Cluster chemistry with sulfur-containing ligands: Reactions of the hexanuclear cluster (μ2-H)Ru6(CO)16(μ 5-S)(μ3,η2-SCNHPhNPh) with two-electron-donor ligands

Article abstract of DOI:10.1021/om950739v

The reaction of the hexanuclear cluster (μ₂-H)Ru₆(CO)₁₆(μ ₅-S)(μ₃,η²-SCNHPhNPh) (1) with triphenylphosphine mainly results in the formation of the substitution product (μ₂-H)RU₆-(CO)₁₅(PPh₃)(μ ₅-S)(μ₃,η²-SCNHPhNPh) (3). X-ray structural analysis of a single crystal of 3 identified the site of substitution as the apical, S-bound Ru atom of the starting cluster and revealed a significant degree of cluster deformation upon substitution. Crystals of 3 are triclinic, space group P1: Z = 2, a = 9.808(2) A?, b = 10.285(2) A?, c = 27.48(1) A?, α= 91.97(2)°, β = 90.02(2)°, γ= 94.52(2)°. Reactions of 1 with other two-electron-donor ligands [Me₂S, P(n-Bu)₃, P(OMe)₃, P(OPh)₃, and ^tBuNC] also resulted in the substitution of a carbonyl group to give (μ₂-H)Ru₆(CO)₁₅(L)(μ ₅-S)(μ₃,η²-SCNHPhNPh) [L = Me₂S, 4; P(n-Bu)₃, 5; P(OMe)₃, 6; P(OPh)₃, 7; ^tBuNC, 8]. The spectra of these substitution products differ from those of 3, indicating an alternative site of substitution. This was verified through X-ray structural analysis of the ^tBuNC derivative 8, which showed the isonitrile ligand to occupy an axial site on the apical Ru atom at the end of the cluster opposite to that found in 3. Crystals of 8 are triclinic, space group P1: Z = 2, a = 9.431(2) A?, b = 15.794(2) A?, c = 16.772(2) A?, α = 87.94(1)°, β= 84.72(1)°, γ= 76.70(1)°.

Full text of DOI:10.1021/om950739v

1122
Organometallics 1996, 15, 1122-1127
Clu ster Ch em istr y w ith Su lfu r -Con ta in in g Liga n d s:
Rea ction s of th e Hexa n u clea r Clu ster
(µ2-H)Ru 6(CO)16(µ5-S)(µ3,η2-SCNHP h NP h ) w ith
Tw o-Electr on -Don or Liga n d s
Lisa A. Hoferkamp, Gerd Rheinwald, Helen Stoeckli-Evans, and
Georg Su¨ss-Fink*
Institut de Chimie, Universite` de Neuchaˆtel, Avenue de Bellevaux 51,
CH-2000 Neuchaˆtel, Switzerland
Received September 15, 1995X
The reaction of the hexanuclear cluster (µ2-H)Ru6(CO)16(µ5-S)(µ3,η2-SCNHPhNPh) (1) with
triphenylphosphine mainly results in the formation of the substitution product (µ2-H)Ru6-
(CO)15(PPh3)(µ5-S)(µ3,η2-SCNHPhNPh) (3). X-ray structural analysis of a single crystal of 3
identified the site of substitution as the apical, S-bound Ru atom of the starting cluster and
revealed a significant degree of cluster deformation upon substitution. Crystals of 3 are
triclinic, space group P1h: Z ) 2, a ) 9.808(2) Å, b ) 10.285(2) Å, c ) 27.48(1) Å, R ) 91.97(2)°,
â ) 90.02(2)°, γ ) 94.52(2)°. Reactions of 1 with other two-electron-donor ligands [Me2S,
t
P(n-Bu)3, P(OMe)3, P(OPh)3, and BuNC] also resulted in the substitution of a carbonyl group
to give (µ2-H)Ru6(CO)15(L)(µ5-S)(µ3,η2-SCNHPhNPh) [L ) Me2S, 4; P(n-Bu)3, 5; P(OMe)3, 6;
t
P(OPh)3, 7; BuNC, 8]. The spectra of these substitution products differ from those of 3,
indicating an alternative site of substitution. This was verified through X-ray structural
t
analysis of the BuNC derivative 8, which showed the isonitrile ligand to occupy an axial
site on the apical Ru atom at the end of the cluster opposite to that found in 3. Crystals of
8 are triclinic, space group P1h: Z ) 2, a ) 9.431(2) Å, b ) 15.794(2) Å, c ) 16.772(2) Å, R
) 87.94(1)°, â ) 84.72(1)°, γ ) 76.70(1)°.
In tr od u ction
skeletal electron pair theory (PSEPT).5 The majority
of M6 clusters adopt an octahedral geometry; therefore,
not surprisingly, clusters of this type were among the
first to be isolated and structurally characterized. The
86-electron octahedral cluster Ru6C(CO)17 is a typical
example.6 Many clusters have been isolated whose
geometries are derived from standard polyhedra by
capping or bridging to provide the additional vertices
necessary to accommodate the additional metal atoms.
These include the bicapped tetrahedral geometry and
the capped square pyramid, as exemplified by Os6(CO)17-
(MeCN) and Os6(CO)17(µ4,η2-HCCEt), respectively.7
These clusters contain a total of 84 and 86 valence
electrons, respectively, which are the numbers predicted
from PSEPT. As opposed to the addition of vertices, the
structures of some hexanuclear clusters must be con-
sidered as larger polyhedra with vertices missing, as
in the case of the edge-bridged diminished pentagonal
bipyramid found for (µ2-H)Ru6C(CO)15(SEt)3.8 Analo-
gous to the boranes and carboranes, the opening up of
the polyhedral geometry requires additional skeletal
electrons; thus, the edge-bridged diminished pentagonal
bipyramid shows an electron count of 92 electrons.
Further variations are planar raft structures including
Cotton’s general definition of a metal cluster as a
group of two or more metal atoms in which substantial
bonding occurs among the metals1 is useful in dif-
ferentiating clusters from Werner-type coordination
complexes. In many of the later reviews of metal
clusters, however, this definition is narrowed to include
only those species containing at least three metal atoms.
