependent. At 373 K in toluene-d8 as solvent, the
spectrum shows in the hydride region a broad resonance
centered at -7.83 ppm. Between 323 and 293 K de-
coalescence occurs, and between 263 and 233 K, two
signals centered at -3.22 and -12.44 ppm are observed.
The NMR spectroscopic data of 3 agree well with
those of 2. The 31P{1H} NMR spectrum shows a singlet
at 12.4 ppm, and the 13C{1H} NMR spectrum contains
a singlet at 133.9 ppm corresponding to the CdN carbon
atom of the oximate group. As for 2, these spectra are
1
temperature-invariant, while the H NMR spectrum is
temperature-dependent. The variation of the hydride
resonances with the temperature is similar to the case
of 2. At 373 K in toluene-d8 as solvent, the spectrum
shows at -7.70 ppm a broad resonance. Between 313
and 273 K, decoalescence occurs, and between 273 and
233 K, two signals centered at -3.24 and -12.60 ppm
are observed.
To estimate the hydrogen-hydrogen separation be-
tween the hydride ligands of 2 and 3 in solution, the T1
values of the hydride resonances were determined over
the temperature range 253-193 K. The same T1(min)
value was found for both compounds and for each
resonance (100 ((3) ms), and it occurs at the same
temperature (213 K). The total relaxation rate for Hn
(Rn ) 1/T1(min)(Hn)) is the addition of the relaxation
rate due to the dipole-dipole interaction (RH-H) and
that due to all other relaxation contributors (R*). Since
F igu r e 3. Experimental (left) and calculated (right) vari-
able-temperature 300 MHz 1H{31P} NMR spectra in the
high-field region of OsH2Cl{κN,κO-[ONdC(CH2)4CH2]}-
(PiPr3)2 (2) in toluene-d8.
Ta ble 2. Ra te Con sta n ts of Dih yd r id e Exch a n ge of
2 a n d 3
rate (s-1
)
rate (s-1
)
the latter one has been estimated as 5.2 s-1 17
we can
,
T (K)
2
3
T (K)
2
3
determine that the relaxation rate due to the hydride-
hydride interaction is 4.8 s-1. Using the standard
equation,18 this value leads to a separation between the
hydride ligands of 2 and 3 of 1.7 Å, which agrees well
with those determinated by X-ray diffraction (1.77(7)
Å) and theoretical methods (1.680 Å).
373 4.53 × 105 4.88 × 105
353 2.95 × 105 2.27 × 105
333 9.40 × 104 1.67 × 105
313 3.51 × 104 3.75 × 104
293 5.85 × 103 9.93 × 103
273 6.20 × 102 1.96 × 103
263 4.95 × 102 3.87 × 102
253 1.46 × 102 2.02 × 102
243 47.3
233 36.0
223 9.37
1.17 × 102
43.8
1
To clarify the mechanism of the hydride site exchange
in 2 and 3, we have calculated the transition state
assuming a structure in which the H2 unit is rotated
by 90° with respect to its position in the minimum. The
B3LYP optimized structure found in this way (4-TS) is
shown in Figure 2b. This transition state has a η2-H2
nature, as is proved by the H(1)-H(2) distance and
H(1)-Os-H(2) angle (0.932 Å and 32.0°, respectively).
The decrease of the distance between the two exchang-
ing hydrides is concomitant with the increase of the
Os-H distances (1.690 Å). Thus, the transition state can
be described as a d6 six-coordinate Os(II) species con-
taining a dihydrogen trans to the oxygen atom of the
oximate ligand. Furthermore, a dramatic shortening of
the Os-O distance occurs in the rotated structure (2.321
Å in 4 vs 2.170 Å in 4-TS).
