Two- and four-electron alkyne ligands in osmium - Cyclopentadienyl chemistry: Consequences of the π⊥→M interaction

Article abstract of DOI:10.1021/om010645n

The complexes Os(η⁵-C₅H₅)Cl{η²-HC≡CC (OH)R₂}(P_iPr₃) (R = Ph (1a), Me (1b)) react with TlPF₆ to give [Os(η⁵-C₅H₅){η²-HC≡CC( OH)R₂}PⁱPr³)]PF₆ (R = Ph (2a), Me (2b)). The structures of 1a and 2a have been determined by X-ray diffraction. The comparative study of the data reveals a shortening of the Os - C(alkyne) distances on going from 1a to 2a, whereas the acetylenic bond lengths remain almost identical. Comparison of their ¹H and ¹³C{¹H} NMR spectra shows that the HC= proton resonances and the chemical shifts of the acetylenic carbon atoms of 2a and 2b are substantially shifted toward lower field than are those of 1a and 1b. DFT calculations were carried out on Os(η⁵-C₅H₅)Cl(η²-HC=CR)(PH ₃) (R = H (A), R = CH₃ (A^CH3)) and [Os(η⁵-C₅H₅)(η²-HC=CR)(PH ₃)⁺ (R = H (B), R = CH₃ (B^CH3)) model systems in order to study the differences in bonding nature of the two parent alkyne complexes, 1 and 2. Calculations give geometries very close to the X-ray-determined ones, and by using the GIAO method we succeed in qualitatively reproducing the experimental ¹H and ¹³C chemical shifts. Both structural and spectroscopic changes can be explained by the participation of the acetylenic second π orbital (π⊥) in the metal - alkyne bonding. As we go from 1 to 2 or from A to B, the extraction of the chloride ligand transforms the 2-electron-donor alkyne ligand to a 4-electron-donor ligand, with both the π∥ and the π⊥ orbitals donating to the metal and stabilizing the otherwise 16-electron unsaturated complex 2. Calculations also predict an increase of dissociation energies of the alkyne, and an enhancement in the energy of rotation of the alkyne, for complex B. Finally, Bader's atoms in molecules (AIM) analysis shows that differences in coordination nature are also reflected in the topological properties of electron density.

Full text of DOI:10.1021/om010645n

Organometallics 2002, 21, 305-314
305
Tw o- a n d F ou r -Electr on Alk yn e Liga n d s in
Osm iu m -Cyclop en ta d ien yl Ch em istr y: Con sequ en ces of
th e π fM In ter a ction
J orge J . Carbo´,† Pascale Crochet,‡ Miguel A. Esteruelas,*,‡ Yves J ean,*,§
Agust´ı Lledo´s,*,† Ana M. Lo´pez,‡ and Enrique On˜ate‡
Departament de Quı´mica, Universitat Auto`noma de Barcelona, 08193 Bellaterra,
Barcelona, Spain, Departamento de Quı´mica Inorga´nica, Instituto de Ciencia de Materiales de
Arago´n-Consejo Superior de Investigaciones Cientı´ficas, 50009 Zaragoza, Spain, and
Laboratoire de Chimie Physique, UMR 8000, Baˆtiment 490, Universite´ de Paris-Sud,
91405 Orsay Cedex, France
Received J uly 19, 2001
The complexes Os(η5-C5H5)Cl{η2-HCtCC(OH)R2}(PiPr3) (R ) Ph (1a ), Me (1b)) react with
TlPF6 to give [Os(η5-C5H5){η2-HCtCC(OH)R2}(PiPr3)]PF6 (R ) Ph (2a ), Me (2b)). The
structures of 1a and 2a have been determined by X-ray diffraction. The comparative study
of the data reveals a shortening of the Os-C(alkyne) distances on going from 1a to 2a ,
1
whereas the acetylenic bond lengths remain almost identical. Comparison of their H and
13C{1H} NMR spectra shows that the HCt proton resonances and the chemical shifts of the
acetylenic carbon atoms of 2a and 2b are substantially shifted toward lower field than are
those of 1a and 1b. DFT calculations were carried out on Os(η5-C5H5)Cl(η2-HCtCR)(PH3)
(R ) H (A), R ) CH3 (ACH3)) and [Os(η5-C5H5)(η2-HCtCR)(PH3)]+ (R ) H (B), R ) CH3
(BCH3)) model systems in order to study the differences in bonding nature of the two parent
alkyne complexes, 1 and 2. Calculations give geometries very close to the X-ray-determined
ones, and by using the GIAO method we succeed in qualitatively reproducing the
1
experimental H and 13C chemical shifts. Both structural and spectroscopic changes can be
explained by the participation of the acetylenic second π orbital (π ) in the metal-alkyne
bonding. As we go from 1 to 2 or from A to B, the extraction of the chloride ligand transforms
the 2-electron-donor alkyne ligand to a 4-electron-donor ligand, with both the π and the π
||
orbitals donating to the metal and stabilizing the otherwise 16-electron unsaturated complex
2. Calculations also predict an increase of dissociation energies of the alkyne, and an
enhancement in the energy of rotation of the alkyne, for complex B. Finally, Bader’s atoms
in molecules (AIM) analysis shows that differences in coordination nature are also reflected
in the topological properties of electron density.
In tr od u ction
hydrosilylation,3 oligomerization,4 polymerization,5 me-
tathesis,6 condensation of terminal alkynes with several
organic molecules (allyl alcohols,7 R,â-unsaturated ke-
tones,8 alkenes,9 dienes,10 and alkynes11), cycloisomer-
ization of 1,6-enynes,12 and hydroamination.13 In addi-
The π-alkyne complexes are some of the most impor-
tant kinds of transition-metal compounds. They are
intermediate species in terminal alkyne to vinylidene
rearrangements1 and in homogeneous and heteroge-
neous catalytic reactions, including hydrogenation,2
(3) Marciniec, B.; Gulinsky, J .; Urbaniak, W.; Kornetka, Z. W. In
Comprehensive Handbook on Hydrosilylation; Marciniec, B., Ed.;
Pergamon: Oxford, U.K., 1992.
† Universitat Auto`noma de Barcelona.
(4) Bruneau, C.; Dixneuf, P. H. Acc. Chem. Res. 1999, 32, 311.
(5) (a) Simionescu, C. I.; Percec, V. Prog. Polym. Sci. 1982, 8, 133.
(b) Masuda, T.; Higashimura, T. Acc. Chem. Res. 1984, 17, 51. (c) Katz,
T. J .; Haker, S. M.; Kendrick, R. D.; Yannoni, C. S. J . Am. Chem. Soc.
1985, 107, 2182. (d) Szymanska-Buzar, T.; Glowiak, T. Polyhedron
1998, 17, 3419. (e) Szymanska-Buzar, T.; Glowiak, T. J . Organomet.
Chem. 1999, 575, 98. (f) Szymanska-Buzar, T.; Glowiak, T. J . Orga-
nomet. Chem. 1999, 585, 215.
‡ Instituto de Ciencia de Materiales de Arago´n-Consejo Superior de
Investigaciones Cient´ıficas.
§ Universite´ de Paris-Sud.
(1) (a) Silvestre, J .; Hoffman, R. Helv. Chim. Acta 1985, 68, 1461.
(b) Lomprey, J . R.; Selegue, J . P. J . Am. Chem. Soc. 1992, 114, 5518.
(c) Wakatsuki, Y.; Koga, N.; Yamazaki, H.; Morokuma, K. J . Am. Chem.
Soc. 1994, 116, 8105. (d) de los R´ıos, I.; J ime´nez-Tenorio, M.; Puerta,
M. C.; Valerga, P. J . Chem. Soc., Chem. Commun. 1995, 1757. (e)
Wakatsuki, Y.; Koga, N.; Werner, H.; Morokuma, K. J . Am. Chem. Soc.
1997, 119, 360. (f) de los R´ıos, I.; J ime´nez-Tenorio, M.; Puerta, M. C.;
Valerga, P. J . Am. Chem. Soc. 1997, 119, 6529. (g) Bustelo, E.; J ime´nez-
Tenorio, M.; Puerta, M. C.; Valerga, P. Organometallics 1999, 18, 950.
(h) Bustelo, E.; J ime´nez-Tenorio, M.; Puerta, M. C.; Valerga, P.
Organometallics 1999, 18, 4563. (i) Puerta, M. C.; Valerga, P. Coord.
Chem. Rev. 1999, 193-195, 977.
(6) Schrock, R. R. Acc. Chem. Res. 1986, 19, 342.
(7) (a) Trost, B. M.; Flygare, J . A. J . Am. Chem. Soc. 1992, 114, 5476.
(b) Trost, B. M.; Kulawiec, R. J . J . Am. Chem. Soc. 1992, 114, 5579.
(c) Trost, B. M. Mart´ınez, J . A.; Kulawiec, R. J .; Indolese, A. F. J . Am.
Chem. Soc. 1993, 115, 10402. (d) Trost, B. M.; Vidal, B.; Thommen,
M. Chem. Eur. J . 1999, 5, 1055.
(8) Trost, B. M.; Pinkerton, A. B. J . Am. Chem. Soc. 1999, 121, 1988.