The next step, from metal clusters to metal surfaces, is
a more difficult distinction. The point at which one
passes from a nonmetallic state to a metallic state has
been the subject of considerable research.2 Localized
versus delocalized bonding within the metal framework
provides a starting point. It has been shown that when
cluster nuclearity approaches six metal atoms, a local-
ized bonding description no longer provides an adequate
description.3 At this point, a delocalized bonding ap-
proach, more similar in nature to the band theory of
metals, becomes necessary. Given such a position
within the cluster hierarchy, the types of reactions and
transformations undertaken by hexanuclear clusters
offer a simplified model of those processes at the metal
surface.
The full range of structural motifs displayed by
hexanuclear cluster compounds is well-documented.4
The relationships between the observed geometries of
these clusters and their closed-shell electronic require-
ments comprise a large portion of the polyhedral
(4) (a) Ma, L.; Williams, G. K.; Shapley, J . R. Coord. Chem. Rev.
1993, 128, 261. (b) Metal Clusters; Moskovits, M., Ed.; J ohn Wiley &
Sons: New York, 1986.
(5) (a) Williams, R. E. Prog. Inorg. Chem. Radiochem. 1976, 18, 67.
(b) Wade, K. Adv. Inorg. Chem. Radiochem. 1976, 18, 1. (c) Mingos,
D. M. P. Acc. Chem. Res. 1984, 17, 311. (d) Rudolph, R. W. Acc. Chem.
Res. 1976, 9, 446.
X Abstract published in Advance ACS Abstracts, J anuary 15, 1996.
(1) Cotton, F. A. Q. Rev. 1966, 20, 389.
(6) J ohnson, B. F. G.; J ohnston, R. D.; Lewis, J . J . Chem. Soc. A
1968, 2856.
(2) Schmid, G. Struct. Bond. 1985, 62, Chapter 2.
(7) Gomez-Sal, M. P.; J ohnson, B. F. G.; Kamarudin, R. A.; Lewis,
J .; Raithby, J . J . Chem. Soc., Chem. Commun. 1985, 1622.
(8) J ohnson, B. F. G.; Lewis, J .; Wong, K.; McPartlin, M. J .
Organomet. Chem. 1980, 185, C17.
(3) Mingos, D. M. P.; Wales, D. J . In Introduction to Cluster
Chemistry; Grimes, R. N., Ed.; Prentice Hall International Editions
Inc.: Englewood Cliffs, NJ , 1990; Chapter 2.
0276-7333/96/2315-1122$12.00/0 © 1996 American Chemical Society

Cluster Chemistry with Sulfur-Containing Ligands
Organometallics, Vol. 15, No. 4, 1996 1123
both the triangular raft, [(µ3-H)Ru6(CO)15(µ3-S)3]- (ref
11d), and the rhombic raft cluster, (µ2-H)4(µ3-H)2Ru6-
(CO)14(µ3,η2-ampy)2.9 The former cluster contains 92
valence electrons, which is 2 greater than the number
predicted from PSEPT. This feature has been used to
explain the elongation of certain M-M bonds via the
population of antibonding cluster orbitals.10 The latter
cluster also contains 92 valence electrons, but this is in
accord with its geometry. Distortion of the metal plane
of the rhombic raft cluster results in alternative core
configurations. Some of these configurations resemble
the various conformers of cyclohexane,11 while others
represent structural intermediates between the stable
conformations.12 The majority of these latter raft-type
clusters are either electron-rich or electron-precise, and
all contain multiply bridging ligands.
nuclear cluster (µ2-H)Ru3(CO)9(µ3,η2-SCNHPhNPh) with
2 equiv of PPh3 or direct thermolysis of the disubstituted
derivative (µ2-H)Ru3(CO)7(PPh3)2(µ3,η2-SCNHPhNPh).14
The IR and NMR data distinguished the yellow band
as a dinuclear Ru complex with the formula Ru2(CO)4-
(PPh3)2(µ3,η2-SCNHPhNPh) (2). Spectral data indicated
the brick red compound to be the monosubstituted, PPh3
derivative of 1, (µ2-H)Ru6(CO)15(PPh3)(µ5-S)(µ3,η2-SCN-
HPhNPh) (3).
Symmetrically substituted, dinuclear Ru complexes
have been isolated and characterized previously.15 The
1
carbonyl region of IR and both H and 31P NMR spectra
of 2 are consistent with a complex of this type, and, on
the basis of these data, compound 2 is identified as Ru2-
(CO)2(SCNHPhNPh)2. In compound 2, two diphenyl-
From a simple thermolysis reaction of the trinuclear
cluster (µ2-H)Ru3(CO)9(µ3,η2-SCNHPhNPh),13 it was
possible to isolate two hexanuclear thioureato clusters,
each with a metal skeleton that mimics a known
conformer of cyclohexane. The first of these is structur-
ally similar to the boat conformer, Ru6(µ2-CO)2(CO)14-
(µ4-S)(µ3,η2-NPhCNHPh)(µ3,η2-SCNHPhNPh), while the
second is a structural analogue of the sofa conformer,
(µ2-H)Ru6(CO)16(µ5-S)(µ3,η2-SCNHPhNPh) (1). The sofa-
like structure is produced in relatively high yield (46%)
for a preparative procedure that is known for its lack
of selectivity. Given the easy access to reasonable
amounts of the second hexanuclear species, it was
considered an ideal opportunity to explore its reactivity
toward a series of organic reagents.
thioureato ligands span the Ru-Ru vector, bonding in
a terminal fashion through the S and the N. One PPh3
and two carbonyl ligands are terminally bonded to each
Ru atom. Only the N-H stretch in the IR spectrum of
2 was not identifiable. If the diphenylthioureato ligand
is considered a three-electron donor, then the dinuclear
species 2 is electron-precise with a total of 34 valence
electrons.
Spectroscopic data for 3 show a relatively simple
carbonyl region; only five bands are observed. The
presence of an undisturbed diphenylthioureato ligand
1
is clear from IR and H NMR data. Integration of the
1
phenyl and the NH regions of the H NMR spectrum,
combined with the 31P NMR spectrum, leads to a single,
σ-bonded PPh3 group. Room temperature 1H NMR
spectra of 3 in CDCl3 show a broad, poorly defined
peak in the hydride region (-17.50 ppm). The downfield
shift of the signal relative to that of 1 (-22.0 ppm)
suggests a chemical environment for this proton that
is less affected by the anisotropy of the metal core.
Upon cooling of a CD2Cl2 sample to -70 °C, two well-
defined singlets at -17.70 and -17.73 ppm (400 MHz)
in an approximately 1:1 ratio are observed. This result
is consistent with the presence of two interconverting
isomers in room temperature solutions of 3.