The behavior of the H NMR spectra of 2 and 3 with
changes in the temperature indicates that the hydride
ligands of these compounds undergo a thermally acti-
vated site exchange process. Line-shape analysis of the
1H{31P} NMR spectra (Figure 3 shows those of 2) allows
the calculation of the rate constants for the processes
at each temperature (Table 2). The activation param-
eters obtained from the corresponding Eyring analysis
are ∆Hq ) 11.9((0.7) kcal mol-1 and ∆Sq ) -0.5((1.4)
cal mol-1 K-1 for 2 and ∆Hq ) 11.7((0.8) kcal mol-1
and ∆Sq ) - 0.8((2.0) cal mol-1 K-1 for 3. The values
for the entropy activation, close to zero, are in agree-
ment with an intramolecular process, while the values
for the enthalpy of activation lie in the range reported
for thermal exchange processes in trihydride and hy-
dride-dihydrogen derivatives.20
The mechanism of the hydride site exchange can be
described as a pairwise exchange through a η2-H2
structure and involves only minor motion of the other
ligands. Such a mechanism has already been shown to
be operative in trihydride complexes.6-13,14e-g The cal-
culated activation barrier for the hydride exchange
process in 4 is 19.1 kcal mol-1, a value higher than that
obtained by NMR spectroscopy in 2 and 3. This discrep-
ancy may be due to the replacement of the bulky PiPr3
(17) Castillo, A.; Esteruelas, M. A.; On˜ate, E.; Ruiz, N. J . Am. Chem.
Soc. 1997, 119, 9691.
(18) At 300 MHz, RH-H ) 129.18/r6.19
(19) (a) Hamilton, D. G.; Crabtree, R. H. J . Am. Chem. Soc. 1988,
110, 4126. (b) Bautista, M. T.; Earl, K. A.; Maltby, P. A.; Morris, R.
H.; Schweitzer, C. T.; Sella, A. J . Am. Chem. Soc. 1988, 110, 7031. (c)
Desrosiers, P. J .; Cai, L.; Lin, Z.; Richards, R.; Halpern, J . J . Am. Chem.
Soc. 1991, 113, 4173. (d) Gusev, D. G.; Kuhlman, R.; Sini, G.;
Eisenstein, O.; Caulton, K. G. J . Am. Chem. Soc. 1994, 116, 2685.
(20) (a) Earl, K. A.; J ia, G.; Maltby, P. A.; Morris, R. H. J . Am. Chem.
Soc. 1991, 113, 3027. (b) J ia, G.; Drouin, S. D.; J essop, P. G.; Lough,
A. J .; Morris, R. H. Organometallics 1993, 12, 906. (c) Esteruelas, M.
A.; Lahoz, F. J .; Lo´pez, A. M.; On˜ate, E.; Oro, L. A.; Ruiz, N.; Sola, E.;
Tolosa, J . I. Inorg. Chem. 1996, 35, 7811.
and ONdCR2 (CR2 ) C(CH2)4CH2, R ) CH3) ligands
by PH3 and ONdCH2.

4300 Organometallics, Vol. 18, No. 21, 1999
Castarlenas et al.
F igu r e 4. CCSD(T)//B3LYP potential energy curves for
the H-H stretching in 4 (b) and 5 (O).
2. An a lysis of th e Os-H2 In ter a ction . In complexes
2 and 3 the H2 unit gives rise to two hydride ligands,
whereas in the related OsCl{NHdC(Ph)C6H4}(η2-H2)-
(PiPr3)2 it forms an elongated dihydrogen ligand.8
Therefore, a fine balance between the different coordi-
nation modes of the H2 moiety can be obtained just
operating upon the choice of the ancillary ligands of the
system.
F igu r e 5. Energy decomposition as a function of the H-H
distance for the OsH2Cl{κN,κO-(ONdCH2)}(PH3)2 (4) and
OsCl(NHdCHCHdCH)(η2-H2)(PH3)2 (5) model complexes.
For comparative purposes, geometry optimization of
OsCl(NHdCHCHdCH)(η2-H2)(PH3)2 (5), taken as a
H2 moiety interacts in different ways with the metal in
complexes 4 and 5. To clarify this point, a previously
successfully used energy decomposition scheme6,21 can
be applied to this case. In this scheme the OsClH2L2-
(PH3)2 complexes are ideally thought to be built from
two separate fragments (the H2 molecule and the
OsClL2(PH3)2 metallic units). In this way, the interac-
tion energy between the two fragments can be decom-
posed at each hydrogen-hydrogen distance into three
terms. Two of them, which are unfavorable, represent
the energy required to distort both fragments from their
isolated equilibrium geometries to the geometry they
model of the OsCl{NHdC(Ph)C6H4}(η2-H2)(PiPr3)2 com-
plex, was performed at the same level of theory as that
used for 4. The optimized H(1)-H(2) distance in 5 (1.494
Å) agrees with the previously calculated H-H distance
in OsCl(NHdCHC6H4)(η2-H2)(PH3)2 (1.483 Å),8 suggest-
ing that complex 5 should be included within the rather
diffuse family of elongated dihydrogen complexes.