(9) Trost, B. M.; Indolese, A. J . Am. Chem. Soc. 1993, 115, 8831.
(10) (a) Trost, B. M.; Imi, K.; Indolese, A. F. J . Am. Chem. Soc. 1993,
115, 8831. (b) Trost, B. M. Chem. Ber. 1996, 129, 1313.
(11) Yi, C. S.; Liu, N. Organometallics 1998, 17, 3158.
(2) (a) Chaloner, P. A.; Esteruelas, M. A.; J oo´, F.; Oro, L. A.;
Homogeneous Hydrogenation; Kluwer Academic: Dordrecht, The
Netherlands, 1994. (b) Esteruelas, M. A.; Oro, L. A. Chem. Rev. 1998,
98, 577.
(12) Trost, B. M.; Toste, D. J . Am. Chem. Soc. 1999, 12, 9728.
10.1021/om010645n CCC: $22.00 © 2002 American Chemical Society
Publication on Web 12/20/2001

306 Organometallics, Vol. 21, No. 2, 2002
Carbo´ et al.
24
tion, they show applications in stoichiometric organic
synthesis such as 2 + 2 + 2 cycloadditions,14 quinone
synthesis,15 and complex condensations with carbenes.16
The broad range of applications of π-alkyne transi-
tion-metal complexes has also attracted the interest of
theoretical chemists. Thus, several aspects of the coor-
dination of alkynes to naked atoms,17 surfaces,18 and
complexes19 have been studied. Frenking and Fro¨hlich
have recently reviewed these investigations.20
The chemical bonding in transition-metal alkyne
complexes can be described in a way similar to that for
the transition-metal alkene complexes. The bonding is
considered to arise either from donor-acceptor interac-
tions between the alkyne ligand and the transition
metal or as a metallacyclic compound. A major differ-
ence between alkene and alkyne complexes is that the
alkyne ligand has a second occupied π orbital orthogonal
to the MC2 plane (π ) which, in some cases, engages in
the transition-metal-alkyne bonding. In that case, the
alkyne is a four-electron-donor ligand by means of its
ML2(alkyne)2 have been also reported. In contrast,
five-coordinate d6 monoalkyne complexes with the gen-
eral formula ML4(alkyne) are very scarce,25 and MCpL-
(alkyne) species are unknown.
Despite the high kinetic inertia of the OsCpL3 com-
pounds,26 we have reported overwhelming evidence
showing that the complex Os(η5-C5H5)Cl(PiPr3)2 is a
labile starting material for the development of new
cyclopentadienyl-osmium chemistry,27 including Os(η5-
C5H5)Cl(η2-alkyne)(PiPr3) complexes,28 where the alkyne
acts as a two-electron-donor ligand. We now show that
these compounds can be converted into stable [Os(η5-
C5H5)(η2-alkyne)(PiPr3)]+ species, containing a four-
electron-donating alkyne.
With regard to the complexes containing two-electron-
donor alkyne ligands, the donation from the π orbital
disturbs the molecular structure and reactivity21,29 as
well as spectroscopic properties30 of the π-alkyne com-
plex. Despite the efforts made toward an understanding
of metal-alkyne bonding,17-20 quantitative theoretical
studies on the two- vs four-electron dichotomy of the
alkyne ligands are still lacking. The discovery of the
complexes Os(η5-C5H5)Cl(η2-alkyne)(PiPr3) and [Os(η5-
C5H5)(η2-alkyne)(PiPr3)]+ has prompted us to carry out
theoretical calculations on the bonding scheme, geom-
etries and bond energies, rotational barriers, and NMR
properties in these unusual osmium compounds.
In this paper, we report the synthesis and spectro-
scopic and X-ray characterization of [Os(η5-C5H5)(η2-
alkyne)(PiPr3)]+, the X-ray characterization of Os(η5-
C5H5)Cl(η2-alkyne)(PiPr3), and the results of the the-
oretical study on the different bonding natures of the
metal-alkyne interaction.
π and π orbitals. The alkyne ligand also acts as an
||
electron acceptor by means of its π* orbitals. The π *
orbital is, however, of local a2 symmetry (within the C2v
group), which prevents it from significant interaction
with the filled d metal orbital (δ type interaction). The
only significant interaction involves the acceptor orbital
lying in the MC2 plane (π * of local b1 symmetry): i.e.,
||
the orbital already at work in the alkene complexes.
The chemistry of four-electron-donating alkynes has
been centered at early transition metals, mainly mo-
lybdenum and tungsten.21 Thus, a wide variety of six-
coordinate Mo(II) and W(II) complexes with four-electron-
donor alkynes have been synthesized.21,22 Four-coor-
dinated d6 complexes of the types ML(alkyne)3 and
23
Resu lts a n d Discu ssion
(13) Mu¨ller, T. E.; Beller, M. Chem. Rev. 1998, 98, 675.
(14) Vollhardt, K. P. C. J . Org. Chem. 1984, 59, 1574.
(15) Liebeskind, L. S.; J ewell, C. F., J r. J . Organomet. Chem. 1985,
285, 305.
(16) (a) Dotz, K. H. Angew. Chem., Int. Ed. Engl. 1984, 23, 587. (b)
Wulf, W. D.; Gilbertson, S. R.; Springer, J . P. J . Am. Chem. Soc. 1986,
108, 520.
(17) (a) Miralles-Sabater, J .; Mercha´n, M.; Nebot-Gil, I.; Viruela-
Mart´ın, P.M. J . Phys. Chem. 1988, 92, 4853. (b) Sodupe, M.; Baus-
chlicher, G. W. J . Phys. Chem. 1991, 95, 8640. (c) Sodupe, M.;
Bauschlicher, G. W., J r.; Langhoff, S. R.; Partridge, H. J . Phys. Chem.
1992, 96, 2118. (d) Mitchell, S. A.; Blitz, M. A.; Fournier, R. Can. J .
Chem. 1994, 72, 587. (e) Bo¨hme, M.; Wagener, T.; Frenking, G. J .
Organomet. Chem. 1996, 520, 31.
1. Syn th esis a n d X-r a y a n d Sp ectr oscop ic Ch a r -
acter ization of Os(η5-C5H5)Cl(η2-alkyn e)(P iP r 3) an d
[Os(η5-C5H5)(η2-a lk yn e)(P iP r 3)]+. As a consequence
of the large steric hindrance experienced by the triiso-
propylphosphine ligands of Os(η5-C5H5)Cl(PiPr3)2, in
pentane, the splitting of a phosphorus-osmium bond
is favored. Thus, the addition at room temperature of
1.2 equiv of 1,1-diphenyl-2-propyn-1-ol and 2-methyl-
(24) See for example: (a) Do¨tz, K. H.; Mu¨klemeier, J . Angew. Chem.,
Int. Ed. Engl. 1982, 21, 929. (b) Salt, J . E.; Girolami, G. J .; Wilkinson,
G.; Motevolli, M.; Thornton-Pett, M.; Hurthouse, M. B. J . Chem. Soc.,
Dalton Trans. 1985, 685. (c) Wink, D. J .; Fox, J . R.; Cooper, J . J . Am.
Chem. Soc. 1985, 107, 5012.
(25) (a) Wink, D. J .; Creagan, T. J . Am. Chem. Soc. 1990, 112, 8585.
(b) Ishiro, H.; Kuwata, S.; Ishii, Y.; Hidai, M. Organometallics 2001,
20, 13.
(18) Sellers, H. J . Phys. Chem. 1990, 94, 8329.
(19) (a) Kitaura, K.; Sakaki, S.; Morokuma, K. Inorg. Chem, 1981,
20, 2292. (b) Ziegler, T. Inorg. Chem. 1985, 24, 1547. (c) Nielson, A.
J .; Boyd, P. D. W.; Clark, G. R.; Hunt, T. A.; Metson, J . B.; Rickard, C.
E. F.; Schwerdtfeger, P. Polyhedron 1992, 11, 1419. (d) Stegmann, R.;
Neuhaus, A.; Frenking, G. J . Am. Chem. Soc. 1993, 115, 11930. (e)
Pidun, U.; Frenking, G. Organometallics 1995, 14, 5325. (f) Li, J .;
Schreckenbach, G.; Ziegler, T. Inorg. Chem. 1995, 34, 3245. (g) Pidun,
U.; Frenking, G. J . Organomet. Chem. 1996, 525, 269. (h) Frenking,
G.; Pidun, U. J . Chem. Soc., Dalton Trans. 1997, 1653. (i) Hyla-
Kriyspin, I.; Koch, J .; Gleiter, R.; Klettke, T.; Walther, D. Organome-
tallics 1998, 17, 4724. (j) Decker, S. A.; Klobukowski, M. J . Am. Chem.
Soc. 1998, 120, 9342. (k) Kova´cs, A.; Frenking, G. Organometallics
1999, 18, 887.
(20) Frenking, G.; Fro¨hlich, N. Chem. Rev. 2000, 100, 717.
(21) (a) Templeton, J . L. Adv. Organomet. Chem. 1989, 29, 1. (b)
Baker, P. B. Adv. Organomet. Chem. 1996, 40, 45.