Resu lts a n d Discu ssion
Rea ction of (µ2-H)Ru 6(CO)16(µ5-S)(µ3,η2-SCNHP h -
NP h ) (1) w ith Tr ip h en ylp h osp h in e. Reaction of 1
with 1 or 2 equiv of PPh3 over a period of 12 h, at 20 °C
in dichloromethane or cyclohexane, gave three different
products. Preparative TLC separated these compounds,
in order of elution, as a bright red band, a yellow band,
and a brick red band. The bright red band was
identified by its IR spectrum as the compound Ru3(CO)6-
(µ2,η2-C6H5)(PPh3)(µ2-PPh2)(µ3-S). This same trinuclear
cluster is available from thermal reactions of the tri-
(9) (a) Cifuentes, M. P.; J eynes, T. P.; Humphrey, M. G.; Skelton,
B. W.; White, A. H. J . Chem. Soc., Dalton Trans. 1994, 925. (b) Cabeza,
J . A.; Ferna´ndez-Colinas, F. M.; Garc´ıa-Granda, S.; Llamazares, A.;
Lo´pez-Ort´ız, F; Riera, V.; Van der Maelen, J . F. Organometallics 1994,
13, 426.
(10) Boag, N. M.; Knobler, C. B.; Kaesz, G. D. Angew. Chem., Int.
Ed. Engl. 1983, 22, 249.
(11) (a) Gervasio, G.; Rossetti, R.; Stanghellini, P. L.; Bor, G. Inorg.
Chem. 1984, 23, 2073. (b) Adams, R. D.; Babin, J . E.; Tasi, M. Inorg.
Chem. 1987, 26, 2561. (c) Cockerton, B. R.; Deeming, A. Polyhedron
1994, 13 (13), 2085. (d) Bodensieck, U.; Hoferkamp, L.; Stoeckli-Evans,
H.; Su¨ss-Fink, G. J . Chem. Soc., Dalton Trans. 1993, 127.
(12) J eannin, S.; J eannin, Y.; Robert, F.; Rosenberger, C. Inorg.
Chem. 1994, 33, 243.
An X-ray structural characterization of 3 provided the
exact location of the PPh3 ligand on the hexanuclear
framework and also revealed some interesting struc-
(13) Hoferkamp, L.; Rheinwald, G.; Stoeckli-Evans, H.; Su¨ss-Fink,
G. Inorg. Chem. 1995, in press.

1124 Organometallics, Vol. 15, No. 4, 1996
Ta ble 1. Cr ysta llogr a p h ic Da ta for Clu ster s 3 a n d 8a
Hoferkamp et al.
compound
3
8
molecular formula
MW
C46H26N2O15PRu6CH2Cl2
1633.1
C33H20N3O15Ru6S2‚C1.5O0.5
1398.1
Z
2
2
crystal dimensions (mm)
crystal color/habit
crystal system
space group
a (Å)
b (Å)
c (Å)
0.38 × 0.38 × 0.15
deep red/plates
triclinic
0.72 × 0.42 × 0.06
red/plates
triclinic
P1h
9.431(2)
15.794(2)
16.772(2)
87.94(1)
84.72(1)
76.70(1)
2420.6(7)
1.928
P1h
9.808(2)
10.285(2)
27.48(1)
91.97(2)
90.02(2)
94.52(2)
2761.8(13)
1.964
R (deg)
â (deg)
γ (deg)
V (Å3)
Dc (g cm-3
)
µ(Mo KR) (mm-1
)
3.66
-60
3104
1.827
20
1346
T of data collection (°C)
F(000)
absorption correction
empirical/psi scans
0.4833/0.2971
9724
empirical/difabs
1.000/0.227
8485
T
max/Tmin
no. of unique data
no. of obs. data used [Fo > 2.5σ(Fo)]
7123
3/50
0.039 (0.052)
8483
3/50
2θmin/2θmax (deg)
R(R′)b,c
R(>2σF2)/R1(all)d,e
0.071 (0.131)
0.171 (0.193)
1.397/-1.203
wR2(>2σF2)/wR2(all)f
residual electron density difference features (max/min) (e Å-3
)
1.29/-1.21
a
Details in common: data were collected on a Stoe-Siemens AED 2 four-circle diffractometer; graphite-monochromated Mo KR radiation,
λ ) 0.710 73 Å; scan mode ω-θ. Refinement was by full-matrix least squares with a weighting scheme of the form w-1 ) σ2(Fo) + k(Fo2).
b
c R ) ∑||Fo| - |Fc||/∑|Fo|, R′ ) [∑w(|Fo| - |Fc|)2/∑w(|Fo|)2]1/2
.
Structure refinement was carried out by using SHELXL93. e R ) ∑||Fo| -
d
|Fc||/∑|Fo|. f wR2 ) [{∑[w(Fo - Fc2)2]/∑[w(Fo)4]]1/2, w-1 ) [σ2(Fo2) + (waP)2 + wbP], P ) (Fo + 2Fc2).
2
2
Ta ble 2. Selected Bon d Len gth s (Å) a n d An gles
(d eg) for 3
Ru(1)-Ru(5)
Ru(1)-Ru(6)
Ru(1)-S(1)
Ru(2)-Ru(3)
Ru(2)-Ru(4)
Ru(2)-S(1)
Ru(2)-S(2)
Ru(3)-Ru(4)
Ru(3)-N(2)
Ru(4)-Ru(5)
Ru(4)-S(1)
Ru(4)-S(2)
2.7904(10)
Ru(5)-Ru(6)
Ru(5)-S(1)
Ru(6)-P(1)
Ru(6)-S(1)
P(1)-C(14)
P(1)-C(20)
P(1)-C(26)
S(2)-C(1)
N(1)-C(2)
N(2)-C(1)
N(2)-C(8)
C(1)-N(1)
2.7885(11)
2.3646(16)
2.3743(18)
2.3381(18)
1.816(6)
1.835(6)
1.830(7)
1.781(7)
1.417(9)
1.272(9)
1.452(9)
1.362(9)
2.9529(10)
2.3762(17)
2.7206(10)
2.9106(10)
2.4089(17)
2.4287(18)
2.7100(10)
2.164(5)
3.0870(11)
2.4285(18)
2.4091(19)
Ru(1)-Ru(2)-Ru(3) 143.01(3)
Ru(1)-Ru(2)-Ru(4) 86.12(3)
Ru(1)-Ru(5)-Ru(4) 94.01(3)
Ru(1)-Ru(5)-Ru(6) 63.92(3)
Ru(1)-Ru(6)-Ru(5) 58.07(3)
Ru(3)-Ru(4)-Ru(5) 148.62(3)
Ru(3)-N(2)-C(1)
Ru(3)-N(2)-C(8)
121.6(4)
119.1(4)
Ru(4)-Ru(5)-Ru(6) 96.87(3)
Ru(4)-S(1)-Ru(5)
Ru(4)-S(2)-C(1)
80.18(5)
103.48(24)
Ru(1)-S(1)-Ru(2)
Ru(1)-S(1)-Ru(6)
89.64(6)
77.56(5)
Ru(5)-Ru(1)-Ru(6) 58.011(24)
Ru(2)-Ru(1)-Ru(5) 88.11(3)
Ru(2)-Ru(1)-Ru(6) 92.75(3)
Ru(2)-Ru(3)-Ru(4) 64.82(3)
Ru(5)-S(1)-Ru(6)
Ru(6)-P(1)-C(14) 120.24(23)
Ru(6)-P(1)-C(20) 111.10(20)
72.73(5)
F igu r e 1. Molecular structure of 3.