The different natures of complexes 4 and 5 were
studied by computing the energy profiles associated with
the H-H bond breaking. In both complexes the H-H
distance was varied from 0.8 to 1.8 Å by steps of 0.2 Å,
and all the other geometrical parameters were opti-
mized at the B3LYP level for each fixed H-H distance.
Single-point energy-only calculations were performed at
the CCSD(T) level on all the optimized structures. The
CCSD(T)//B3LYP potential energy curves are shown in
Figure 4.
have at each point of the curve (∆Estretch (H2) and ∆Edef
-
(OsClL2(PH3)2)). The third term is the energy gain
arising from the interaction between the two distorted
fragments to restore the whole complex (∆Eint). At each
point, the variation in the total energy (∆Etot) can be
obtained according to eq 1. The results of this analysis
In both cases there is only one minimum, and the
dihydrogen form is not available. However, as expected,
the minimum of 4 is slightly shifted to the dihydride
region (long H-H distances). More remarkable is the
fact that the shape of the two curves is clearly different.
The flatness of the curve associated with the elongated
dihydrogen complex 5 in the 1.2-1.8 Å region suggests
that, with the inclusion of zero-point energy, the two
hydrogen atoms are delocalized to a certain extent. Such
a phenomenon is usual for this kind of complex,3-6 and
to give an accurate theoretical prediction of the actual
H-H distance which could be obtained via neutron
diffraction is not so easy. As shown previously, electronic
structure calculations are not sufficient to reproduce the
experimental values; nuclear wave function calculations
are required.4,5 Things are much easier for complex 4,
which is unambiguously a classical dihydride without
H-H interaction. From these results, it is clear that the
∆Etot ) ∆Estretch(H2) + ∆Edef(OsClL2(PH3)2) + ∆Eint
(1)
at the B3LYP level of theory are shown in Figure 5.
Because we want to establish the differences between
4 and 5, the ∆Estretch (H2) term has been omitted in this
figure.
The distortion term (∆Edef (OsClL2(PH3)2)) increases
along the dihydrogen addition path for both complexes,
since it is obviously more difficult to accommodate an
elongated H2 moiety than a dihydrogen molecule in the
coordination sphere of the metal. The distortion term
is larger for 4 than for 5, because the isolated OsClL2-
(PH3)2 optimized fragment is further from the geometry
it has in the final structure, in the case of the ONdCH2
ligand. However, this is not the reason for the different
(21) Toma`s, J .; Lledo´s, A.; J ean, Y. Organometallics 1998, 17, 4932.

Ligand Influence on H2 in Osmium Complexes
Organometallics, Vol. 18, No. 21, 1999 4301
Ta ble 3. En er getica l P a r a m eter s Rela ted to H-H
equilibrium distance in complex 5, as it was actually
found in our geometry optimizations.