(22) (a) Frohnapfel, D. S.; Reinartz, S., White, P. S.; Templeton, J .
L. Organometallics 1998, 17, 3759. (b) Frohnapfel, D. S.; Enriquez, A.
E.; Templeton, J . L. Organometallics 2000, 19, 221.
(26) Atwood, J . D. Inorganic and Organometallic Reaction Mecha-
nisms; VCH: New York, 1997; Chapter 3.
(27) (a) Esteruelas, M. A.; Go´mez, A. V.; Lo´pez, A. M.; Oro, L. A.
Organometallics 1996, 15, 878. (b) Crochet, P.; Esteruelas, M. A.;
Lo´pez, A. M.; Ruiz, N.; Tolosa, J . I. Organometallics 1998, 17, 3479.
(c) Crochet, P. Esteruelas, M. A.; Gutie´rrez-Puebla, E. Organometallics
1998, 17, 3141. (d) Baya, M.; Crochet, P.; Esteruelas, M. A.; Gutie´rrez-
Puebla, E.; Lo´pez, A. M.; On˜ate, E.; Tolosa, J . I. Organometallics 2000,
19, 275. (e) Baya, M.; Crochet, P.; Esteruelas, M. A.; Gutie´rrez-Puebla,
E.; Lo´pez, A. M.; Modrego, J .; On˜ate, E.; Vela, N. Organometallics 2000,
19, 2585. (f) Esteruelas, M. A.; Lo´pez, A. M.; Tolosa, J . I.; Vela, N.
Organometallics 2000, 19, 4650. (g) Baya, M.; Crochet, P.; Esteruelas,
M. A.; On˜ate, E. Organometallics 2001, 20, 240.
(23) See for example: (a) Tate, D. P.; Augl, J . M. J . Am. Chem. Soc.
1963, 85, 2174. (b) Wink, D. J .; Creagan, T. Organometallics 1990, 9,
328. (c) Szymanska-Buzar, T.; Glowiak, T. J . Organomet. Chem. 1994,
467, 233. (d) Yeh, W.-Y.; Ting, C.-S. Organometallics 1995, 14, 1417.
(e) Mealli, C.; Masi, D.; Galindo, A.; Pastor, A. J . Organomet. Chem.
1998, 569, 21.
(28) Esteruelas, M. A.; Lo´pez, A. M.; Ruiz, N.; Tolosa, J . I. Orga-
nometallics 1997, 16, 4657.
(29) Frohnapfel, D. S.; Templeton, J . L. Coord. Chem. Rev. 2000,
206-207, 199.
(30) Templetom, J . L.; Ward, B. C. J . Am. Chem. Soc. 1980, 102,
3288.

Osmium-Cyclopentadienyl Chemistry
Organometallics, Vol. 21, No. 2, 2002 307
The geometry around the osmium atom of 1a can be
described as a three-legged piano stool. The angles
P-Os-Cl, P-Os-M(2) (M(2) is the midpoint of the
carbon-carbon triple bond of the alkyne), and Cl-Os-
M(2) are 86.85(6), 86.73(18), and 103.8(2)°, respectively.
The carbon-carbon triple bond (C(1)-C(2)) forms an
angle of 28° with the Os-Cl bond. The torsion angle
Cl-Os-C(1)-C(2) is 156.3°, whereas the torsion angle
P-Os-C(1)-C(2) is 112°. As expected, the coordination
of the alkyne to the metal has a slight effect on the
acetylenic bond length. Thus, the C(1)-C(2) distance
(1.222(8) Å) is about 0.04 Å longer than the average
value in free alkynes (1.18 Å).31 The Os-C(1) (2.142(7)
Å) and Os-C(2) (2.163(6) Å) bond lengths are statisti-
cally identical. In addition, it should be mentioned that
the substituted carbon atom of the alkyne (C(2)) is
pointed away from the bulky triisopropylphosphine
ligand. Although two isomers of 1a could be formed in
the solid state, this indicates that only one of them is
obtained, the isomer with less steric hindrance.
The geometry around the osmium atom of 2a can be
rationalized as a two-legged piano stool with the acetyl-
enic C(1)-C(2) bond and the Os-P axis in the same
plane. The P-Os-C(1)-C(2) torsion angle is 183.7°.
Interestingly, although the C(1)-C(2) bond lengths in
2a (1.26(2) Å) and 1a are very similar, there are
substantial differences between the respective Os-
carbon distances of both compounds. The Os-C(1)
distance in 2a (1.992(9) Å) is about 0.15 Å shorter than
the related parameter in 1a , whereas the Os-C(2) bond
length in 2a (1.981(8) Å) is about 0.18 Å shorter than
that in 1a . As in 1a , the substituted C(2) carbon atom
of 2a is pointed away from the phosphine ligand.
F igu r e 1. Molecular diagrams for the complex Os(η5-
C5H5)Cl{η2-HCtCC(OH)Ph2}(PiPr3) (1a ) and for the cation
of [Os(η5-C5H5){η2-HCtCC(OH)Ph2}(PiPr3)]PF6 (2a ). Ther-
mal ellipsoids are shown at 50% probability.
There are also substantial differences between the 1H,
13C{1H}, and 31P{1H} NMR spectra for complexes of
3-butyn-2-ol to pentane solutions of this complex gives
rise to the formation of the π-alkyne derivatives Os(η5-
C5H5)Cl{η2-HCtCC(OH)R2}(PiPr3) (R ) Ph (1a ), Me
(1b)). In toluene solutions, at 85 °C, complexes 1a and
1b evolve into the allenylidene (Os(η5-C5H5)Cl(dCdCd
CPh2}(PiPr3))27b and alkenylvinylidene (Os(η5-C5H5)Cl-
{dCdCHC(CH3)dCH2}(PiPr3)) complexes,28 respec-
tively, with loss of a water molecule from the alkyne.
Treatment at room temperature of dichloromethane
solutions of 1a and 1b with TlPF6 produces the extrac-
tion of the chloride ligand and the formation of [Os(η5-
C5H5){η2-HCtCC(OH)R2}(PiPr3)]PF6 (R ) Ph (2a ), Me
(2b)), which are isolated in high yield (eq 1). In contrast
1
types 1 and 2. In the H NMR spectra of 1a and 1b the
HCt resonances of the alkynes appear at 4.3 and 3.73
ppm, respectively, as doublets with H-P coupling
constants of about 9 Hz. The same HCt resonances for
complexes 2a and 2b appear also as doublets, but at
lower field (about 9.3 ppm) and with H-P coupling
constants (26.4 Hz for both compounds) much higher
than those found in 1a and 1b. A similar relationship
is observed in the 13C{1H} NMR spectra. In the 13C-
{1H} NMR spectra of 1a and 1b, the resonances corre-
sponding to the acetylenic carbon atoms appear at 82.2
and 69.1 ppm (tCR) and at 57.532 and 49.6 ppm (HCt
), while in the spectra of 2a and 2b they are observed
at 179.0 and 182.8 ppm (tCR) and at 146.0 and 143.3
ppm (HCt): i.e., shifted about 100 ppm toward low
field. The 31P{1H} NMR spectra also indicate that the
electronic properties of osmium atoms in compounds of
types 1 and 2 are different. While the chemical shifts
of the singlets of 1a and 1b are 10.0 and 9.5 ppm,
respectively, those of 2a and 2b are 38.0 and 36.3 ppm.
These differences indicate that the alkyne ligand of 1a
and 1b acts as a two-electron-donor ligand, while in 2a
and 2b it acts as a four-electron-donor ligand.30
to 1a and 1b, 2a and 2b are stable in solution for a long
time. The dehydration of the alkynes, as well as their
transformation into the corresponding allenylidene and
alkenylvinylidene species, is not observed.
2. Com p u ta tion a l Stu d ies on th e Alk yn e Com -
p lexes. The differences in the alkyne-osmium bonding
(31) Allen, F. H.; Kennard, O.; Watson, D. G.; Brammer, L.; Orpen,
A. G.; Taylor, R. J . Chem. Soc., Perkin Trans. 1987, S1.
(32) This chemical shift was erroneously reported as 122.9 ppm in
ref 27b.
Figure 1 shows the X-ray structures of 1a and 2a ,
whereas selected bond distances and angles for both
compounds are listed in Table 1.

308 Organometallics, Vol. 21, No. 2, 2002
Carbo´ et al.
Ta ble 1. Selected Bon d Dista n ces (Å) a n d An gles (d eg) for th e Com p lexes
Os(η5-C5H5)Cl{η2-HCtCC(OH)P h 2}(P iP r 3) (1a ) a n d [Os(η5-C5H5){η2-HCtCC(OH)P h 2}(P iP r 3)]P F 6 (2a ).