Ru(2)-Ru(4)-Ru(3) 57.767(25) Ru(6)-P(1)-C(26) 113.29(21)
tural features. Deep red plates of 3 were grown from
CH2Cl2/hexanes solutions. Crystals of 3 are triclinic and
belong to the space group P1h, Z ) 2. Additional crystal
data are available in Table 1. The molecular structure
of 3 is illustrated in Figure 1, while selected bond
lengths and angles are summarized in Table 2. The
metal framework of 3 consists of six Ru atoms, arranged
analogously to those of 1. A pentacoordinate sulfur
ligand S1 remains bonded through five σ-bonds to Ru1,
Ru2, Ru4, Ru5, and Ru6. A µ3,η2-diphenylthioureato
ligand spans the Ru2-Ru4 triangular substructure; the
S (S2) bridges Ru2 and Ru4, and the N (N2) is bound
terminally to Ru3. An apical CO ligand on Ru6 has
been replaced with a PPh3 ligand. An appropriate
number of terminal CO ligands were located on each of
the six Ru atoms.
Ru(2)-Ru(4)-Ru(5) 91.73(3)
S(1)-Ru(2)-S(2)
S(2)-C(1)-N(1)
S(2)-C(1)-N(2)
85.61(6)
114.5(5)
120.3(5)
125.2(6)
119.3(6)
Ru(2)-S(1)-Ru(4)
Ru(2)-S(2)-Ru(4)
Ru(2)-S(2)-C(1)
73.98(5)
73.97(5)
103.87(23) N(2)-C(1)-N(1)
Ru(3)-Ru(2)-Ru(4) 57.415(24) C(1)-N(2)-C(8)
bond lengths vary significantly among bonds of the same
order,16 the distance between Ru1 and Ru2 clearly does
not allow for a formal Ru-Ru bond. Lengthening of
M-M bonds in cluster complexes upon substitution of
a CO with the more basic phosphine has been noted
previously17 and demonstrates the inherent weakness
of the Ru1-Ru2 interaction. Within the Ru1-Ru5-
Ru6 triangular substructure, the sequence of long-short
bond lengths has changed with respect to that of 1,
(14) Hoferkamp, L.; Rheinwald, G.; Stoeckli-Evans, H.; Su¨ss-Fink,
G. Organometallics 1995, in press.
However, the original sofa-like configuration of 1 has
been somewhat distorted in 3. The distance between
Ru1 and Ru2 (3.3729(11) Å) has increased by ca. 0.20
Å from that of 1 (3.1713(6) Å). While, in general, M-M
(15) Beguin, A.; Rheinwald, G.; Stoeckli-Evans, H.; Su¨ss-Fink, G.
Helv. Chim. Acta 1994, 77, 525.
(16) J ohnson, B. F. G.; Rodgers, A. In The Chemistry of Metal Cluster
Complexes; Shriver, D. F., Kaesz, H. D., Adams, R. D., Eds.; VCH
Publishers Inc.: New York, 1990; Chapter 6.

Cluster Chemistry with Sulfur-Containing Ligands
Organometallics, Vol. 15, No. 4, 1996 1125
Ta ble 3. Selected Bon d Len gth s (Å) a n d An gles
(d eg) for 8
Ru(1)-Ru(2)
Ru(1)-Ru(3)
Ru(1)-Ru(5)
Ru(1)-S(1)
Ru(1)-S(2)
Ru(2)-Ru(3)
Ru(2)-Ru(4)
Ru(2)-S(1)
Ru(2)-S(2)
Ru(3)-N(1)
Ru(3)-C(91)
Ru(4)-Ru(5)
Ru(4)-Ru(6)
2.929(2)
2.728(2)
3.190(2)
2.392(3)
2.420(3)
2.720(2)
3.152(2)
2.401(3)
2.417(3)
2.166(11)
1.96(2)
Ru(4)-S(1)
Ru(5)-Ru(6)
Ru(5)-S(1)
Ru(6)-S(1)
S(2)-C(1)
N(1)-C(1)
N(1)-C(2)
N(2)-C(1)
N(2)-C(8)
N(91)-C(92)
C(91)-N(91)
C(98)-C(97)
2.394(3)
2.784(2)
2.393(3)
2.312(3)
1.802(12)
1.28(2)
1.45(2)
1.34(2)
1.44(2)
1.49(2)
1.14(2)
1.70(6)
2.907(2)
2.783(2)
Ru(1)-Ru(2)-Ru(4) 90.56(4) Ru(6)-Ru(5)-Ru(4) 58.50(4)
Ru(1)-S(1)-Ru(2) 75.34(9) Ru(6)-S(1)-Ru(1) 142.29(14)
Ru(1)-S(1)-Ru(4) 129.22(14) Ru(6)-S(1)-Ru(2) 142.35(14)
Ru(1)-S(1)-Ru(5)
83.64(10) Ru(6)-S(1)-Ru(4)
72.49(9)
72.56(9)
85.50(11)
85.36(11)
118.7(9)
125.8(11)
115.6(10)
Ru(2)-Ru(1)-Ru(5) 89.04(4) Ru(6)-S(1)-Ru(5)
Ru(2)-Ru(3)-Ru(1) 65.04(4) S(1)-Ru(1)-S(2)
F igu r e 2. Molecular structure of 8.
