Neutron Diffraction Analysis of H
2
Os
6
(CO)
18
Robert Bau,* Sax A. Mason, Li Li, and Wing-Tak Wong
Department of Chemistry, UniVersity of Southern California
Los Angeles, California 90089
Institut Laue-LangeVin, Grenoble F-38042, France
Chemistry Department, UniVersity of Hong Kong
Pokfulam Road, Hong Kong
ReceiVed May 20, 1997
H
2
Os
6
(CO)
18
is one of the more intriguing compounds in
metal cluster chemistry. A rare example of a non-octahedral
hexanuclear cluster, it was first prepared in 1976 from the
vacuum pyrolysis of Os
3
(CO)
12
and subsequent acidification.
1
The structure, reported later that year by McPartlin and co-
workers,
2
consists of a capped square pyramid of osmium atoms
(Figure 1) augmented by three terminal carbonyl ligands on each
Os atom. The unconventional shape of the cluster contrasts
markedly with its isoelectronic analogues, [Os
6
(CO)
18
]
2-
and
[HOs
6
(CO)
18
]
-
, which have the usual octahedral structure.
2
In this paper, we report a single-crystal neutron structure
determination of H
2
Os
6
(CO)
18
and the location of the two
hydrogen atoms of this molecule, which were not found in the
original X-ray study and have been the subject of much
speculation. The original authors
2
postulated that one of the H
atoms is located on either the Os(2)-Os(5) edge or the Os-
(3)-Os(4) edge, and the second H atom is associated with either
the Os(1)-Os(3)-Os(4) triangular face or the Os(2)-Os(3)-
Os(4)-Os(5) square face. This assignment was based in part
on the
13
C and
1
H NMR spectra of the compound, which
indicated that the two H ligands are associated with the plane
of symmetry of the molecule.
This was followed soon thereafter by a suggestion by
Pauling,
3
in one of his infrequent papers on metal cluster
compounds, that the two hydride ligands are terminal, bonded
to the atoms Os(3) and Os(4). Finally, in 1976, Orpen
introduced a potential energy calculational method for predicting
the positions of H atoms in clusters
4
and applied it to H
2
Os
6
-
(CO)
18
to predict that both H atoms should be edge-bridging,
located on the Os(2)-Os(5) and Os(3)-Os(4) edges of the
cluster. We consequently undertook a single-crystal neutron
structure determination of H
2
Os
6
(CO)
18
to settle this issue.
A sample of the title compound was prepared as reported
1
and recrystallized from CH
2
Cl
2
to yield a crystal (volume 0.9
mm
3
) used in the single-crystal neutron analysis. Data were
collected at 20 K on the Instrument D19 at the Institut Laue-
Langevin equipped with a Displex cryostat and a position-
sensitive area detector.
5a
The unit cell parameters (at 20 K)
for H
2
Os
6
(CO)
18
‚0.5CH
2
Cl
2
are a ) 16.3535(6) Å, b )
15.3170(6) Å, c ) 22.0871(8) Å, ) 90.437(2)°, space group
P2
1
/c (monoclinic). A total of 7561 reflections were collected
and merged into 4415 independent reflections with positive F
2
values.
5b
Phasing of the neutron data was carried out using the
atomic coordinates of the non-hydrogen atoms obtained from
the earlier X-ray analysis.
2
Exhaustive least-squares refinement
of all the atomic coordinates resulted in final agreement factors
of R(F) ) 4.5% for the 4008 reflections with F > 4σ(F), and
R(F) ) 5.3% for all 4415 reflections.
The structure of H
2
Os
6
(CO)
18
is shown in Figure 2, and
selected distances and angles are listed in Table 1. There are
two independent molecules in the unit cell, which have
essentially the same geometry, and one dichloromethane solvent
molecule. The most significant result is that both H atoms are
in edge-bridging positions, bridging the Os(2)-Os(5) and
Os(3)-Os(4) edges, as predicted by Orpen.
4
The average
Os-H distance [1.807(4) Å] and the average Os-H-Os angle
[108.4(4)°] are very close to standard values found in other
neutron diffraction analyses of edge-bridging hydride ligands.
6
An alternative view of the cluster is given in Figure 3, which
shows that atom H(2) is coplanar with Os(2), Os(5), and four
(1) Eady, C. R.; Johnson, B. F. G.; Lewis, J. Chem. Commun. 1976,
302.
(2) McPartlin, M.; Eady, C. R.; Johnson, B. F. G.; Lewis, J. Chem.
Commun. 1976, 883.
(3) Pauling, L. Proc. Nat. Acad. Sci. U.S.A. 1977, 74, 5235.
(4) (a) Orpen, A. G. J. Organomet. Chem. 1978, 159, C1. (b) Orpen, A.
G. J. Chem. Soc., Dalton Trans. 1980, 2509.
(5) (a) Thomas, M.; Stansfield, R. F. D.; Berneron, M.; Filhol, A.;
Greenwood, G.; Jacobe, J.; Feltin, D; Mason, S. A. In Position-SensitiVe
Detection of Thermal Neutrons; Convert, P., Forsyth, J. B., Eds.; Academic
Press: London, 1983; p 344. (b) Neutron data reduction was carried out
using the program RETREAT: Wilkinson, C.; Khamis, H. W.; Stansfield,
R. F. D.; McIntyre, G. J. J. Appl. Crystallogr. 1988, 21, 471. Details of the
Displex cryostat used are given in the following: Archer, J.; Lehmann, M.
S. J. Appl. Crystallogr. 1986, 19, 456.
Figure 1. Os6 skelton of H2Os6(CO)18, as determined from the earlier
X-ray analysis (ref 2).
Figure 2. Molecular structure of H2Os6(CO)18, as determined by
neutron diffraction.
Table 1. Selected Distances (Å) and Angles (deg) in H2Os6(CO)18
molecule 1 molecule 2
Os(3)-H(1) 1.820(8) 1.796(8)
Os(4)-H(1) 1.798(8) 1.798(8)
Os(2)-H(2) 1.811(8) 1.816(8)
Os(5)-H(2) 1.803(8) 1.812(7)
Os(1)-Os(2) 2.797(3) 2.828(3)
Os(1)-Os(3) 2.881(3) 2.852(3)
Os(1)-Os(4) 2.841(3) 2.841(3)
Os(1)-Os(5) 2.830(3) 2.827(3)
Os(1)-Os(6) 2.882(3) 2.845(3)
Os(2)-Os(3) 2.869(3) 2.860(3)
Os(2)-Os(5) 2.901(3) 2.893(3)
Os(2)-Os(6) 2.801(3) 2.805(3)
Os(3)-Os(4) 2.970(3) 2.962(3)
Os(4)-Os(5) 2.859(3) 2.875(3)
Os(5)-Os(6) 2.781(3) 2.813(3)
Os(3)-H(1)-Os(4) 110.4(4) 111.0(4)
Os(2)-H(2)-Os(5) 106.7(4) 105.7(4)
11992 J. Am. Chem. Soc. 1997, 119, 11992-11993
S0002-7863(97)01632-6 CCC: $14.00 © 1997 American Chemical Society