1a
2a
1a
2a
Os-Cl
2.4445(16)
2.3444(17)
2.142(7)
2.163(6)
2.237(6)
2.246(7)
Os-C(27)
Os-C(28)
Os-C(29)
C(1)-C(2)
C(2)-C(3)
2.190(7)
2.203(7)
2.246(7)
1.222(8)
1.499(9)
2.180(11)
2.205(9)
2.205(9)
1.26(2)
Os-P
2.410(2)
1.992(9)
1.981(8)
2.222(7)
2.220(9)
Os-C(1)
Os-C(2)
Os-C(25)
Os-C(26)
1.513(11)
Cl-Os-P
86.85(6)
115.5(3)
103.8(2)
129.6(2)
86.73(18)
121.0(3)
Os-C(1)-C(2)
Os-C(1)-H(1)
Os-C(2)-C(1)
Os-C(2)-C(3)
H(1)-C(1)-C(2)
C(1)-C(2)-C(3)
74.4(4)
125(5)
72.6(5)
137.8(5)
154(5)
71.1(6)
141(4)
72.0(5)
147.6(7)
145(4)
Cl-Os-M(1)a
Cl-Os-M(2)a
P-Os-M(1)
P-Os-M(2)
M(1)-Os-M(2)
119.2(4)
99.2(4)
141.7(4)
148.8(7)
140.3(9)
a
M(1) and M(2) are the midpoints of the C(25)-C(29) Cp and C(1)-C(2) acetylenic ligands.
in compounds 1 and 2 has been studied by means of
DFT calculations (B3LYP functional), in conjunction
with Bader’s atoms in molecules (AIM) theory. The
study has been performed using Os(η5-C5H5)Cl(η2-HCt
CH)(PH3) (A) and [Os(η5-C5H5)(η2-HCtCH)(PH3)]+ (B)
as model systems of complexes of types 1 and 2,
respectively. Additionally, we have also considered Os-
(η5-C5H5)Cl(η2-HCtCCH3)(PH3) (ACH3) and [Os(η5-C5H5)-
(η2-HCtCCH3)(PH3)]+ (BCH3) in order to model the
substituted alkynes.
F igu r e 2. Schematic representation of the donative (a and
b) and back-donative (c) interactions for metal-alkyne
bonding in the complex [Os(η5-C5H5)(η2-HCtCH)(PH3)]+
(B).
Bon d in g Sch em e. The way the alkyne ligand binds
the metal center in complexes A and B can be analyzed
first by using a fragment MO analysis.33 In the former,
the alkyne interacts with a 16-electron metal fragment
of d6-[Os(η5-C5H5)LL′] type. Schilling et al. have shown
that, in such complexes, only the π| orbital of the alkyne
interacts with a vacant d orbital on the metal center,34
this 2-electron-donor behavior leading to an 18-electron
alkyne complex. A back-donation interaction, involving
π|*, is also at work and was found to be responsible for
the orientation of the alkyne with respect to the metal
fragment. Complex B can be described as a 14-electron
fragment of d6-[Os(η5-C5H5)L]+ type interacting with an
alkyne ligand in a geometry where the P, Os, C1, and
C2 atoms are coplanar. There are now two vacant d
orbitals on the metal fragment, one symmetrical and
one antisymmetrical with respect to the molecular
symmetry plane. The symmetry properties of these two
empty d orbitals match those of the occupied π| and π
orbitals (Figure 2a,b) so that the alkyne acts as a
4-electron-donor ligand, leading to an 18-electron com-
plex. Note that a further stabilization results from the
back-donation interaction which involves the π|* orbital
(Figure 2c). Similar orbital interaction schemes have
been derived by Hoffmann and co-workers for d4 mo-
lybdenum systems35 and, more recently, by Decker and
Klobukowski for the M(CO)3(C2H2) complexes (M ) Fe,
Ru).19j In each case, the participation of both the π| and
π orbitals in donating electrons to the metal allows the
complex to reach the 18-electron count. This qualitative
analysis, derived from the DFT-computed molecular
orbitals, is further supported by NBO (natural bonding
orbitals) population analysis carried out on complexes
A and B. The population of the alkyne π NBO orbital
is significantly lower in B (1.679e) than in A (1.982e),
showing that π is involved in the alkyne-metal bond-
ing in B but not in A. Note that, in both complexes, the
participation of π * is almost negligible, the NBO
populations being equal to 0.017e and 0.042 e in A and
B, respectively.
This qualitative analysis shows that the alkyne ligand
acts as a two- and a four-electron donor in complexes A
and B, respectively. Therefore, these two parent com-
plexes provide the opportunity to study how the geo-
metrical, electronic, and energetic properties of an
alkyne complex depend on the donor behavior of that
ligand.
Geom etr ies a n d Bon d En er gies. The computed
optimized geometries of complexes Os(η5-C5H5)Cl(η2-
HCtCR)(PH3) (R ) H (A), CH3 (ACH3)) and [Os(η5-
C5H5)(η2-HCtCR)(PH3)]+ (R ) H (B), CH3 (BCH3)) are
presented in Figure 3. In A, the CtC acetylene bond
forms an angle of about 25° with the Os-Cl bond. The
computed torsion angle between P-Os-C1-C2 atoms
is 106.0° for complex A, while for B it is 179.9°,
indicating that acetylenic carbon, osmium, and phos-
phorus atoms are almost coplanar in complex B. These
results agree with the alkyne conformations of the
X-ray-determined structures. The differences in confor-
mational preferences of the two complexes will be
commented upon later. In the case of substituted
alkynes (R ) CH3), two different isomers have been
considered for each complex (ACH3 and BCH3), depending
on which acetylenic carbon is substituted. Thus, the
methyl substituent can lie in the phosphine ligand side
(endo) or in the opposite side (exo). The endo isomers
(An CH3 and Bn CH3) are 0.8 and 2.8 kcal mol-1 higher
in energy than their respective exo forms (AxCH3 and
BxCH3) for complexes ACH3 and BCH3, respectively. This
(33) Albright, T. A.; Burdett, J . K.; Whangbo, M. H. Orbital
Interactions in Chemistry; Wiley: New York, 1985.
(34) (a) Schilling, B. E. R.; Hoffmann, R.; Faller, J . W. J . Am. Chem.
Soc. 1979, 101, 585. (b) Schilling, B. E. R.; Hoffmann, R.; Faller, J . W.
J . Am. Chem. Soc. 1979, 101, 592.
(35) Tatsumi, K.; Hoffmann, R.; Templeton, J . L. Inorg. Chem. 1982,
21, 466.

Osmium-Cyclopentadienyl Chemistry
Organometallics, Vol. 21, No. 2, 2002 309
It compares nicely with the geometry changes previously
reported between Os(CO)4(C2H2) and Os(CO)3(C2H2)
complexes19j at similar levels of calculation. In the
former (two-electron-donor acetylene ligand), the Os-
C(acetylene) bond distance is longer than in the latter
(four-electron-donor acetylene ligand) by 0.189 Å (2.220
vs 2.031 Å), while the C-C distance is shorter by 0.053
Å (1.276 vs 1.329 Å). The bond length values for tetra-
and tricarbonyl species are actually close to our com-
puted values for complexes A and B, respectively (Table
2).
The coordination of the alkyne ligand induces not only
a lengthening of the C1-C2 distance (1.18 Å in free
alkyne)31 but also a bending of the substituents away
from the metal. The calculated bond angles of C2-
C1-H moieties are 153° for complex A and 146° for B,
in excellent agreement with the experimental values for
1a (154(5)°) and 2a (145(4)°). Finally, note that no
significant differences in geometrical parameters were
observed upon introduction of a methyl substituent onto
the acetylene ligand (AxCH3 and BxCH3).
It may be stated from the geometrical analysis that
the behavior of the alkyne ligand as a donor of two or
four electrons (2e or 4e) is reflected in both metal-
carbon and carbon-carbon bond distances. Thus, when
the alkyne acts as a 4e-donor ligand there is a contrac-
tion of the M-C bond, and the C-C bond is slightly
elongated. The average experimental values of C-C
distances in the coordinate terminal alkynes are 1.271
and 1.309 Å for 2e- and 4e-donor ligands, respectively.36
It is interesting to notice that the X-ray-determined
C-C distances for our osmium systems are shorter than
these average values. Furthermore, the C-C distance
of complex 2a , in which the alkyne acts as a 4e-donor
ligand, is in the range of the average value for 2e-donor
ligands. The reliability of the C-C alkyne distance as
an indicator of 2e- or 4e-donor ligand was studied by
computing the monodimensional potential energy sur-
face (PES) for the C-C coordinate in A and B com-
plexes. The PES was built by varying the C-C distances
from their equilibrium values and optimizing the rest
of the complex at each point. The C-C distances that
can be reached within an energy excess of 3 kcal mol-1
with respect to the minimum energy structure range
from 1.209 to 1.333 Å for complex A and from 1.243 to
1.378 Å for complex B. This is what we might call the
flexibility range of the CtC bond. The calculated PES
is flat (Figure 4), and therefore, a wide range of C-C
distances can be reached with low energy cost. Thus,
the equilibrium distance of A (1.265 Å) is within the
range of flexibility of B, and at the same time, the
equilibrium distance of B (1.299 Å) is within the range
of flexibility of complex A. The calculations show clearly
that the CtC bond lengths are not a reliable indicator
of the metal-alkyne coordination mode.