Ru(2)-S(2)-Ru(1)
74.53(10) S(1)-Ru(2)-S(2)
Ru(3)-Ru(1)-Ru(2) 57.35(4) N(1)-C(1)-S(2)
Ru(3)-Ru(1)-Ru(5) 145.65(5) N(1)-C(1)-N(2)
Ru(3)-Ru(2)-Ru(1) 57.62(4) N(2)-C(1)-S(2)
further suggesting alterations in the Ru-Ru bonding
scheme. The Ru1-Ru6 vector elongates by ca. 0.17 Å,
while Ru1-Ru5 shortens by ca. 0.13 Å. However, these
changes do not affect the proposed M-M bond orders
among those bonds. In opening the Ru1-Ru2 bond, the
cluster now assumes the appearance of two trinuclear
units linked through the µ5-S and a single long, but not
unreasonable, Ru-Ru bond (Ru4-Ru5, 3.0870(11) Å).
Although a bridging hydride could not be located in
a difference map, the NMR data are consistent with the
placement of this ligand spanning the Ru1-Ru5 vector;
no coupling to the phosphorus atom is observed. As
Ru(3)-Ru(2)-Ru(4) 147.09(5) N(91)-C(91)-Ru(3) 179.5(13)
Ru(4)-Ru(5)-Ru(1) 90.20(4) N(91)-C(92)-C(94) 106.3(14)
Ru(4)-Ru(6)-Ru(5) 62.96(4) N(91)-C(92)-C(95) 107(2)
Ru(4)-S(1)-Ru(2)
82.20(10) N(91)-C(92)-C(93) 108(2)
Ru(5)-Ru(4)-Ru(2) 90.17(4) C(1)-N(1)-Ru(3)
Ru(5)-S(1)-Ru(2) 127.26(14) C(1)-N(1)-C(2)
123.2(8)
117.9(11)
127.5(11)
118.9(9)
Ru(5)-S(1)-Ru(4)
74.80(10) C(1)-N(2)-C(8)
Ru(6)-Ru(4)-Ru(2) 97.25(4) C(2)-N(1)-Ru(3)
Ru(6)-Ru(4)-Ru(5) 58.54(4) C(91)-Ru(3)-Ru(1) 87.9(4)
Ru(6)-Ru(5)-Ru(1) 96.08(4) C(91)-N(91)-C(92) 177(2)
addition of P(n-Bu)3, either of the phosphites P(OMe)3
or P(OPh)3, (CH3)2S, or tBuNC gives, as the major
product, an intact, monosubstituted derivative of 1, (µ2-
H)Ru6(CO)15(L)(µ5-S)(µ3,η2-SCNHPhNPh) [L ) P(n-
C4H9)3 (4), L ) P(OMe)3 (5), L ) P(OPh)3 (6), L )
(CH3)2S (7), and L ) (CH)3CNC (8)]. The duration of
1
stated earlier, room temperature H NMR spectra of 3
show a broad, poorly defined peak in the hydride region.
Low temperature spectra, on the other hand, reveal two
closely spaced peaks in an approximately 1:1 ratio. The
two separate peaks, centered at -17.72 ppm (CD2Cl2,
400 MHz), reflect the presence of two isomers in
solutions of 3. One of these isomers (3a , δ ) -17.70
ppm) is believed to have the same structure as that
observed in the solid state (Figure 2), with the PPh3
ligand on Ru6 directed away from the bridging hydride.
The second isomer (3b, δ ) -17.73 ppm) presumably
contains the PPh3 ligand at the same ruthenium atom
(Ru6), but directed toward the bridging hydride.
With seven Ru-Ru bonds, the cluster 3 requires 94
electrons to achieve electronic saturation. No changes
in the bond lengths or angles of the diphenylthioureato
ligand indicate an alteration in its role as a five-electron
donor. The same is true for the sulfur ligand, which
continues to donate all six of its electrons to the cluster.
Like the CO ligands, PPh3 is a two-electron donor.
Overall, cluster 3 contains 92 electrons, the same
number as 1; thus, it must be considered an electroni-
cally unsaturated cluster.
the reaction has very little effect on the product yields.
These derivatives are all easily isolated by using
preparative TLC. Spectral data (see Experimental
Section) clearly show that the site of substitution in
these derivatives differs from that of 3. Insignificant
spectral shifts of relevant signals for the diphenylthio-
ureato ligand, as compared to the same data for 1,
establish its bonding mode as unchanged. Only in the
1
case of cluster 8 do the NH and CH3 signals in the H
When exposed to CO, 3 does not reform 1. Instead,
cluster fragmentation occurs to generate the trinuclear
species (µ2-H)Ru3(CO)9(µ3,η2-SCNHPhNPh).18 Presum-
ably, the CO coordinates at the highly unsaturated Ru2,
causing the Ru2-S1 bond to open. Further bond
scission must follow, with the eventual abstraction of
an ambient hydride to produce the trinuclear species.
Rea ction s of 1 w ith Oth er Tw o-Electr on -Don or
Liga n d s. In contrast to reactions of 1 with PPh3,
NMR spectra indicate the presence of isomers.
Verification of the site of substitution in 4-8 was
t
provided by structural analysis of the BuNC-substitut-
ed derivative, 8. Deep red plates of 8 could be isolated
from 3:1 acetone/heptane solutions maintained at 5 °C
for 3 days. These crystals were triclinic of the space
group P1h. Additional crystal data are summarized in
Table 1. An Ortep plot of 8 is presented in Figure 2,
and selected bond lengths and angles are given in Table
3. An area of disordered solvent was associated with
each molecule of 8.
(17) Bruce, M. I.; Liddell, M. J .; Hughes, C. A.; Skelton, B. W.; White,
A. H. J . Organomet. Chem. 1988, 347, 157.
(18) Bodensieck, U.; Stoeckli-Evans, H.; Su¨ss-Fink, G. Chem. Ber.
1990, 123, 1603.