Coor d in a tion for OsClH2L2(P H3)2 Com p lexesa
Experimental values of Os-H binding energies be-
tween 63 and 73 kcal mol-1 have been recently reported
in a series of [CpOsHL]+ compounds.22a The computed
values of De(Os-H) in 4 and 5 are 10 kcal mol-1 higher
and similar to those determined in [OsH(η2-H2)L2]+ (L
) depe, 76 kcal mol-1; L ) dppe, 80 kcal mol-1; L )
dtfpe, 81 kcal mol-1).22b They can be placed among the
strongest reported values for M-H bonds.22c However,
this term cannot account for the different behavior of 4
and 5: the De(Os-H) values are very similar in both
complexes. As we have found in our previous work on
Kubas-like complexes,21 the M-H binding energy de-
pends mainly on the metal atom and remains almost
unaffected by changes in the ligand (the slight difference
should even favor the dihydride form in complex 5). It
is thus clear that the different behavior toward the H2
coordination of the OsCl2L2(PH3)2 complex fragments
is related to the ∆ES/T term. All other things being equal,
the smaller singlet-triplet energy promotion gives rise
to the easier formation of the dihydride form. The
singlet/triplet gap is lower for L2 ) ONdCH2 than for
L2 ) NHdCHCHdCH (26.7 vs 34.9 kcal mol-1), making
the dihydride formation easier in the former. This
difference in the ∆ES/T can be understood by taking into
account the structural differences between the two types
of complex fragments. NHdCHCHdCH is a bidentate
ligand with a bite angle of 75°. It occupies two sites in
the coordination sphere of the metal, and the corre-
sponding complex fragment can be described as a d6 ML5
distorted-square-pyramidal system. The N-Os-Cl and
C-Os-Cl angles (117 and 167°, respectively) agree with
this description. On the other hand, the ONdCR2
ligands have a bite angle of about 37°, making the OsCl-
(ONdCR2)(PR3)2 fragment better described as a square-
planar d6 ML4 system, with N-Os-Cl and O-Os-Cl
angles of about 157 and 166°, respectively. Since the
HOMO/LUMO gap is lower in a square-planar d6 ML4
fragment than in a square-pyramidal d6 ML5 fragment,
the observed trend found for the singlet-triplet energy
separation can be easily rationalized on orbital grounds.
L2
ONdCH2
NHdCHCHdCH
De(M-H2)
∆ES/T
De(M-H)
14.3
26.7
81.5
19.5
34.9
83.6
a
All the energies (in kcal/mol) have been calculated at the
CCSD(T) level of theory on B3LYP optimized structures.
natures of the H2 unit in 4 and 5. In fact, since the slope
of ∆Edef (OsClL2(PH3)2) is slightly larger for 4 than for
5, one should expect a dihydride form more destabilized
for 4 than for 5, and this is not observed. Thus, the
understanding of the results comes from the interaction
term. This term reveals that the interaction of the H2
moiety with the complex fragment is stronger in com-
plex 4 than in complex 5. Furthermore, with the H-H
lengthening, this difference increases, compensating the
unfavorable trend found for complex 4 in the distortion
term. From the curves it can also be stated that in the
short H-H distances region the interaction term is too
weak to allow the possibility of a dihydrogen minimum
in this particular system.
3. Ability of OsClL2(P iP r 3)2 (L2 ) ONdCR2, NHd
C(P h )C6H4) Com p lex F r a gm en ts To Br ea k th e
H-H Bon d . In a previous work we have shown that
the nature of the product of dihydrogen addition to
M(CO)n(PH3)5-n (M ) Cr, Mo, W; n ) 0, 3, 5) can be
related to the energetics of the M-H2 and M-H bonds
and to the singlet-triplet separation (∆ES/T) in the ML5
complex fragment. Dihydride structures are favored by
a small singlet/triplet separation and by large M-H
bonding energies (De(M-H)). We will now use this
analysis to compare the complexes 4 and 5.
The ∆ES/T, De(M-H2), and De(M-H) terms were
calculated at the CCSD(T)//B3LYP level for both com-
plexes. With regard to the ∆ES/T term it should be noted
that the excitation involves the orbitals required for the
further formation of the M-H bonds. The M-H2 bond
energies were calculated as the energy difference be-
tween the OsClL2(PH3)2 fragments optimized in their
lowest singlet state plus a dihydrogen molecule, and the
η2-H2 complexes optimized with the H-H distance hold
fixed to 0.8 Å. Finally, the De(M-H) values were
calculated as the energy difference between the
OsClL2(PH3)2 fragments optimized in their triplet state
plus two hydrogen atoms and a structure which was the
actual dihydride minimum for complex 4, while for
complex 5, a hypothetical structure with the H-H
distance fixed at 1.8 Å was taken as the dihydride. The
results are collected in Table 3.