F igu r e 3. Optimized B3LYP geometries of the alkyne
complexes Os(η5-C5H5)Cl(η2-HCtCR)(PH3) (R ) H (A), CH3
(ACH3)) and [Os(η5-C5H5)(η2-HCtCR)(PH3)]+ (R ) H (B),
CH3 (BCH3)).
Ta ble 2. Exp er im en ta l a n d Ca lcu la ted Geom etr ic
P a r a m eter sa a n d Ca lcu la ted Bon d Dissocia tion
En er giesb
param
1a
A
AxCH3
2a
B
BxCH3
Os-C1
2.142(7) 2.176 2.168 1.992
2.014 2.000
Os-C2
2.163(6) 2.150 2.182 1.981(8) 1.991 2.004
1.222(8) 1.265 1.266 1.26(2) 1.299 1.305
148.8(7) 152.6 152.4 140.3(9) 146.2 145.9
C1-C2
C1-C2-R
C2-C1-H
154(5)
152.6 152.0 145(4)
106.0 105.7 183.7
23.0 21.1
146.0 145.5
179.9 180.1
69.6 71.1
P-Os-C1-C2 112
De
a
b
Bond legths in Å and bond angles in degrees. Alkyne-
osmium bond dissociation energies (De) in kcal mol-1
.
result agrees with the experimental data, since the
X-ray structures of complexes 1a and 2a correspond to
exo forms. In the following, the discussion on substituted
acetylenes will therefore concentrate on the exo isomers.
In Table 2 are summarized the most relevant theo-
retical parameters, together with those reported for
experimental complexes 1a and 2a . Optimized geom-
etries were found to be close to experimental ones for
both alkyne complexes. The most interesting aspect of
our computed geometrical parameters is that we suc-
ceeded in reproducing structural changes between com-
plexes 1a and 2a . As we go from A to B the Os-C1 and
Os-C2 distances decrease by 0.162 and 0.159 Å, re-
spectively, and C1-C2 increases by only 0.034 Å. These
results reflect the two-electron vs four-electron behavior
of the alkyne ligand in complexes A and B, respectively.
Table 2 gives also the theoretically predicted alkyne
bond dissociation energies (De) of A, B, AxCH3, and
BxCH3. The bond energies are calculated as the energy
difference between the complex, on one hand, and the
ligand and the metal fragment at their respective
optimized geometries, on the other hand. The predicted
(36) Orpen, A. G.; Brammer, L.; Allen, F. H.; Kennard, O.; Watson,
D. G.; Taylor, R. J . Chem. Soc., Perkin Trans. 1989, S1.

310 Organometallics, Vol. 21, No. 2, 2002
Carbo´ et al.
decrease of the back-donation to the π|* orbital. Ar ot
remains, however, an 18-electron species in which the
acetylene acts as a 2-electron-donor ligand.
The situation is more complicated for complex B, since
two saddle points were characterized for the rotational
process of acetylene. In the first one (Br ot, Figure 5),
the optimized geometrical parameters are rather similar
to those given just above for Ar ot, with Os-C(acetylene)
distances of 2.186 and 2.283 Å and a C-C distance of
1.249 Å. Despite this geometrical similarity, the com-
puted energy barrier is much higher, since Br ot is
located 32.7 kcal mol-1 above the minimum energy
structure B. This remarkably high value for the rota-
tional barrier can be explained by using the symmetry
properties of the fragment molecular orbitals displayed
in Figure 2. We have previously mentioned that in the
equilibrium structure B the filled π orbitals, π| and π ,
are respectively symmetric (a′) and antisymmetric (a′′)
with respect to the molecular symmetry plane, so that
each of them can interact with one of the two low-lying
d orbitals of the metal fragment (π with a′′ and π| with
1a′; Figure 2). Therefore, in the equilibrium structure,
the acetylene ligand plays the role of a four-electron
donor. In the transition state structure, the acetylene
is rotated by 90°. Assuming an idealized Cs symmetry,
both π| and π become symmetric, so that the overlap
between π and the a′′ metal fragment orbital vanishes.
The electron donation from π is thus lost, and the only
significant interaction which remains is that between
π| and the 1a′ orbital. The high value computed for the
rotational barrier can thus be traced to the change of
the electron donor character of the acetylene ligand on
going from B (4-electron, 18-electron species) to Br ot
(2-electron, 16-electron species). This qualitative analy-
sis, consistent with the evolution of the geometrical
parameters, is further supported by the NBO analysis.
The population of the alkyne π NBO is significantly
smaller for complex B (1.679e) than for the transition
state Br ot (1.984e), in which this orbital is almost full.
These results show that π is involved in alkyne-metal
bonding in the stable complex, B, but not in the Br ot
structure.
F igu r e 4. Potential energy surface for the CtC distance
in Os(η5-C5H5)Cl(η2-HCtCH)(PH3) (O, A) and [Os(η5-C5H5)-
(η2-HCtCH)(PH3)]+ (b, B) model systems.
De for B (69.6 kcal mol-1) is substantially higher than
De for the parent complex A (23.0 kcal mol-1). The
stronger De of acetylene in B than in A can be attributed
to the bond contribution of the acetylene second π
system (π ) in the former. Similar bond energies were
calculated for methyl-substituted acetylene: 21.1 and
71.1 kcal mol-1 for AxCH3 and BxCH3, respectively. It
should be mentioned that the computed value of De for
complex B (69.6 kcal mol-1) is higher than those usually
reported in previous theoretical studies on alkyne-
metal bonding.20 However, in a recent contribution
similar values were calculated at the CCSD(T) level for
Ni(C2H2)2 and Ni(PH3)2(C2H2)2 complexes: 66.9 and 62.6
kcal mol-1, respectively.19i For both nickel complexes,
this suggested a bonding contribution of acetylene’s
second π system. A bond-length/bond-strength correla-
tion between the two acetylene complexes A and B is
observed. The Os-C1 and Os-C2 bond distances of B
are shorter than those of A, and the metal-acetylene
De value of B is much higher than that of A (Table 2).
Thus, a shortening of the osmium-alkyne distance
increases the metal-ligand interaction. Therefore, we
can state that the 4e alkyne ligands are more strongly
bonded to the metal than their respective 2e alkyne
ligands, due to the bonding contribution of the second
π system of the alkyne.
As mentioned above, a second transition state was
found for the acetylene rotation (Br ot′, Figure 5). Its
geometry strongly differs from that of Br ot: the Os-
C(acetylene) distances (1.930 and 1.929 Å) are shortened
about 0.3 Å, and the C-C distance (1.362 Å) is length-
ened by 0.113 Å. Note that this latter value is greater
than that for free ethylene (1.336 Å). These geometrical
features clearly indicate that the nature of the metal-
acetylene bonding in Br ot′ cannot be described in terms
of donor-acceptor interactions following the Dewar-
Chatt-Duncanson model.37 It is better to consider Br ot′
as a metallacycle with two Os-C single bonds and a
double bond between the carbon atoms, instead of an
alkyne complex. Last but not least, Br ot′ was found to
Rota tion a l Ba r r ier s. The transition states for the
acetylene rotation were located for both A and B
complexes and characterized by calculation of the Hes-
sian matrix.
In complex A, the transition state (Ar ot, Figure 5)
was found to be located 11.4 kcal mol-1 above the
minimum. In this structure, the dihedral angle between
CtC and Os-Cl bonds is equal to 50.7°. The values of
the Os-C(acetylene) distances are 2.300 and 2.198 Å,
and the CtC distance is 1.245 Å. Comparison with the
geometry of the minimum energy structure A (Table 2)
shows that the rotation induces a lengthening of the
M-C distances accompanied by a shortening of the C-C
distance. These changes between A and Ar ot can be
rationalized by the orbital arguments developed by
Hoffmann et al.:34 the lengthening of the Os-C dis-
tances upon rotation of the acetylene ligand results from
the increase of the Os-π repulsive interaction, while
the shortening of the CtC bond is related to the
be lower in energy than Br ot by 8.4 kcal mol-1
.
Consequently, the lowest energy path for the rotation
of the alkyne ligand in complex B involves the metal-
lacycle Br ot′ as a transition state structure. The activa-
(37) (a) Dewar, M. J . S. Bull. Soc. Chim. Fr. 1951, 18, C79. (b) Chatt,
J .; Duncanson, L. A. J . Chem. Soc. 1953, 2939.

Osmium-Cyclopentadienyl Chemistry
Organometallics, Vol. 21, No. 2, 2002 311
F igu r e 5. Optimized B3LYP geometries of Ar ot, Br ot, and Br ot′ structures. Bond distances are given in Å.
Ta ble 3. NMR 13C a n d 1H Ch em ica l Sh ifts (p p m )
Rela tive to TMS, a n d NP A Ch a r gesa
param
1a
1b
A
AxCH3
2a
2b
B
BxCH3
δ(13C) C1 57.5 49.6 68.7
57.7
146.0 143.3 145.3 144.8
δ(13C) C2 82.2 69.1 104.2 102.6 179.0 182.8 157.3 175.0
δ(1H) Hb 4.32 3.72 4.4
3.7
-0.08 -0.10
0.04
0.25
9.43 9.9
8.9
8.8
q(C1)c
q(C2)c
q(H)
-0.04 -0.05
0.00
0.25
0.05
0.28
0.10
0.28
a
b
Charges in atomic units. Chemical shifts for terminal acetyl-
enic hydrogens. c The charges of the substituents have been added
to the charges of acetylenic carbons.
shortening of the M-C(acetylene) distances (antibond-
ing M-C f bonding M-C) and a lengthening of the
C-C distance (bonding C-C f antibonding C-C) on
going from Br ot to Br ot′.