The molecular structure of 8 shows a derivative of 1
resulting from CO substitution by the two-electron-

1126 Organometallics, Vol. 15, No. 4, 1996
Hoferkamp et al.
t
donor group BuNC. The cluster skeleton of 8, like that
of 1, consists of five Ru atoms arranged in a pentagonal-
shaped polyhedron with a sixth Ru atom bridging the
Ru-Ru edge opposite to the apex. The sixth Ru atom
rises above the planar base at an angle of 85°. The
pentacoordinate S remains bonded to five metal atoms,
four from the square base (Ru1, Ru2, Ru4, and Ru5)
and the out-of-plane, apical Ru atom (Ru6). The trian-
gular substructure composed of Ru1, Ru2, and Ru3 is
spanned by a µ3,η2-diphenylthioureato ligand. In con-
trast to the monosubstituted PPh3 derivative 3, the
isonitrile ligand in 8 occupies an axial position on the
in-plane apical Ru atom of the hexanuclear framework
(Ru3). Previous studies of trinuclear ruthenium clusters
containing µ3,η2-bridging ligands have shown regiospe-
cific substitution of carbonyl groups at the bridged metal
atoms,19 but substitution at the metal bound to the
nonbridging nitrogen atom of similar ligands is not
unfounded.20 The C91-N91-C92 pole of the ligand is
almost perfectly vertical, bending slightly toward (∼2°)
the interior of the cluster. Terminal carbonyl ligands
are found on each of the Ru atoms. A bridging hydride
could not be located in the refinement of the structural
F igu r e 3. Axial-equatorial exchange of the isonitrile
ligands of 8.
31P NMR experiments on the formation of compounds
3 and 7 was carried out. The 31P NMR spectrum of a
solution of 1 and P(OPh)3, maintained at -50 °C, shows
only the free ligand and a small amount of the substitu-
tion product, 7. Subsequent spectra obtained at in-
creasingly higher temperatures show the free ligand
signal intensity to diminish and the signal correspond-
ing to 7 to increase. At room temperature, only the
signal corresponding to the monosubstituted product is
found. From these data, it appears that the formation
of 7 is straightforward and involves no intermediates.
The reaction of 1 with any of P(n-Bu)3, P(OMe)3, Me2S,
t
or BuNC is assumed to follow an analogous course.
In contrast, the formation of 3 is more complex. The
-50 °C 31P NMR spectrum of a solution of 1 and PPh3
shows two signals, both in the region of metal-bound
phosphine groups, neither of which corresponds to the
substitution product 3. If the solution is allowed to
warm slowly, both of these signals deteriorate and
others grow in, until at room temperature a series of
signals associated with coordinated PPh3 groups is
observed as well as a signal for the free ligand. Ad-
ditional spectra obtained over a period of 48 h show no
indication of the product 3. However, chromatographic
workup of the test solution produces 3 in typical yields,
suggesting that the monosubstituted product is formed
during workup of the reaction solution, presumably on
the TLC plates.
1
data of 8, but the H NMR spectral data show a single
peak in the hydride region essentially unchanged from
that of 1. Thus, it is likely that a hydride ligand bridges
the Ru4-Ru5 vector of 8 (Ru1-Ru5 in 1).
The bond lengths and angles determined for 8 show
little change from those of 1. The two triangular
substructures consist of Ru-Ru single bonds, wherein
the multiply bridged sides (Ru4-Ru5 and Ru1-Ru2)
are elongated relative to the remaining two bonds. The
trinuclear units are connected by long Ru-Ru bonds
(Ru1-Ru5 and Ru2-Ru4); however, the bond lengths
lie within acceptable limits for Ru-Ru single bonds. As
in the case of 1, these interactions relieve the unsat-
uration predicted by standard electron counting rules
at Ru1 and Ru2 (Ru2 and Ru4 in 1). The noticeable
changes in the analogous Ru-Ru bond lengths apparent
upon comparison of 3 and 1 are absent from the
derivative 8.
Con clu sion s
It is established that the weakest points in transition
metal carbonyl clusters are the metal-metal bonds.3
Reactions of the hexanuclear “sofa” cluster 1 with PPh3
demonstrate the truly sensitive nature of these struc-
tural features. In this reaction, the effects of replacing
CO with the isoelectronic PPh3 manifest themselves in
metal-metal bond length changes relatively far re-
moved from the substitution site. Other two-electron-
donor groups, including some alternative phosphorus-
based reagents, also result in substitution, but not at
the same Ru atom, and moreover, these ligands do not
significantly affect the Ru framework relative to that
of 1. Steric interaction is most likely responsible for the
variation in the site of substitution.
The presence of isomers in solutions of 8, as implied
by the 1H NMR spectrum, is due to differing orientations
t
of the BuNC group. The situation is similar to that
described for 3. Steric requirements should favor an
orientation in which the tBuNC group occupies an axial
coordinate site on Ru3, as determined crystallographi-
cally. The phenyl group on N2 would interact to a lesser
extent with an axial tBuNC group than with that group
in an equatorial position. This axial isomer produces
the NH signal at 6.76 ppm and the CH3 signal at 1.50
t
ppm. The isomer containing the BuNC group in an
equatorial coordination site still occurs, however, and
is most likely responsible for the NH peak at 6.74 ppm
and the CH3 peak at 1.54 ppm. That binding still occurs
in the equatorial isomer can be traced to the large
Exp er im en ta l Section
Manipulations were carried out by using standard Schlenk
techniques21 under an N2 atmosphere. Thermolysis reactions
were performed in a high-pressure Schlenk tube able to
withstand 8 bars of internal pressure. TLC plates were
prepared by placing a uniform 0.5 mm layer of the appropriate
support (Al2O3 or SiO2, G or G/UV254, Macherey-Nagel) on 20
× 20 cm glass plates. Laboratory solvents were purified and
dried according to standard laboratory practices.22 Distillation
of solvents was carried out under an N2 atmosphere; the inert
atmosphere over the solvent was maintained thereafter. The
t
separation between the N2-bound phenyl and the Bu
methyls provided by the C-N-tBu linkage. This situ-
ation is illustrated in Figure 3. No NMR evidence was
found for the existence of isomers in solutions of 4-7.
In an effort to gain insight into the nature of these
substitution reactions, a series of variable temperature
(19) (a) Andreu, P. L.; Cabeza, J . A.; Riera, V.; Bois, C.; J eannin, Y.
J . Chem. Soc., Dalton Trans. 1990, 3347. (b) Andreu, P. L.; Cabeza,
J . A.; Riera, V. J . Organomet. Chem. 1990, 393, C30.
(21) Herzog, S.; Dehnert, J . Z. Chem. 1969, 4, 1.
(20) Cabeza, J . A.; Franco, R. J .; Llamazares, A.; Riera, V.; Pe´rez-
Carren˜o, E.; Van der Maelen, J . F. Organometallics 1994, 13, 55.