The computed values of De(Os-H2) are in the range
of the binding energies of dihydrogen ligands in stable
dihydrogen complexes (15-20 kcal mol-1).21 Thus, this
magnitude by itself cannot explain why a dihydrogen
structure is not found in these systems. The potential
energy curves depicted in Figures 4 and 5 give a better
insight into this fact: there is no energetic barrier in
the H-H stretching process, so that the H-H bond
breaking is not arrested at this stage. It must be
stressed that the dihydrogen binding energy is higher
in 5 than in 4, indicating that at the initial stage of the
oxidative addition, the Os-H2 bond is stronger in the
former. This factor works in favor of a shorter H-H
Con clu d in g Rem a r k s
We have previously shown that the H2 moiety of the
complex OsCl{NHdC(Ph)C6H4}(η2-H2)(PiPr3)2 forms an
elongated dihydrogen ligand. This study reveals that a
fine balance between different coordination modes of the
H2 moiety in OsClH2L2(PiPr3)2 systems can be obtained
just operating upon the choice of the L2 ancillary
ligands. Thus, we now show that in the presence of Et3N
the dihydride-dichloro complex OsH2Cl2(PiPr3)2 reacts
with oximes to give the related compounds OsH2Cl-
{κN,κO-(ONdCR2)}(PiPr3)2, where, in contrast to OsCl-
{NHdC(Ph)C6H4}(η2-H2)(PiPr3)2, the H2 moiety gives
rise to two hydride ligands.
Theoretical calculations suggest that the interaction
of the H2 moiety with the OsCl{κN,κO-(ONdCR2)}-
(22) (a) Cappellani, E. P.; Drouin, S. D.; J ia, G.; Maltby, P. A.;
Morris, R. H.; Schweitzer, C. T. J . Am. Chem. Soc. 1994, 116, 3375.
(b) Wang, D.; Angelici, R. J . J . Am. Chem. Soc. 1996, 118, 935. (c)
Martinho Simo˜es, J . A.; Beauchamp, J . L. Chem. Rev. 1990, 90, 629
and references therein.

4302 Organometallics, Vol. 18, No. 21, 1999
Castarlenas et al.
(PiPr3)2 complex fragments is stronger than that with
Ta ble 4. Cr ysta l Da ta a n d Da ta Collection a n d
Refin em en t for
OsCl{NHdC(Ph)C6H4}(PiPr3)2. Moreover, in agreement
with the experimental results, the interaction of the H2
moiety with the model system OsCl{κN,κO-(ONdCH2)}-
(PH3)2 leads to a dihydride minimum structure with a
complete breaking of the H-H bond, while the inter-
OsH2Cl{KN,KO-[ONdC(CH2)4CH2]}(P iP r 3)2
formula
mol wt
C24H54ClNOOsP2
660.27
color, habit
space group
a, Å
b, Å
c, Å
orange
monoclinic, P21/c
14.8951(9)
9.5382(6)
20.8395(12)
104.369(1)
2868.1(3)
4
action of the H2 moiety with the model system OsCl-
(NHdCHCHdCH)(PH3)2 makes the H-H bond break-
ing/forming process to occur almost without energy
barrier.
â, deg
V, Å3
Z
D(calcd), g cm-3
µ, mm-1
1.529
4.666
The different behavior of OsCl{κN,κO-(ONdCR2)}-
(PiPr3)2 and OsCl{NHdC(Ph)C6H4}(PiPr3)2 complex frag-
ments toward the H2 coordination is related to their
different singlet-triplet separation. The oximate groups
give rise to complex fragments with a singlet-triplet
energy promotion smaller than that of the NHdC(Ph)-
C6H4 ligand, and since the excitation involves the
orbitals required for the formation of the M-H bonds,
the oximate groups favor the formation of the dihydride
form.
scan type
θ range, deg
temp, K
ω scans at different æ values
2 e θ e 28.4
153.0(2)
no. of data collected
no. of unique data
no. of params refined
R1a (F2 > 2σ(F2))
wR2b (all data)
Sc (all data)
10 239
5463 (Rint ) 0.0343)
379
0.0325
0.0759
1.056
R1(F) ) ∑||Fo| - |Fc||/∑|Fo|. wR2(F2) ) [∑[w(Fo - Fc2)2]/
a
b
2
∑[w(Fo2)2]]1/2; w-1 ) [σ2(Fo2) + (0.0341P)2 + 1.9443P], where P )
[max(Fo2,0) + 2Fc2)]/3. S ) [∑[w(Fo - Fc2)2]/(n - p)]1/2, where n
is the number of reflections and p the number of refined param-
eters.