NMR P r op er ties. We have carried out theoretical
studies on the NMR properties of model complexes A,
AxCH3, B, and BxCH3 by using the gauge-including
atomic orbitals (GIAO) method.38 The 13C and 1H
chemical shifts were calculated with respect to tetra-
methylsilane. Recently, the GIAO method has been
successfully used in the study of NMR properties of
acetylenes coordinated to a transition metal by Walther
and co-workers.19i
Table 3 collects the experimental and calculated
chemical shifts, as well as the NPA charges on acetyl-
enic carbons. The calculated chemical shifts are in the
range found experimentally and reproduce the trend
observed on going from 1 to 2. Calculated δ(13C) values
for B (145.3 and 157.3 ppm) are significantly higher
than those for A (68.7 and 104.2 ppm). The same trend
is observed for the calculated δ(1H) values: 4.4 ppm for
A and 8.9 ppm for B. When a methyl group replaces
one of the acetylenic hydrogens (AxCH3 and BxCH3), δ-
(13C) and δ(1H) still better fit the experimental values.
The NMR properties are very sensitive to electronic
variations, and therefore, the use of a simple alkyne
model could be one source of error on computed chemical
shifts, especially for substituted acetylenic carbons.
However, we have succeeded in reproducing qualita-
tively the differences between a two-electron-donor and
a four-electron-donor alkyne complex.
F igu r e 6. Schematic drawing of the frontier molecular
orbital crossing found on going from Br ot to Br ot′.
tion energy remains high (24.3 kcal mol-1) compared
to that for complex A, because Br ot′ is still a 16-electron
species.
Br ot and Br ot′ can be seen as two structures along
the reaction path for the perpendicular approach of an
acetylene molecule toward the CpOs(PH3)+ metal frag-
ment. Since both have been characterized as saddle
points for the acetylene rotational process, this means
that an energy barrier should be encountered on going
from Br ot to Br ot′ by decreasing the Os-acetylene
distance. The origin of this barrier, which makes it
possible to optimize separately an acetylene (Br ot) and
a metallacyclopropene (Br ot′) complex, lies in a change
of the ground electronic configuration, schematically
depicted in Figure 6, which in turn reflects the different
chemical natures of the two complexes. Let us assume
an idealized Cs symmetry to analyze the origin of this
orbital crossing. In Br ot (“long” Os-C distance), the
HOMO (a′) is the antibonding combination of the filled
π
ligand orbital with a filled metal fragment orbital
and the LUMO (a′′) a bonding combination of π|* with
the filled a′′ orbital on the metal fragment (Figure 6,
right-hand side). Going from Br ot to Br ot′ entails a
shortening of Os-C distances and a lengthening of the
C-C distance. The HOMO (a′), Os-C antibonding and
C-C bonding, is thus destabilized, while the LUMO
(a′′), Os-C bonding and C-C antibonding, is stabilized.
A crossing occurs between these occupied and vacant
molecular orbitals, which results in an energy barrier
between these two structures (reaction path “forbidden
by symmetry”). Note that in turn the change from a′2
to a′′2 ground-state configuration is consistent with a
Additionally, we have computed the 13C and 1H
chemical shifts of the saddle points for acetylene rota-
tion (Ar ot, Br ot and Br ot′). The average values of the
calculated δ(13C) for acetylenic carbons of Ar ot species
(77.8 ppm) and Br ot species (69.7 ppm) are similar and
are more closely related to complex A than to complex
B (Table 3). On the other hand, the average value for
(38) Shreckenbach, G.; Ziegler, T. J . Phys. Chem. 1995, 99, 606.

312 Organometallics, Vol. 21, No. 2, 2002
Carbo´ et al.
Ta ble 4. Top ologica l P r op er ties of th e Electr on
Den sity a t th e Bon d a n d Rin g Cr itica l P oin tsa
Br ot′ (249.3 ppm) is much higher than those computed
for A and B complexes. These results indicate that the
bonding scheme in Ar ot and Br ot structures resembles
that in complex A, in which the acetylene ligand acts
as a two-electron donor, while the scheme in Br ot′
differs from those in A and B. Thus, computed NMR
properties further support the arguments stated in the
previous section about the bonding nature of rotational
structures.
A
AxCH3 Ar ot
B
BxCH3 Br ot Br ot′
Os-C1 Fcp
0.097 0.099 0.075 0.138 0.142 0.075 0.165
2
∇ Fcp 0.158 0.143 0.161 0.185 0.177 0.170 0.272
Os-C2 Fcp 0.101 0.096 0.090 0.143 0.140 0.091 0.164
∇ Fcp 0.180 0.174 0.153 0.212 0.205 0.155 0.276
C1-Os- Fcp 0.090 0.089 0.074 0.124 0.124 0.075 0.144
C2b
2
2
∇ Fcp 0.360 0.348 0.276 0.472 0.469 0.248 0.505
C1-C2 Fcp 0.377 0.376 0.388 0.363 0.359 0.373 0.328
The 13C chemical shift values are roughly indicative
of the electronic density around that atom. The atom
most shifted is the least shielded one and, consequently,
the poorest in electrons. Thus, one could expect a
correlation between the atomic charges and the chemi-
cal shift of the acetylenic coordinated carbons, in such
a way that the least charged carbon is the most shifted.
The calculated NPA charges of acetylenic carbons are
consistent with this argument for a given complex
(Table 3). Also, NPA charges show that acetylenic
carbons are less charged for the 4e donor acetylene
complex B (-0.04e and 0.05e) than for the 2e donor A
(-0.08e and 0.04e), the B complex exhibiting the most
shifted carbons. Despite variations in atomic carbon
charges, the small differences (e0.04e) do not seem to
justify the considerable variation in chemical shifts
observed for the two bonding situations. Note, however,
that the two complexes carry different total net charges,
which could mask the differences in atomic charges,
hindering a direct comparison between them. Apart
from atomic charges, chemical shifts may be also related
to the occupancies of acetylenic molecular orbitals. From
the NBO analysis we found that in complex B the π
orbital is significantly depopulated (1.679e), while that
for complex A is full (1.982e). Thus, in line with previous
arguments, when the alkyne ligand acts as a four-
electron donor there is a decrease of electron population
on acetylenic carbons, which become more deshielded
and, consequently, appear at low-field resonance: i.e.,
highly shifted.
2
∇ Fcp -1.041 -1.095 -1.156 -1.041 -1.021 -1.096 -0.856
ꢀ
0.189 0.210 0.183 0.024 0.030 0.145 0.075
a
2
Electron charge density Fcp, Laplacian ∇
Fcp, and ellipticity
b
ꢀ, in atomic units. Ring critical point.
Br ot′ structure. Thus, according to AIM theory, the
relatively low and positive value of the Laplacian at the
bcp indicates a closed-shell interaction. On the other
2
hand, for CtC bonds the values of ∇ Fcp (large and
negative) are indicative of shared interactions, charac-
terized by a large accumulation of charge between the
nuclei.
The two kinds of acetylene complexes, A and B,
present differences in the topological properties of
electron density for the osmium-acetylene interactions.
Larger values of F(r) at the bcp (FCP) for Os-C bonds
are observed in the four-electron-donor acetylene com-
plex B (Table 4). The value of FCP is related to the bond
order and can be considered a measure of the bond
strength, in such a way that the larger FCP is, the
stronger the bond. Thus, these results are consistent
with our previous findings, which indicated an increase
in the strength of the metal-acetylene bond when the
π orbital participated in the bonding. The calculated
values of FCP for Os-C bonds increase by 0.04e, from
0.097e and 0.101e in A to 0.138e and 0.143e in B. These
values compare nicely with those reported by Decker
and Klobukowski for Ru(CO)4(C2H2) and Ru(CO)3(C2H2)
complexes,19j in which it was found that the acetylene
ligand acts as a two-electron and four-electron donor,
respectively. In the Ru complexes the FCP values in-
crease also by 0.04e, from 0.075e in the tetracarbonyl
complex to 0.110e in the tricarbonyl species. Moreover,
ring critical points were found between the Os and the
acetylenic carbons, and a larger value of FCP was
observed in complex B for the metallacyclopropene-like
ring. For C-C acetylene bonds the value of FCP de-
creases on going from complex A to B, suggesting a
weakening of the acetylene bond. However, the decrease
in the amount of FCP is small, only 0.01e (from 0.377e
in A to 0.363e in B). This is not surprising, since we
have shown in a previous section that the CtC bond is
relatively flexible: i.e., that a wide range of C-C
distances can be reached with low energy cost. Neither
the C-C bond lengths nor the values of F(r) at the C-C
bcp are reliable indicators of the metal-alkyne coordi-
nation mode. In spite of this, significant differences were
found in the values of ellipticity (ꢀ) for the C-C bonds
between the two complexes (Table 4). The ellipticity
provides a measure for the π character of the bond; thus,
large values of ellipticity are indicative of high π bond
character. The low value of ꢀ at the C-C bcp in B
(0.024e) can be related to the participation of two π
orbitals instead of one in the bonding, which will
decrease the π character of the acetylene. We have
focused our discussion on the topological analysis of
Ba d er An a lysis. The metal-alkyne bonding has
been also investigated by means of Bader analysis of
the electron density. Bader’s atoms in molecules (AIM)
theory provides a set of practical tools for the study of
bonding properties.39 The Os-C and CtC interactions
were characterized by critical points (cp’s) in the elec-
tronic charge density (F(r)), which are points in the space
where ∇F(r) vanishes. According to Bader’s theory, the
localization of a bond critical point (bcp) between two
atoms proves the existence of an interaction between
them, while the ring critical point (rcp) is characteristic
2
of atomic rings. The values of F(r), Laplacian (∇ F(r)),
and ellipticity (ꢀ) at the critical points were used to
gauge the variation in charge density and bonding
properties for the complexes under study. In Table 4
are summarized the results of topological analysis of
electron density, for acetylene and propyne complexes,
as well as for the saddle points of the rotational process.