(22) Perrin, D. D.; Armarego, W. L. F. Purification of Laboratory
Chemicals, 3rd ed.; Pergamon Press: Oxford, England, 1988.

Cluster Chemistry with Sulfur-Containing Ligands
Organometallics, Vol. 15, No. 4, 1996 1127
cluster (µ2-H)Ru6(CO)16(µ5-S)(µ3,η2-SCNHPhNPh) (1) was pre-
pared according to a previously published procedure.11d Other
chemicals [tert-butylisonitrile (Fluka, 98%), methyl sulfide
(Fluka, 99%), tri-n-butylphosphine (Fluka, 99%), trimethyl
phosphite (Fluka, 99%), triphenylphosphine (Merck, 98%),
triphenyl phosphite (Fluka, 99%)] were purchased from the
appropriate vendors and used as received. Infrared spectra
were obtained by using a Perkin-Elmer 1720X spectrometer.
Room temperature 1H and 31P NMR spectra were acquired
with either a Bruker AMX 400 or a Varian Gemini 200 BB
spectrometer, while all low temperature 31P NMR spectra were
acquired by using the Bruker AMX 400 instrument with
controlled temperature nitrogen gas flow. Elemental analyses
were performed by the Mikroelementaranalytisches Labora-
torium der ETH, Zu¨rich, Switzerland. FAB mass spectra were
measured by Professor T. A. J enny of the University of
Fribourg, Fribourg, Switzerland, using 3-nitrobenzyl alcohol
(NBA) as the matrix.
unreacted 1, while the dark red band just below that contained
4 (12.6 mg, 40.6%). Spectral and analytical data for 4: IR (c-
hex, 298 K, νCO (cm-1)) 2085m, 2058vs, 2043s, 2026vs, 2013w,
2002m, 1988m, 1981w, 1956m, 1930w; IR (KBr, νNH (cm-1))
3355m; IR (KBr, νNCN (cm-1)) 1565m; 1H NMR (CDCl3, 298 K,
200 MHz) δ 7.10-7.70 (m, 10H, Ph-H), 6.85 (s, br, 1H, N-H),
2.06 (s, 6H, CH2-H); FAB-MS (NBA) M+, 1349. Anal. Calcd
for C30H18N2O15Ru6S3 (1353.4): C, 26.71; H, 1.34; N, 2.08.
Found: C, 26.58; H, 1.50; N, 1.98.
Syn th eses of (µ2-H)Ru 6(CO)15(L)(µ5-S)(µ3,η2-SCNHP h -
NP h ) [L ) P (n -C4H9)3 (5), P (OMe)3 (6), P (OP h )3 (7), (CH)3-
CNC (8)]. A solution of 1 (5, 59 mg, 0.045 mmol; 6, 33.5 mg,
0.025 mmol; 7, 67.4 mg, 0.051 mmol; 8, 62.0 mg, 0.047 mmol)
in a solvent (cyclohexane, CH2Cl2, or THF produced similar
results) (20 mL) was prepared in a dried Schlenk tube. Under
N2 flow, a solution of the appropriate reagent [5, P(n-C4H9)3,
12.0 µL, 0.048 mmol; 6, P(OMe)3, 3.0 µL, 0.027 mmol; 7
P(OPh)3, 14 µL, 0.054 mmol; 8, (CH)3CNC, 6 µL, 0.053 mmol]
in the appropriate solvent (10 mL) was added dropwise to the
stirring solution. The solution color immediately turned a
deeper shade of red. The reaction solution was stirred at room
temperature for 4-6 h. After removal of the solvent, the
residue was redissolved in CH2Cl2 and chromatographed on
TLC plates (Al2O3, 40:60 Et2O/cyclohexane). Compounds 5-8
were isolated from prominent red bands located in the lower
half of the TLC plates: 5, 15.3 mg, 22.8%; 6, 12.2 mg, 34.0%;
7, 53.4 mg, 65.2%; 8, 37.0 mg, 53.0%. Samples for elemental
analyses were prepared by recrystallization of the products
from CH2Cl2 layered with an excess of hexane or heptane.
Spectral and analytical data for 5: IR (c-hex, 298 K, νCO (cm-1))
2083m, 2056vs, 2041s, 2023vs, 2011w, 2001m, 1985m,br,
1953m, 1925w; IR (KBr, νNH (cm-1)) 3362m; IR (KBr, νNCN
(cm-1)) 1552m; 1H NMR (CDCl3, 298 K, 200 MHz) δ 7.40-
7.10 (m, Ph-H), 6.81 (s, br, N-H), 1.43-0.84 (m, C4H9), -22.07
(s, Ru2-H); 31P NMR (CDCl3, 298 K) δ 20.81(s); FAB-MS (NBA)
M+, 1486. Anal. Calcd for C40H38N2O15PRu6S2‚1/2C6H14 (1535.6):
C, 33.6; H, 2.95; N, 1.82. Found: C, 33.0; H, 3.03; N, 2.29.
Spectral and analytical data for 6: IR (c-hex, 298 K, νCO (cm-1))
2084m, 2057vs, 2041s, 2030s, 2015m, 2001m, 1989m, 1966w,
br, 1937w,br; IR (KBr, νNH (cm-1)) 3339m; IR (KBr, νNCN (cm-1))
1554m; 1H NMR (CDCl3, 298 K, 200 MHz) δ 7.57-7.10 (m,
10H, Ph-H), 6.78 (s, br, 1H, N-H), 3.45 (d, J ) 11.2 Hz, 9H,
CH2-H), -22.0 (s, 1H, Ru2-H); 31P NMR (CDCl3, 298 K) δ
X-ray structural data were collected on a Stoe-Siemens AED
2 four-circle diffractometer using graphite-monochromatized
Mo KR radiation (λ ) 0.710 73 Å). Low temperature measure-
ments were carried out using controlled temperature nitrogen
gas flow. Crystal structure data were solved by using
SHELXS8623 and refined by using either SHELXL9324 or the
NRCVAX25 program packages of the VAX-Cluster of the
De´partment de Calcul de l’Universite´ de Neuchaˆtel. Illustra-
tions showing thermal motion ellipsoids were drawn by using
either the PLATON26 or ZORTEP27 program. Additional data
for structures have been deposited with the Cambridge
Crystallographic Data Centre, Union Road, GB-Cambridge
CB2 1EW, UK.