c
2
Exp er im en ta l Section
P r ep a r a tion of OsClH2{KN,KO-[ONdC(CH3)2]}(P iP r 3)2
(3). This complex was prepared as described for 2, by starting
from 1 (200 mg, 0.34 mmol) with 25 mg (0.34 mmol) of acetone
oxime and 0.25 mL (1.8 mmol) of triethylamine. An orange
microcrystalline solid was obtained. Yield: 182 mg (86%). Anal.
All reactions were carried out with rigorous exclusion of air
using Schlenk-tube techniques. Solvents were dried by the
usual procedures and distilled under argon prior to use. The
starting material OsH2Cl2(PiPr3)2 (1) was prepared by the
published method.11
Calcd for
C21H50NOClOsP2: C, 40.66; H, 8.12; N, 2.25.
In the NMR spectra, chemical shifts are expressed in ppm
downfield from Me4Si (1H and 13C) and 85% H3PO4 (31P).
Coupling constants, J , are given in hertz.
Found: C, 40.43; H, 7.82; N, 2.14. IR (KBr, cm-1): ν(Os-H)
2189 (m), 2165 (s); ν(CdN) 1642 (m). 1H NMR (300 MHz,
toluene-d8, 243 K): δ 2.20 (m, 6H, PCH); 1.95 and 1.47 (both
s, 6H, CH3CdN); 1.26 and 1.12 (both dvt, 36H, J H-H ) 6.3
Hz, N ) 12.9 Hz, PCHCH3); -3.24 (br, 1H, OsH); -12.6 (br,
1H, OsH). 13C{1H} NMR (75.4 MHz, C6D6, 293 K): δ 133.9 (s,
CdN); 24.7 (vt, N ) 23.7 Hz, PCH); 21.2 and 19.2 (both s,
CH3CdN); 19.7 and 18.6 (both s, PCHCH3). 31P{1H} NMR
(121.4 MHz, C6D6, 293 K): δ 12.4 (s). MS (FAB+): m/z ) 586
(M+).
Kin etic An a lysis. Complete line-shape analyses of the
1H{31P} NMR spectra of 2 and 3 were achieved using the
program gNMR v3.6 for Macintosh (Cherwell Scientific Pub-
lishing Limited). The transverse relaxation time T2 used was
common for the two signals for all temperatures recorded and
was obtained from the line width of the exchange-averaged
resonance above the fast-exchange limit. The activation pa-
rameters ∆Hq and ∆Sq were calculated by a least-squares fit
of ln(k/T) vs 1/T (Eyring equation). Error analysis assumed a
2% error in the rate constant and 1 K in the temperature.
Errors were computed by published methods.23
X-r a y Str u ctu r e An a lysis of OsH2Cl{KN,KO-[ONdC-
(CH2)4CH2]}(P iP r 3)2 (2). Crystals suitable for the X-ray
diffraction study were obtained by cooling at 258 K a saturated
solution of 2 in diethyl ether. A summary of crystal data and
refinement parameters is reported in Table 4. The orange,
prismatic crystal, of approximate dimensions 0.08 × 0.06 ×
0.01 mm, was glued on a glass fiber and mounted on a
Siemens-CCD diffractometer (sealed tube 2.4 kW, λ ) 0.710 73
Å). A total of 10 239 data were collected via runs of 0.3° ω
scans at different æ values, over a θ range of 2.5-28.4°. All
data were corrected for absorption using the SADABS pro-
gram.24 The structure was solved by direct methods25 and
conventional Fourier techniques and refined by full-matrix
least squares on F2 (SHELXL97).25 Anisotropic parameters
were used in the last cycles of refinement for all non-hydrogen
atoms. Hydrogen atoms were included in observed positions
and refined freely or refined riding on their respective carbon
atoms. Atomic scattering factors, corrected for anomalous
dispersion, were implemented by the program. The refinement
converge to R1 ) 0.0325 (F2 > 2σ(F2)) and wR2 ) 0.0759 (all
data), with weighting parameters x ) 0.0341 and y ) 1.9443.