In a first inspection of the data collected in Table 4,
some general trends can be observed. The values of the
2
Laplacian at the bcp (∇ FCP) are relatively low and
positive for all the Os-C(alkyne) bonds, except for the
(39) (a) Bader, R. F. W. Atoms in Molecules: A Quantum Theory;
Clarendon Press: Oxford, U.K., 1990. (b) Bader, R. F. W. Chem. Rev.
1992, 92, 893.

Osmium-Cyclopentadienyl Chemistry
Organometallics, Vol. 21, No. 2, 2002 313
acetylenic complexes; however, the same argument
could be drawn for the methyl-substituted acetylenic
complexes AxCH3 and BxCH3. In summary, the differ-
ences in coordination mode of the alkyne complexes are
reflected in the properties of the electron density topol-
ogy, in both the values of FCP at the Os-C bonds and of
ꢀ at the C-C bcp. Therefore, when the two π orbitals of
alkyne participate in the bonding, higher values of FCP
and lower values of ꢀ are expected.
agreement with previous findings. As we go from A to
B, we observe an increase of electron density at the bcp
between the metal and the acetylenic carbon atoms,
which is accompanied by a decrease of ꢀ at the bcp
between the two acetylenic carbons. These results are
consistent with an increase in the amount of bonding
between the alkyne ligand and the metal.
Exp er im en ta l Section
Additionally, we have performed the topological analy-
sis of electron density for osmium-acetylene interac-
tions in the saddle points for acetylene rotation (Ar ot,
Br ot, and Br ot′). The topological properties in Ar ot and
Br ot species are very similar for both F(r) and ꢀ at the
bcp’s. The values of FCP at the Os-C bonds and ꢀ at the
C-C bcp’s better compare to the values found in the
acetylene complex A than in B, further proving the two-
electron coordination of acetylene in these rotational
species. However, somewhat lower values of FCP at the
Os-C bonds were observed in Ar ot and Br ot, indicating
a weakening of the Os-acetylene bonding. The Br ot′
structure presents a significant increase in the values
of FCP at the Os-C bonds and a decrease at the C-C
acetylenic bonds, reaching respectively higher and lower
values than those of complex B. In addition relatively
Syn th esis. All reactions were carried out with exclusion of
air using standard Schlenk techniques. Solvents were dried
by known procedures and distilled under argon prior to use.
The complexes Os(η5-C5H5)Cl{η2-HCtCC(OH)Ph2}(PiPr3)27b
(1a ) and Os(η5-C5H5)Cl{η2-HCtCC(OH)(CH3)2}(PiPr3)28 (1b)
were prepared as previously reported.
In the NMR spectra, chemical shifts are expressed in ppm
downfield from tetramethylsilane (1H and 13C{1H}) and H3-
PO4 (85%) (31P{1H}).
P r epar ation of [Os(η5-C5H5){η2-HCtCC(OH)P h2}(P iP r 3)]-
P F 6 (2a ). A solution of Os(η5-C5H5)Cl{η2-HCtCC(OH)Ph2}(Pi-
Pr3) (1a ; 125 mg, 0.19 mmol) in 10 mL of dichloromethane was
treated with TlPF6 (67 mg, 0.19 mmol). The mixture was
stirred for 2 min and then filtered through Kieselguhr and
concentrated to dryness. The residue was washed with diethyl
ether to yield a pale brown solid. Yield: 118 mg (81%). IR
(Nujol): ν(OH) 3529, ν(PF6) 840 cm-1. 1H NMR (300 MHz, CD2-
Cl2, 293 K): δ 9.43 (d, 1 H, J (PH) ) 26.4 Hz, tCH), 7.30 (br
s, 10 H, Ph), 5.39 (s, 5 H, Cp), 3.62 (s, 1 H. OH), 2.65 (m, 3 H,
PCH), 1.16 (dd, 18 H, J (HH) ) 7.2 Hz, J (PH) ) 14.7 Hz,
PCCH3). 31P{1H} NMR (121.4 MHz, CD2Cl2, 293 K): δ 38.0
(s), -144.7 (sept, J (PF) ) 713 Hz, PF6). 13C{1H} NMR (75.4
MHz, (CD3)2CO, 223 K): δ 179.0 (s, tC-), 147.7 (s, ipso-Ph),
146.0 (s, tCH), 129.0, 128.3, 126.8 (all s, Ph), 84.9 (s, COH),
77.1 (s, Cp), 26.2 (br, PCH), 19.7 (br, PCCH3). Anal. Calcd for
2
large values of (∇ FCP) at the Os-C bonds were ob-
served, suggesting a change in the nature of the metal-
alkyne interaction for Br ot′ species, in agreement with
previous findings.
Con clu d in g Rem a r k s
Treatment of the π-alkyne complexes Os(η5-C5H5)Cl-
{η2-HCtCC(OH)R2}(PiPr3) (1) with TlPF6 produces the
extraction of the chloride ligand and the formation of
[Os(η5-C5H5){η2-HCtCC(OH)R2}(PiPr3)]PF6 (2). The ex-
traction of the chloride ligand from 1 generates an
interaction between an empty d orbital of the osmium
atom and the π orbital of the alkyne. As a result, the
structural parameters and the spectroscopic properties
of the alkyne undergo significant disturbances. The Os-
alkyne distances are shortened, and in the 13C{1H} and
1H NMR spectra, the chemical shifts of the acetylenic
carbon and HCt resonances are shifted toward lower
field. Theoretical calculations on the model compounds
Os(η5-C5H5)Cl(η2-HCtCR)(PH3) (A) and [Os(η5-C5H5)-
{η2-HCtCR}(PH3)]+ (B) suggest that the disturbance
in the chemical shifts is a consequence of the depopula-
tion of the π orbital of the alkyne, on going from 1 to 2
or from A to B. Both structural and spectroscopic
changes are in accord with the rationale of an increased
donation from the alkyne, as a consequence of the
participation of the acetylenic second π orbital (π ) in
the bonding.
The theoretical calculations also predict that, in these
types of systems, the interaction between the π orbital
of the alkyne and an empty d orbital of the osmium gives
rise to an increase of the dissociation energy of the
alkyne and an increase of the energy for the rotation of
the alkyne around the osmium-alkyne axis. The en-
hancement in the rotational barrier is due to the loss
of the π fM interaction, which makes rotation proceed
via a formally unsaturated 16-electron path. The results
of topological properties of the electron density of the
system also reflect the π fM interaction and are in full
C
29H38F6OOsP2: C, 45.31; H, 4.98. Found: C, 45.08; H, 5.09.
MS (FAB+): m/e 625 (M+).
P r ep a r a tion of [Os(η5-C5H5){η2-HtCC(OH)(CH3)2}(P i-
P r 3)]P F 6 (2b). A solution of Os(η5-C5H5)Cl{η2-HCtCC(OH)-
(CH3)2}(PiPr3) (1b; 128 mg, 0.24 mmol) in 10 mL of dichlo-
romethane was treated with TlPF6 (84 mg, 0.24 mmol). The
mixture was stirred for 2 min and then filtered through
Kieselguhr and concentrated to dryness. The residue was
washed with diethyl ether to yield a pale brown solid. Yield:
135 mg (87%). IR (Nujol): ν(OH) 3575, ν(PF6) 840 cm-1 1H
.
NMR (300 MHz, CD2Cl2, 293 K): δ 9.17 (br d, 1 H, J (PH) )
26.4 Hz, tCH), 5.70 (s, 5 H, Cp), 5.67 (s, 1 H. OH), 2.70 (m, 3
H, PCH), 1.70 (s, 6 H, CH3), 1.23 (dd, 18 H, J (HH) ) 7.2 Hz,
J (PH) ) 14.4 Hz, PCCH3). 31P{1H} NMR (121.4 MHz, CD2Cl2,
293 K): δ 36.3 (s), -144.7 (sept, J (PF) ) 713 Hz, PF6). 13C-
{1H} NMR (75.4 MHz, CD2Cl2, 223 K): δ 182.8 (s, tC-), 143.3
(s, tCH), 77.6 (s, COH), 76.4 (s, Cp), 31.6 (s, CH3), 27.3 (br d,
J (PC) ) 29.7 Hz, PCH), 20.0 (s, PCCH3). Anal. Calcd for
C19H34F6OOsP2: C, 35.40; H, 5.32. Found: C, 35.35; H, 5.41.