Syn th eses of Ru 2(CO)4(P P h 3)2(µ2,η2-SCNHP h NP h ) (2)
a n d (µ2-H)Ru 6(CO)15(P P h 3)(µ5-S)(µ3,η2-SCNHP h NP h ) (3).
A solution of 1 (64 mg, 0.049 mmol) and PPh3 (26.8 mg, 0.102
mmol) in C6H12 (20 mL) was stirred for 24 h. The solvent was
removed, the residue was dissolved in CH2Cl2, and the
products were separated by TLC (Al2O3, 40:60 Et2O/cyclohex-
ane). Compound 2 was isolated from a leading yellow band
(<3% yield), while compound 3 was isolated from a red brown
band (29 mg, 38.2%) located just below a faint red band
consisting of the trinuclear compound Ru3(CO)6(µ2-C6H5)(µ2-
PPh2)(µ3,η2-SCNHPhNPh)14 (trace). Crystals of 3 were grown
at room temperature from a 3:1 CH2Cl2/C7H16 solution. Spec-
troscopic and analytical data for 2: IR (c-hex, 298 K, νCO (cm-1))
2019vs, 1987m, 1956s; IR (KBr, νNCN (cm-1)) 1624m, 1588m;
1H NMR (CDCl3 298 K, 200 MHz) δ 7.48-6.85 (m, 25H, Ph-
H), 5.72 (s, br, 1H, N-H); 31P NMR (CDCl3, 298 K) δ 24.47 (s).
Anal. Calcd for C66H52N4O4P2Ru2S2‚CH2Cl2 (1378): C, 58.39;
H, 3.95; N, 4.06. Found: C, 59.0; H, 4.17; N, 2.77. Spectro-
scopic and analytical data for 3: IR (c-hex, 298 K, νCO (cm-1))
2061vs, 2040s, 2004s, 1989w,br, 1943m,br; IR (KBr, νNH (cm-1))
3333m; IR (KBr, νNCN (cm-1)) 1564m; 1H NMR (CDCl3, 298 K,
200 MHz) δ 7.60-7.07 (m, 25H, Ph-H), 6.74 (s, br, 1H, N-H),
-17.50 (s, br, 1H, Ru2-H); 31P NMR (CDCl3, 298 K) δ 34.48(s).
Anal. Calcd for C46H28N2O15PRu6S2 (1550.3): C, 35.64; H, 1.82;
N, 1.81. Found: C, 35.92; H, 1.62; N, 1.86.
121.8(s); FAB-MS (NBA) M+, 1409. Anal. Calcd for C31H21
-
N2O18PRu6S2 (1411.0): C, 26.4; H, 1.50; N, 1.99. Found: C,
27.15; H, 1.58; N, 2.05. Spectral and analytical data for 7:
IR (c-hex, 298 K, νCO (cm-1) 2085m, 2058vs, 2042s, 2031s,sh,
2016m, 2002m, 1990m, 1976w,br, 1942w,br; IR (KBr, νNH
(cm-1)) 3355m; IR (KBr, νNCN (cm-1)) 1556m; 1H NMR (CDCl3,
298 K, 200 MHz) δ 7.43-6.67 (m, 25H, Ph-H), 6.78 (s, br, 1H,
N-H), -22.1 (s, 1H, Ru2-H); 31P NMR (CDCl3, 298 K) δ 107.7(s);
FAB-MS (NBA) M+, 1598. Anal. Calcd for C46H27N2O18PRu6S2
(1597.2): C, 34.6; H, 1.70; N, 1.75. Found: C, 34.8; H, 1.69; N,
1.72. Spectral and analytical data for 8: IR (c-hex, 298 K,
ν
NC (cm-1)) 2166w; IR (c-hex, 298 K, νCO (cm-1)) 2084m, 2057vs,
2042s, 2033s, 2016m, 2001m, 1989s,br, 1976w,br, 1939w,br;
IR (KBr, νNH (cm-1)) 3361w; IR (KBr, νNCN (cm-1)) 1564m; H
1
Syn th eses of (µ2-H)Ru 6(CO)15(µ5-S)(Me2S)(µ3,η2-SCN-
HP h NP h ) (4). A solution of 1 (30 mg, 0.023 mmol) and Me2S
(0.075 mmol) in CH2Cl2 (10 mL) was stirred at room temper-
ature for 7 days. After removal of the solvent, the residue was
redissolved in CH2Cl2 and chromatographed on Al2O3 TLC
plates in 1:3 CH2Cl2/cyclohexane. The first band contained
NMR (CDCl3, 298 K, 200 MHz) δ 7.60-7.12 (m, 10H, Ph-H),
6.76 (s, br, 0.8H), 6.74 (s, br, 0.2H, N-H), 1.54 (s, 0.2H) 1.50
(s, 0.8H, C-H3), -21.91 (s, 1H, Ru2-H). Anal. Calcd for
C33H21N3O15S2Ru6‚0.5C7H16 (1420.1): C, 30.87; H, 2.06; N, 2.96.
Found: C, 29.92; H, 2.02; N, 2.96.
Ack n ow led gm en t. We thank the Fonds National
Suisse pour la Recherche Scientifique for financial
support and the J ohnson Matthey Technology Centre
for a generous loan of ruthenium trichloride hydrate.
(23) Sheldrick, G. M. SHELXS-86, Program for Crystal Structure
Determination; University of Go¨ttingen: Go¨ttingen, Germany, 1986.
(24) Sheldrick, G. M. SHELXL-93, Program for Refinement of
Crystal Structure Data; University of Go¨ttingen: Go¨ttingen, Germany,
1993.
(25) Gabe, E. J.; Le Page, Y.; Charland, J.-P.; Lee, F. L. NRCVAXsan
Interactive Program System for Structure Analysis. J . Appl. Crystal-
logr. 1989, 22, 384.
Su p p or tin g In for m a tion Ava ila ble: Tables of positional
and thermal parameters, bond lengths and angles, and least
squares planes for 3 and 8 (18 pages). Ordering information
is available on any current masthead page.
(26) Spek, A. L. Acta Crystallogr. 1990, A46, C34.
(27) J ohnson, C. K. ORTEP; Oak Ridge National Laboratory: Oak
Ridge, TN [modified for PC (ZORTEP) by L. Zsolnai and H. Pritzkow,
University of Heidelberg, Heidelberg, Germany, 1994].
OM950739V