P r ep a r a t ion of OsH2Cl{KN,KO-[ONdC(CH2)4CH 2]}-
(P iP r 3)2 (2). A brown solution of 1 (200 mg, 0.34 mmol) in 15
mL of toluene was treated with cyclohexanone oxime (40 mg,
0,35 mmol) and 0.25 mL (1.8 mmol) of triethylamine. After it
was stirred for 20 min at room temperature, the orange
suspension was filtered through Kieselguhr and was concen-
trated to dryness. Addition of methanol yielded a yellow
precipitate that was washed (3 × 2 mL) with methanol and
dried in vacuo. Yield: 185 mg (82%). Anal. Calcd. for C24H54
-
NOClOsP2: C, 43.65; H, 8.24; N, 2.12. Found: C, 43.27; H,
8.32; N, 2.05. IR (KBr, cm-1): ν(Os-H) 2220 (m), 2157 (s); ν-
(CdN) 1637 (m). 1H NMR (300 MHz, toluene-d8, 253 K): δ
2.66 (m, 2H, CH2CdN); 2.26 (m, 6H, PCH); 2.00 (m, 2H,
CH2CdN); 1.24 (m, 6H, CH2); 1.30 and 1.16 (both dvt, 36H,
J H-H ) 6.6 Hz, N ) 13.2 Hz, PCHCH3); -3.22 (br, 1H, OsH);
-12.44 (br, 1H, OsH). 13C{1H} NMR (75.4 MHz, C6D6, 293 K):
δ 139.4 (s, CdN); 32.1 and 30.5 (both s, CH2CdN); 27.7, 26.0
and 25.4 (all s, CH2); 25.0 (vt, N ) 24 Hz, PCH); 19.8 and 18.1
(both s, PCHCH3). 31P{1H} NMR (121.4 MHz, C6D6, 293 K): δ
12.3 (s). MS (FAB+): m/z 659 (M+) and 623 (M+ - Cl).
(24) SADABS program: Sheldrick, G. SADABS User Guide; Institu¨t
fu¨r Anorganische Chemie der Universita¨t: Go¨ttingen, Germany, 1995.
(25) Sheldrick, G. SHELXL-97: Program for Crystal Structure
Refinement; Institu¨t fu¨r Anorganische Chemie der Universita¨t, Go¨t-
tingen, Germany, 1997.
(23) Morse, P. M.; Spencer, M. D.; Wilson, S. R.; Girolami, G. S.
Organometallics 1994, 13, 1646 and references therein.

Ligand Influence on H2 in Osmium Complexes
Organometallics, Vol. 18, No. 21, 1999 4303
Com p u ta tion a l Deta ils. Calculations were performed with
the GAUSSIAN94 series of programs.26 Geometry optimiza-
tions were carried out using the density functional theory
(DFT)27 with the B3LYP functional.28 An effective core poten-
tial operator was used to represent the innermost electrons of
the osmium atom.29 The basis set for the metal was that
associated with the pseudopotential,29 with a standard valence
double-ú LANL2DZ contraction.26 The basis set for the hydro-
gen atoms directly attached to the metal was double-ú supple-
mented with a polarization p shell.30a,b A 6-31G(d) basis was
used for Cl, P, N, and O atoms, as well as for the C coordinated
to the metal in complex 5.30c The rest of the atoms were
described using a 6-31G basis set.30a
To obtain accurate estimations for the oxidative addition
energy profiles, the geometries were optimized at the B3LYP
level and energies recalculated using coupled-cluster theory
with single, double, and a noniterative estimation of triple
excitations (CCSD(T)//B3LYP).31 The metal-dihydrogen and
metal-hydride dissociation energies and the singlet/triplet
energy differences in the fragments were also calculated at
the CCSD(T)//B3LYP level.
Ackn owledgm en t. Financial support from the DGES
of Spain (Projects PB95-0639-C02-01 and PB95-0806)
is acknowledged. R.C. thanks the Ministerio de Educa-
cio´n y Ciencia for a grant. J .T. acknowledges also the
“Direccio´ General de Recerca de la Generalitat de
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