MS (FAB+): m/e 501 (M+).
Cr ysta l Da ta . Crystals suitable for X-ray diffraction were
obtained by slow diffusion of pentane into a concentrated
solution of 1a in toluene or by diffusion of diethyl ether into a
concentrated solution of 2a in CH2Cl2. A summary of crystal
data and refinement parameters is listed in Table 5. Data were
collected on
a Siemens Stoe AED-2 diffractometer, with
graphite-monochromated Mo KR radiation (λ ) 0.710 73 Å),
using the ω/2θ scan method. Three standard reflections were
monitored every 55 min throughout data collection; no impor-
tant variations were observed. Both data were corrected for
Lorentz and polarization effects and for absorption using the
ψ-scan method. Both structures were solved by Patterson and
Fourier techniques and refined by full-matrix least-squares

314 Organometallics, Vol. 21, No. 2, 2002
Carbo´ et al.
Ta ble 5. Cr ysta llogr a p h ic Da ta for
was used to represent the 60 innermost electrons of the
osmium atom.44 The basis set for the metal atom was that
associated with the pseudopotential,44 with a standard dou-
ble-ú LANL2DZ contraction.41 The 6-31G(d,p) basis set was
used for the P, Cl, and C atoms directly attached to the metal,
whereas the 6-31G basis set was used for the hydrogen
atoms.45 In the case of propyne model complexes (ACH3 and
BCH3) the C and H atoms of the methyl substituents were
described using a 6-31G basis set.45 Geometry optimizations
were carried out without any symmetry restrictions, and all
stationary points were optimized with analytical first deriva-
tives. Saddle points were located by means of approximate
Hessians and synchronous transit-guided quasi-Newtonian
methods.46 All saddle points were characterized by means of
normal mode analysis, with one imaginary frequency corre-
sponding to rotation of the acetylene ligand. In the case of Br ot
and Br ot′, additional calculations were performed in order to
further confirm that both saddle points correspond to the
acetylene rotational process. Displacements in atomic coordi-
nates following the normal mode of the imaginary frequencies
were made, and in both species, the subsequent optimization
processes led to the reactant and product of the process.
Os(η5-C5H5)Cl{η2-HCtCC(OH)P h 2}(P iP r 3) (1a ) a n d
[Os(η5-C5H5){η2-HCtCC(OH)P h 2}(P iP r 3)][P F 6] (2a )
1a
2a
formula
Mr
T (K)
C
29H38ClOOsP
C29H38F6OOsP2
768.63
295
659.21
295
cryst syst
space group
a, Å
orthorhombic
Pbca
orthorhombic
Pna21
13.924(2)
17.670(2)
22.157(3)
5451.4(12)
8
16.842(2)
9.077(2)
19.643(3)
3002.9(6)
4
b, Å
c, Å
V, Å3
Z
Fcalcd (g cm-3
)
1.606
4.85
1.700
4.41
µ, mm-1
θ, range data collecn (deg)
no. of measd rflns
no. of unique rflns
2-25
10 237
4798
2-25
8554
5282
no. of restraints/params
R(F) (F2 > 2σ(F2))a
Rw(F2) (all data)b
282/2
0.0351
0.0351
333/5
0.0788
0.0340
R(F) ) ∑||Fo| - |Fc||/∑|Fo|. Rw(F2) ) (∑[w(Fo - Fc2)2]/
a
b
2
∑[w(Fo2)2])1/2
.
Nuclear magnetic resonance (NMR) properties have been
computed by using the gauge-independent atomic orbital
(GIAO) method.38 The bonding situation of the complexes has
been analyzed with the help of the NBO partitioning scheme.47
Furthermore, atomic charges have been calculated by means
of the natural population analysis (NPA) method.47 The
topological properties of the electron density39 were investi-
gated using the XAIM 1.0 program.48
on F2.40 Atomic scattering factors, corrected for anomalous
dispersion, were used as implemented in the refinement
program.
Crystals of approximate dimensions 0.37 × 0.19 × 0.11 mm
(1a ) and 0.22 × 0.20 × 0.19 mm (2a ) were used for data
collection. Cell constants were obtained by the least-squares
fit on the setting angles of 60 reflections in the range 20 e 2θ
e 40° (1a and 2a ). Data were collected at room temperature
in the range 4 e 2θ e 50° (-16 e h e 16, 0 e k e 21, 0 e l e
26; 10 237 measured reflections, 4798 unique (Rint ) 0.0377))
for 1a ; or in the ranger 4 e 2θ e 50° (0 e h e 20, -10 e k e
4, -23 e l e 23; 8554 measured reflections, 5282 unique (Rint
) 0.0377)) for 2a . In both molecules a phenyl group of the
alkynol ligand was observed to be disordered, and both were
refined with two moieties (A and B), with complementary
occupancy factors and restrained geometry. In 2a , the PF6
anion was observed to be disordered in the same way, also.
Anisotropic displacement parameters were used in the last
cycles of refinement for all hydrogen atoms, except those
involved in disorder. Hydrogen atoms, except the alkynol ones,
were placed in calculated positions and refined riding on the
corresponding carbon atoms.
Ack n ow led gm en t. We acknowledge financial sup-
port from the DGES of Spain (Projects PB98-1591 and
PB98-0916-C02-01, Programa de Promocio´n General del
Conocimiento). The use of the computational facilities
of the Centre de Supercomputacio´ i Comunicacions de
Catalunya is gratefully appreciated as well.
Su p p or tin g In for m a tion Ava ila ble: Tables of atomic
coordinates and equivalent isotropic displacement coefficients,
anisotropic thermal parameters, experimental details of the
X-ray studies, and bond distances and angles for 1a and 2a .
This material is available free of charge via the Internet at
http://pubs.acs.org.
OM010645N
Com p u ta tion a l Deta ils. Calculations were performed with
the GAUSSIAN 98 series of programs41 within the framework
of density functional theory (DFT)42 using the B3LYP func-
tional.43 A quasi-relativistic effective core potential operator
(42) (a) Parr, R. G.; Yang, W. Density Functional Theory of Atoms
and Molecules; Oxford University Press: Oxford, U.K., 1989. (b)
Ziegler, T. Chem. Rev. 1991, 91, 651.
(43) (a) Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B 1988, 37, 785.
(b) Becke, A. D. J . Chem. Phys. 1993, 98, 5648. (c) Stephens, P. J .;
Devlin, F. J .; Chabalowski, C. F.; Frisch, M. J . J . Phys. Chem. 1994,
98, 11623.
(44) Hay, P. J .; Wadt, W. R. J . Chem. Phys. 1985, 82, 299.
(45) (a) Francl, M. M.; Pietro, W. J .; Hehre, W. J .; Binkley, J . S.;
Gordon, M. S.; Defrees, D. J .; Pople, J . A. J . Chem. Phys. 1982, 77,
3654. (b) Hehre, W. J .; Ditchfield, R.; Pople, J . A. J . Chem. Phys. 1972,
56, 2257. (c) Hariharan, P. C.; Pople, J . A. Theor. Chim. Acta 1973,
28, 213.
(46) Peng, C.; Ayala, P. Y.; Schlegel, H. B.; Frisch, M. J . J . Comput.
Chem. 1996, 17, 49.
(47) Reed, A. E.; Curtiss, L. A.; Weinhold, F. Chem. Rev. 1988, 88,
899.
(40) Sheldrick, G. M. University of Go¨ttingen, Go¨ttingen, Germany,
1997.
(41) Frisch, M. J .; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;
Robb, M. A.; Cheeseman, J . R.; Zakrzewski, V. G.; Montgomery, J . A.,
J r.; Stratmann, R. E.; Burant, J . C.; Dapprich, S.; Millam, J . M.;
Daniels, A. D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J .;
Barone, V.; Cossi, M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo,
C.; Clifford, S.; Ochterski, J .; Petersson, G. A.; Ayala, P. Y.; Cui, Q.;
Morokuma, K.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.;
Foresman, J . B.; Cioslowski, J .; Ortiz, J . V.; Stefanov, B. B.; Liu, G.;
Liashenko, A.; Piskorz, P.; Komaromi, I.; Gomperts, R.; Martin, R. L.;
Fox, D. J .; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.;
Gonzalez, C.; Challacombe, M.; Gill, P. M. W.; J ohnson, B. G.; Chen,
W.; Wong, M. W.; Andres, J . L.; Head-Gordon, M.; Replogle, E. S.;
Pople, J . A. Gaussian 98; Gaussian, Inc.: Pittsburgh, PA, 1998.
(48) This program was developed by J ose Carlos Ortiz and Carles
Bo, Universitat Rovira i Virgili, Tarragona, Spain.